All posts by Daniel Stenberg

Working open source

I work full time on open source and this is how.


I started learning how to program in my teens, well over thirty years ago and I’ve worked as a software engineer and developer since the early 1990s. My first employment as a developer was in 1993. I’ve since worked for and with lots of companies and I’ve worked on a huge amount of (proprietary) software products and devices over many years. Meaning: I certainly didn’t start my life open source. I had to earn it.

When I was 20 years old I did my (then mandatory) military service in Sweden. After having endured that, I applied to the university while at the same time I was offered a job at IBM. I hesitated, but took the job. I figured I could always go to university later – but life took other turns and I never did. I didn’t do a single day of university. I haven’t regretted it.

I learned to code in the mid 80s on a Commodore 64 and software development has been one of my primary hobbies ever since. One thing it taught me well, that I still carry with me, is to spend a few hours per day in front of my home computer.

And then I shipped curl

In the spring of 1998 I renamed my little pet project of the time again and I released the first ever curl release. I have told this story many times, but since then I have spent two hours or so of my spare time on that project – every day for over twenty years. While still working as a software engineer by day.

Over time, curl gradually grew popular and attracted more users. There was no sudden moment in time where I struck gold and everything took off. It was just slowly gaining ground while me and my fellow project members kept improving and polishing curl. At some point in time I happened to notice that curl and libcurl would appear in more and more acknowledgements and in open source license collections in products and devices.

It was still just a spare time project.

Proprietary Software for years

I’d like to emphasize that I worked as a contract and consultant developer for many years (over 20!), primarily on proprietary software and custom solutions, before I managed to land myself a position where I could primarily write open source as part of my job.


In 2014 I joined Mozilla and got the opportunity to work on the open source project Firefox for a living – and doing it entirely from my home. This was the first time in my career I actually spent most of my days on code that was made public and available to the world. They even allowed me to spend a part of my work hours on curl, even if that didn’t really help them and curl was not a fundamental part of any Mozilla work or products. It was still great.

I landed that job for Mozilla a lot thanks to my many years and long experience with portable network coding and running a successful open source project at this level.

My work setup with Mozilla made it possible for me to spend even more time on curl, apart from the (still going) two daily spare time hours. Nobody at Mozilla cared much about (my work with) curl and no one there even asked me about it. I worked on Firefox for a living.

For anyone wanting to do open source as part of their work, getting a job at a company that already does a lot of open source is probably the best path forward. Even if that might not be easy either, and it might also mean that you would have to accept working on some open source projects that you might not yourself be completely sold on.

In late 2018 I quit Mozilla, in part because I wanted to try to work with curl “for real” (and part other reasons that I’ll leave out here). curl was then already over twenty years old and was used more than ever before.


I now work for wolfSSL. We sell curl support and related services to companies. Companies pay wolfSSL, wolfSSL pays me a salary and I get food on the table. This works as long as we can convince enough companies that this is a good idea.

The vast majority of curl users out there of course don’t pay anything and will never pay anything. We just need a small number of companies to do it – and it seems to be working. We help customers use curl better, we make curl better for them and we make them ship better products this way. It’s a win win. And I can work on open source all day long thanks to this.

My open source life-style

A normal day in the work week, I get up before 7 in the morning and I have breakfast with my family members: my wife and my two kids. Then I grab my first cup of coffee for the day and take the thirteen steps up the stairs to my “office”.

I sit down in front of my main development (Linux) machine with two 27″ screens and get to work.

Photo of my work desk from a few years ago but it looks very similar still today.

What work and in what order?

I lead the curl project. It means many questions and decisions fall down to me to have an opinion about or say on, and it’s a priority for me to make sure that I unblock such situations as soon as possible so that developers wanting to do things with curl can continue doing that.

Thus, I read and respond to email about curl all hours I’m awake and have network access. Of course incoming messages actually rarely require immediate responses and then I can queue them up and instead do them later. I also try to read and assess all new incoming curl issues as soon as possible to see if there’s something urgent I should deal with immediately, or otherwise figure out how to deal with them going forward.

I usually have a set of bugs or features to work on so when there’s no alarming email or GitHub issue left, I context-switch over to the curl source code tree and the particular branch in which I work on right now. I typically have 20-30 different branches of development of various stages and maturity going on. If I get stuck on something, or if I create a pull-request for one of them that needs time to get all the CI jobs done, I switch over to one of the others.

Customers and their needs of course have priority when I decide what to work on. The exception would perhaps be security vulnerabilities or other really serious bugs being reported, but thankfully they are rare. But after that, I go by ear and work on what I think is fun and what I think users might appreciate.

If I want to go forward with something, for my own sake or for a customer’s, and that entails touching or improving other software in other projects, then I don’t shy away from submitting pull requests for them – or at least filing an issue.

Spare time open source

Yes, I still spend my spare time hours on open source, mostly curl. This means I often end up spending 50-55 hours per week on curl and curl related activities. But I don’t count or measure work hours and I rarely have to report any to anyone. This is a work of love.

Lots of people will say that they don’t have time because of life, family, kids etc. I have of course been very fortunate over the years to have had the opportunity and ability to spend all this time on what I want to do, but let’s not forget that people in general spend lots of time on their hobbies; on watching TV, on playing computer games and on socializing with friends and why not: to sleep. If you cut down on all of those things (yes, including the sleeping) there could very well be opportunities. It’s often a question of priorities. I’ve made spare time development a priority in my life.

curl support?

Any company that uses curl or libcurl – and they are plenty – could benefit from buying support from us instead of wasting their own time and resources. We at wolfSSL are probably much better at curl already and we can find and fix the issues much faster, which ends up cheaper and better long-term.


The top photo is taken by Anja Stenberg, my wife. It’s me in a local forest, summer 2020.

A server transition

The main physical server (we call it giant) we’ve been using at Haxx for a very long time to host sites and services for 20+ domains and even more mailing lists. The machine – a physical one – has been colocated in an ISP server room for over a decade and has served us very well. It has started to show its age.

Some of the more known sites and services it hosts are perhaps curl, c-ares, libssh2 and this blog (my entire site). Some of these services are however primarily accessed via fronting CDN servers.

giant is a physical Dell PowerEdge 1850 server from 2005, which has undergone upgrades of CPU, disks and memory through the years.

giant featured an Intel X3440 Xeon CPU at 2.53GHz with 8GB of ram when decommissioned.

New host

The new host is of course entirely virtual and we’ve finally taken the step into the modern world of VPSes. The new machine is hosted by the same provider as before but as an entirely new instance.

We’ve upgraded the OS, all packages and we’ve remodeled how we run the web services and all our jobs and services from before have been moved into this new fresh server in an attempt to leave some of the worst legacies behind.

The former server will not be used anymore and will be powered down and sent for recycling.

Glitches in this new world

We’ve tried really hard to make this transition transparent and ideally not many users will notice anything or have a reason to bother about this, but of course we also realize that we probably have not managed this to 100% perfection. If you detect something on any of the services we run that used to work or exist but isn’t anymore, do let us know so that become aware of it and can work on a fix!

This site ( already moved weeks ago and nobody noticed. The curl site changed on October 23 and are much more likely to get glitches because of all the many more scripts and automatic things setup for it. Both sites are served via Fastly so ordinary users will not detect or spot that there’s a new host in the back end.

Three years since the Polhem prize

Today, exactly three years ago, I received flowers, money and a gold medal at a grand prize ceremony that will forever live on in my mind and memory. I was awarded the Polhem Prize for my decades of work on curl. The prize itself was handed over to me by no one else than the Swedish king himself. One of the absolute top honors I can imagine in my little home country.

In some aspects, my life is divided into the life before this event and the life after. The prize has even made little me being presented on a poster in the Technical Museum in Stockholm. The medal itself still sits on my work desk and if I just stop starring at my monitors for a moment and glance a little over to the left – I can see it. I think the prize made my surroundings, my family and friends get a slightly different view and realization of what I actually do all these hours in front of my screens.

In the tree years since I received the prize, we’ve increased the total number of contributors and authors in curl by 50%. We’ve done over 3,700 commits and 25 releases since then. Upwards and onward.

Life moved on. It was not “peak curl”. There was no “prize curse” that left us unable to keep up the pace and development. It was possibly a “peak life moment” there for me personally. As an open source maintainer, I can’t imagine many bigger honors or awards to come my way ever again, but I’m not complaining. I got the prize and I still smile when I think about it.

curl 7.73.0 – more options

In international curling competitions, each team is given 73 minutes to complete all of its throws. Welcome to curl 7.73.0.

Release presentation


the 195th release
9 changes
56 days (total: 8,2XX)

135 bug fixes (total: 6,462)
238 commits (total: 26,316)
3 new public libcurl function (total: 85)
1 new curl_easy_setopt() option (total: 278)

2 new curl command line option (total: 234)
63 contributors, 31 new (total: 2,270)
35 authors, 17 new (total: 836)
0 security fix (total: 95)
0 USD paid in Bug Bounties (total: 2,800 USD)


We have to look back almost two years to find a previous release packed with more new features than this! The nine changes we’re bringing to the world this time are…


Tell curl where to store the file when you use -o or -O! Requested by users for a long time. Now available!

find .curlrc in $XDG_CONFIG_HOME

If this environment variable is set and there’s a .curlrc in there, it will now be used in preference to the other places curl will look for it.

--help has categories

The huge wall of text that --help previously gave us is now history. “curl --help” will now only show a few important options and the rest is provided if you tell curl which category you want to list help for. Requested by users for a long time.

API for meta-data about easy options

With three new functions, libcurl now provides an API for getting meta-data and information about all existing easy options that an application (possibly via a binding) can set for a transfer in libcurl.


We’ve introduced a new error code, and what makes this a little extra note-worthy besides just being a new way to signal a particular error, is that applications that receive this error code can also query libcurl for extended error information about what exactly in the proxy use or handshake that failed. The extended explanation is extracted with the new CURLINFO_PROXY_ERROR.

MQTT by default

This is the first curl release where MQTT support is enabled by default. We’ve had it marked experimental for a while, which had the effect that virtually nobody actually used it, tried it or even knew it existed. I’m very eager to get some actual feedback on this…

SFTP commands: atime and mtime

The “quote” functionality in curl which can sends custom commands to the SFTP server to perform various instructions now also supports atime and mtime for setting the access and modification times of a remote file.

knownhosts “fine replace” instruction

The “known hosts” feature is a SSH “thing” that lets an application get told if libcurl connects to a host that isn’t already listed in the known hosts file and what to do about it. This new feature lets the application return CURLKHSTAT_FINE_REPLACE which makes libcurl treat the host as “fine” and it will then replace the host key for that host in the known hosts file.

Elliptic curves

Assuming you use the supported backend, you can now select which curves libcurl should use in the TLS handshake, with CURLOPT_SSL_EC_CURVES for libcurl applications and with --curves for the command line tool.


As always, here follows a small selection of the many bug-fixes done this cycle.

buildconf: invoke ‘autoreconf -fi’ instead

Born in May 2001, this script was introduced to build configure etc so that you can run configure etc, when you’re building code straight from git (even if it was CVS back in 2001).

Starting now, the script just runs ‘autoreconf -fi‘ and no extra custom magic is needed.

checksrc: now even stricter

Our homegrown code style checker got better and now also verifies:

  1. Mandatory space after do and before while in “do { } while
  2. No space allowed after an exclamation mark in if(!true) expressions
  3. // comments are not allowed even on column 0

cmake: remove scary warning

The text that was previously output, saying that the cmake build has glitches has been removed. Not really because it has gotten much better, but to not scare people away, let them proceed and use cmake and then report problems and submit pull requests that fix them!

curl: lots of dynbuf in the tool

The libcurl internal system for doing “dynamic buffers” (strings) is now also used by the curl tool and a lot of custom realloc logic has been converted over.

curl: retry and delays in parallel mode

The --retry option didn’t work when doing parallel transfers, and when fixed, the retry delay option didn’t work either so that was also fixed!

etag: save and use the full received contents

We changed the handling of saving and loading etags a little bit, so that it now stores and loads the full etag contents and not just the bytes between double quotes. This has the added benefit that the etag functionality now also works with weak etags and when working with servers that don’t really follow the syntax rules for etags.

ftp: a 550 SIZE response returns CURLE_REMOTE_FILE_NOT_FOUND

Getting a 550 response back from a SIZE command will now make curl return at once saying the file is missing. Previously it would put that into the generic “SIZE doesn’t work” pool of errors, but that could lead to some inconsistent return codes.

ftp: make a 552 response return CURLE_REMOTE_DISK_FULL

Turns out that when uploading data to an FTP server, there’s a dedicated response code for the server to tell the client when the disk system is full (there was not room enough to store the file) and since there was also an existing libcurl return code for it, this new mapping thus made perfect sense…

ftp: separate FTPS from FTP over “HTTPS proxy”

The FTP code would previously wrongly consider a HTTPS proxy to euqal *using TLS” as if it was using TLS for its control channel, which made it wrongly use commands that are reserved for TLS connections.

libssh2: SSH protocols over HTTPS proxy

Some adjustments were necessary to make sure the SCP and SFTP work when spoken over an HTTPS proxy. Also note that this fix is only done for the libssh2 backend (primarily because I didn’t find out to do it in the others when I did a quick check).

pingpong: use dynbuf sending commands

I blogged about this malloc reducing adventure separately. The command sending function in curl for FTP, IMAP, POP3 and SMTP now reuses the same memory buffer much better over a transfer’s lifetime.

runtests: add %repeat[]% and %hex[]hex% for test files

One of the really nice benefits of “preprocessing” the text files and generate another file that is actually used for running tests, is that we now can provide meta-instructions to generate content when the test starts up. For example, we had tests that had a megabyte or several hundred kilobytes of text for test purposes, and how we can replace those with just a single %repeat% instruction. Makes test cases much smaller and much easier to read. %hex% can generate binary output given a hex string.

runtests: curl’s version as %VERSION

The test suite now knows which version of curl it tests, which means it can properly verify the User-Agent: headers in curl’s outgoing HTTP requests. We’ve previously had a filter system that would filter out such headers before comparing. Yours truly then got the pleasure of updating no less than 619 test cases to longer filter off that header for the comparison and instead use %VERSION!

win32: drop support for WinSock version 1

Apparently version 2 has been supported in Windows since Windows 95…


Unless we’ve screwed up somewhere, the next release will be 7.74.0 and ship on December 9, 2020. We have several pending pull-requests already in the queue with new features and changes!

rust in curl with hyper

tldr: work has started to make Hyper work as a backend in curl for HTTP.

curl and its data transfer core, libcurl, is all written in C. The language C is known and infamous for not being memory safe and for being easy to mess up and as a result accidentally cause security problems.

At the same time, C compilers are very widely used and available and you can compile C programs for virtually every operating system and CPU out there. A C program can be made far more portable than code written in just about any other programming language.

curl is a piece of “insecure” C code installed in some ten billion installations world-wide. I’m saying insecure within quotes because I don’t think curl is insecure. We have our share of security vulnerabilities of course, even if I think the rate of them getting found has been drastically reduced over the last few years, but we have never had a critical one and with the help of busloads of tools and humans we find and fix most issues in the code before they ever land in the hands of users. (And “memory safety” is not the single explanation for getting security issues.)

I believe that curl and libcurl will remain in wide use for a long time ahead: curl is an established component and companion in scripts and setups everywhere. libcurl is almost a de facto standard in places for doing internet transfers.

A rewrite of curl to another language is not considered. Porting an old, established and well-used code base such as libcurl, which to a far degree has gained its popularity and spread due to a stable API, not breaking the ABI and not changing behavior of existing functionality, is a massive and daunting task. To the degree that so far it hasn’t been attempted seriously and even giant corporations who have considered it, have backpedaled such ideas.

Change, but not change

This preface above might make it seem like we’re stuck with exactly what we have for as long as curl and libcurl are used. But fear not: things are more complicated, or perhaps brighter, than it first seems.

What’s important to users of libcurl needs to be kept intact. We keep the API, the ABI, the behavior and all the documented options and features remain. We also need to continuously add stuff and keep up with the world going forward.

But we can change the internals! Refactor as the kids say.

Backends, backends, backends

Already today, you can build libcurl to use different “backends” for TLS, SSH, name resolving, LDAP, IDN, GSSAPI and HTTP/3.

A “backend” in this context is a piece of code in curl that lets you use a particular solution, often involving a specific third party library, for a certain libcurl functionality. Using this setup you can, for example, opt to build libcurl with one or more out of thirteen different TLS libraries. You simply pick the one(s) you prefer when you build it. The libcurl API remains the same to users, it’s just that some features and functionality might differ a bit. The number of TLS backends is of course also fluid over time as we add support for more libraries in the future, or even drop support for old ones as they fade away.

When building curl, you can right now make it use up to 33 different third party libraries for different functions. Many of them of course mutually exclusive, so no single build can use all 33.

Said differently: you can improve your curl and libcurl binaries without changing any code, by simply rebuilding it to use another backend combination.

Green boxes are possible third-party dependencies curl can be told to use. No Hyper in this map yet…

libcurl as a glorified switch

With an extensive set of backends that use third party libraries, the job of libcurl to a large extent becomes to act as a switch between the provided stable external API and the particular third party library that does the heavy lifting.

API <=> glue code in C <=> backend library

libcurl as the rock, with a door and the entry rules written in stone. The backends can come and go, change and improve, but the applications outside the entrance won’t notice that. They get a stable API and ABI that they know and trust.

Safe backends

This setup provides a foundation and infrastructure to offer backends written in other languages as part of the package. As long as those libraries have APIs that are accessible to libcurl, libraries used by the backends can be written in any language – but since we’re talking about memory safety in this blog post the most obvious choices would probably be one of the modern and safe languages. For example Rust.

With a backend library written in Rust , libcurl would lean on such a component to do low level protocol work and presumably, by doing this it increases the chances of the implementations to be safe and secure.

Two of the already supported third party libraries in the world map image above are written in Rust: quiche and Mesalink.

Hyper as a backend for HTTP

Hyper is a HTTP library written in Rust. It is meant to be fast, accurate and safe, and it supports both HTTP/1 and HTTP/2.

As another step into this world of an ever-growing number of backends to libcurl, work has begun to make sure curl (optionally) can get built to use Hyper.

This work is gracefully funded by ISRG, perhaps mostly known as the organization behind Let’s Encrypt. Thanks!

Many challenges remain

I want to emphasize that this is early days. We know what we want to do, we know basically how to do it but from there to actually getting it done and providing it in source code to the world is a little bit of work that hasn’t been done. I’m set out to do it.

Hyper didn’t have a C API, they’re working on making one so that C based applications such as curl can actually use it. I do my best at providing feedback from my point of view, but as I’m not really into Rust much I can’t assist much with the implementation parts there.

Once there’s an early/alpha version of the API to try out, I will first make sure curl can get built to use Hyper, and then start poking on the code to start using it.

In that work I expect me to have to go back to the API with questions, feedback and perhaps documentation suggestions. I also anticipate challenges in switching libcurl internals to using this. Mostly small ones, but possibly also larger ones.

I have created a git branch and make my work on this public and accessible early on to let everyone who wants to, to keep up with the development. A first milestone will be the ability to run a single curl test case (any test case) successfully – unmodified. The branch is here: – beware that it will be rebased frequently.

There’s no deadline for this project and I don’t yet have any guesses as when there will be anything to test.

Rust itself is not there yet

This project is truly ground work for future developers to build upon as some of the issues dealt with in here should benefit others as well down the road. For example it immediately became obvious that Rust in general encourages to abort on out-of-memory issues, while this is a big nono when the code is used in a system library (such as curl).

I’m a bit vague on the details here because it’s not my expertise, but Rust itself can’t even properly clean up its memory and just returns error when it hits such a condition. Clearly something to fix before a libcurl with hyper could claim identical behavior and never to leak memory.

By default?

Will Hyper be used by default in a future curl build near you?

We’re going to work on the project to make that future a possibility with the mindset that it could benefit users.

If it truly happens involve many different factors (for example maturity, feature set, memory footprint, performance, portability and on-disk footprint…) and in particular it will depend a lot on the people that build and ship the curl packages you use – which isn’t the curl project itself as we only ship source code. I’m thinking of Linux and operating system distributions etc.

When it might happen we can’t tell yet as we’re still much too early in this process.

Still a lot of C

This is not converting curl to Rust.

Don’t be fooled into believing that we are getting rid of C in curl by taking this step. With the introduction of a Hyper powered backend, we will certainly reduce the share of C code that is executed in a typical HTTP transfer by a measurable amount (for those builds), but curl is much more than that.

It’s not even a given that the Hyper backend will “win” the competition for users against the C implementation on the platforms you care about. The future is not set.

More backends in safe languages?

Sure, why not? There are efforts to provide more backends written in Rust. Gradually, we might move into a future where less and less of the final curl and libcurl executable code was compiled from C.

How and if that will happen will of course depend on a lot of factors – in particular funding of the necessary work.

Can we drive the development in this direction even further? I think it is much too early to speculate on that. Let’s first see how these first few episodes into the coming decades turn out.


ISRG’s blog post: Memory Safe ‘curl’ for a More Secure Internet and the hacker news discussion.


Image by Peter H from Pixabay

everything curl five years

The first content to the book Everything curl was committed on September 24, 2015 but I didn’t blog about it until several months later in December 2015: Everything curl – work in progress.

At the time of that blog post, the book was already at 13,000 words and 115 written subsections. I still had that naive hope that I would have it nearly “complete” by the summer of 2016. Always the optimist.

Today, the book is at over 72,000 words with content in 600 subsections – with just 21 subtitles noted “TBD” to signal that there’s still content to add there. The PDF version of it now clocks in at over 400 pages.

I’ve come to realize and accept that it will never be “complete” and that we will just keep on working on it indefinitely since curl itself keeps changing and we keep improving and expanding texts in the book.

Right now, we have 21 sections marked as not done, but then we’ve also added features through these five years that we haven’t described in the book yet. And there are probably other areas still missing too that would benefit the book to add. There’s no hurry, we’ll just add more content when we get around to it.

Everything curl is quite clearly the most complete book and resource about curl, libcurl, the project and how all of it works. We have merged contributions from 39 different authors and we’re always interested in getting more help!

Printed version

We’ve printed two editions of the book. The 2017 and the 2018 versions. As of 2020, the latest edition is out of print. If you really want one, email Dan Fandrich as mention on the web page this link takes you to. Maybe we can make another edition reality again.

The book was always meant to remain open and free, we only sell the printed version because it costs actual money to produce it.

For a long time we also offered e-book versions of everything curl, but sadly gitbooks removed those options in a site upgrade a while ago so now unfortunately we only offer a web version and a PDF version.

Other books?

There are many books that mention curl and that have sections or parts devoted to various aspects of curl but there are not many books about just curl. curl programming (by Dan Gookin) is one of those rare ones.

Reducing mallocs for fun

Everyone needs something fun to do in their spare time. And digging deep into curl internals is mighty fun!

One of the things I do in curl every now and then is to run a few typical command lines and count how much memory is allocated and how many memory allocation calls that are made. This is good project hygiene and is a basic check that we didn’t accidentally slip in a malloc/free sequence in the transfer path or something.

We have extensive memory checks for leaks etc in the test suite so I’m not worried about that. Those things we detect and fix immediately, even when the leaks occur in error paths – thanks to our fancy “torture tests” that do error injections.

The amount of memory needed or number of mallocs used is more of a boiling frog problem. We add one now, then another months later and a third the following year. Each added malloc call is motivated within the scope of that particular change. But taken all together, does the pattern of memory use make sense? Can we make it better?


Now this is easy because when we build curl debug enabled, we have a fancy logging system (we call it memdebug) that logs all calls to “fallible” system functions so after the test is completed we can just easily grep for them and count. It also logs the exact source code and line number.

cd tests
./runtests -n [number]
egrep -c 'alloc|strdup' log/memdump

Let’s start

Let me start out with a look at the history and how many allocations (calloc, malloc, realloc or strdup) we do to complete test 103. The reason I picked 103 is somewhat random, but I wanted to look at FTP and this test happens to do an “active” transfer of content and makes a total of 10 FTP commands in the process.

The reason I decided to take a closer look at FTP this time is because I fixed an issue in the main ftp source code file the other day and that made me remember the Curl_pp_send() function we have. It is the function that sends FTP commands (and IMAP, SMTP and POP3 commands too, the family of protocols we refer to as the “ping pong protocols” internally because of their command-response nature and that’s why it has “pp” in the name).

When I reviewed the function now with my malloc police hat on, I noticed how it made two calls to aprintf(). Our printf version that returns a freshly malloced area – which can even cause several reallocs in the worst case. But this meant at least two mallocs per issued command. That’s a bit unnecessary, isn’t it?

What about a few older versions

I picked a few random older versions, checked them out from git, built them and counted the number of allocs they did for test 103:

7.52.1: 141
7.68.0: 134
7.70.0: 137
7.72.0: 123

It’s been up but it has gone down too. Nothing alarming, Is that a good amount or a bad amount? We shall see…

Cleanup step one

The function gets printf style arguments and sends them to the server. The sent command also needs to append CRLF to the data. It was easy to make sure the CRLF appending wouldn’t need an extra malloc. That was just sloppy of us to have there in the first place. Instead of mallocing the new printf format string with CRLF appended, it could use one in a stack based buffer. I landed that as a first commit.

This trimmed off 10 mallocs for test 103.

Step two, bump it up a notch

The remaining malloc allocated the memory block for protocol content to send. It can be up to several kilobytes but is usually just a few bytes. It gets allocated in case it needs to be held on to if the entire thing cannot be sent off over the wire immediately. Remember, curl is non-blocking internally so it cannot just sit waiting for the data to get transferred.

I switched the malloc’ed buffer to instead use a ‘dynbuf’. That’s our internal “dynamic buffer” system that was introduced earlier this year and that we’re gradually switching all internals over to use instead of doing “custom” buffer management in various places. The internal API for dynbuf is documented here.

The internal API Curl_dyn_addf() adds a printf()-style string at the end of a “dynbuf”, and it seemed perfectly suitable to use here. I only needed to provide a vprintf() alternative since the printf() format was already received by Curl_pp_sendf()… I created Curl_dyn_vaddf() for this.

This single dynbuf is kept for the entire transfer so that it can be reused for subsequent commands and grow only if needed. Usually the initial 32 bytes malloc should be sufficient for all commands.

Not good enough

It didn’t help!

Counting the mallocs showed me with brutal clarity that my job wasn’t done there. Having dug this deep already I wasn’t ready to give this up just yet…

Why? Because Curl_dyn_addf() was still doing a separate alloc of the printf string that it then appended to the dynamic buffer. But okay, having our own printf() implementation in the code has its perks.

Add a printf() string without extra malloc

Back in May 2020 when I introduced this dynbuf thing, I converted the aprintf() code over to use dynbuf to truly unify our use of dynamically growing buffers. That was a main point with it after all.

As all the separate individual pieces I needed for this next step were already there, all I had to do was to add a new entry point to the printf() code that would accept a dynbuf as input and write directly into that (and grow it if needed), and then use that new function (Curl_dyn_vprintf) from the Curl_dyn_addf().

Phew. Now let’s see what we get…

There are 10 FTP commands that previously did 2 mallocs each: 20 mallocs were spent in this function when test 103 was executed. Now we are down to the ideal case of one alloc in there for the entire transfer.

Test 103 after polish

The code right now in master (to eventually get released as 7.73.0 in a few weeks), now shows a total of 104 allocations. Down from 123 in the previous release, which not entirely surprising is 19 fewer and thus perfectly matching the logic above.

All tests and CI ran fine. I merged it. This is a change that benefits all transfers done with any of the “ping pong protocols”. And it also makes the code easier to understand!

Compared to curl 7.52.1, this is a 26% reduction in number of allocation; pretty good, but even compared to 7.72.0 it is still a 15% reduction.


There is always more to do, but there’s also a question of diminishing returns. I will continue to look at curl’s memory use going forward too and make sure everything is motivated and reasonable. At least every once in a while.

I have some additional ideas for further improvements in the memory use area to look into. We’ll see if they pan out…

Don’t count on me to blog about every such finding with this level of detail! If you want to make sure you don’t miss any of these fine-tunes in the future, follow the curl github repo.


Image by Julio César Velásquez Mejía from Pixabay

a Google grant for libcurl work

Earlier this year I was the recipient of a monetary Google patch grant with the expressed purpose of improving security in libcurl.

This was an upfront payout under this Google program describing itself as “an experimental program that rewards proactive security improvements to select open-source projects”.

I accepted this grant for the curl project and I intend to keep working fiercely on securing curl. I recognize the importance of curl security as curl remains one of the most widely used software components in the world, and even one that is doing network data transfers which typically is a risky business. curl is responsible for a measurable share of all Internet transfers done over the Internet an average day. My job is to make sure those transfers are done as safe and secure as possible. It isn’t my only responsibility of course, as I have other tasks to attend to as well, but still.

Do more

Security is already and always a top priority in the curl project and for myself personally. This grant will of course further my efforts to strengthen curl and by association, all the many users of it.

What I will not do

When security comes up in relation to curl, some people like to mention and propagate for other programming languages, But curl will not be rewritten in another language. Instead we will increase our efforts in writing good C and detecting problems in our code earlier and better.

Proactive counter-measures

Things we have done lately and working on to enforce everywhere:

String and buffer size limits – all string inputs and all buffers in libcurl that are allowed to grow now have a maximum allowed size, that makes sense. This stops malicious uses that could make things grow out of control and it helps detecting programming mistakes that would lead to the same problems. Also, by making sure strings and buffers are never ridiculously large, we avoid a whole class of integer overflow risks better.

Unified dynamic buffer functions – by reducing the number of different implementations that handle “growing buffers” we reduce the risk of a bug in one of them, even if it is used rarely or the spot is hard to reach with and “exercise” by the fuzzers. The “dynbuf” internal API first shipped in curl 7.71.0 (June 2020).

Realloc buffer growth unification – pretty much the same point as the previous, but we have earlier in our history had several issues when we had silly realloc() treatment that could lead to bad things. By limiting string sizes and unifying the buffer functions, we have reduced the number of places we use realloc and thus we reduce the number of places risking new realloc mistakes. The realloc mistakes were usually in combination with integer overflows.

Code style – we’ve gradually improved our code style checker ( over time and we’ve also gradually made our code style more strict, leading to less variations in code, in white spacing and in naming. I’m a firm believer this makes the code look more coherent and therefore become more readable which leads to fewer bugs and easier to debug code. It also makes it easier to grep and search for code as you have fewer variations to scan for.

More code analyzers – we run every commit and PR through a large number of code analyzers to help us catch mistakes early, and we always remove detected problems. Analyzers used at the time of this writing:, Codacy, Deepcode AI, Monocle AI, clang tidy, scan-build, CodeQL, Muse and Coverity. That’s of course in addition to the regular run-time tools such as valgrind and sanitizer builds that run the entire test suite.

Memory-safe components – curl already supports getting built with a plethora of different libraries and “backends” to cater for users’ needs and desires. By properly supporting and offering users to build with components that are written in for example rust – or other languages that help developers avoid pitfalls – future curl and libcurl builds could potentially avoid a whole section of risks. (Stay tuned for more on this topic in a near future.)

Reactive measures

Recognizing that whatever we do and however tight ship we run, we will continue to slip every once in a while, is important and we should make sure we find and fix such slip-ups as good and early as possible.

Raising bounty rewards. While not directly fixing things, offering more money in our bug-bounty program helps us get more attention from security researchers. Our ambition is to gently drive up the reward amounts progressively to perhaps multi-thousand dollars per flaw, as long as we have funds to pay for them and we mange keep the security vulnerabilities at a reasonably low frequency.

More fuzzing. I’ve said it before but let me say it again: fuzzing is really the top method to find problems in curl once we’ve fixed all flaws that the static analyzers we use have pointed out. The primary fuzzing for curl is done by OSS-Fuzz, that tirelessly keeps hammering on the most recent curl code.

Good fuzzing needs a certain degree of “hand-holding” to allow it to really test all the APIs and dig into the dustiest corners, and we should work on adding more “probes” and entry-points into libcurl for the fuzzer to make it exercise more code paths to potentially detect more mistakes.

See also my presentation testing curl for security.

My first 15,000 curl commits

I’ve long maintained that persistence is one of the main qualities you need in order to succeed with your (software) project. In order to manage to ship a product that truly conquers the world. By continuously and never-ending keeping at it: polishing away flaws and adding good features. On and on and on.

Today marks the day when I landed my 15,000th commit in the master branch in curl’s git repository – and we don’t do merge commits so this number doesn’t include such. Funnily enough, GitHub can’t count and shows a marginally lower number.

This is of course a totally meaningless number and I’m only mentioning it here because it’s even and an opportunity for me to celebrate something. To cross off an imaginary milestone. This is not even a year since we passed 25,000 total number of commits. Another meaningless number.

15,000 commits equals 57% of all commits done in curl so far and it makes me the only committer in the curl project with over 10% of the commits.

The curl git history starts on December 29 1999, so the first 19 months of commits from the early curl history are lost. 15,000 commits over this period equals a little less than 2 commits per day on average. I reached 10,000 commits in December 2011, so the latest 5,000 commits were done at a slower pace than the first 10,000.

I estimate that I’ve spent more than 15,000 hours working on curl over this period, so it would mean that I spend more than one hour of “curl time” per commit on average. According to gitstats, these 15,000 commits were done on 4,271 different days.

We also have other curl repositories that aren’t included in this commit number. For example, I have done over 4,400 commits in curl’s website repository.

With these my first 15,000 commits I’ve added 627,000 lines and removed 425,000, making an average commit adding 42 and removing 28 lines. (Feels pretty big but I figure the really large ones skew the average.)

The largest time gap ever between two of my commits in the curl tree is almost 35 days back in June 2000. If we limit the check to “modern times”, as in 2010 or later, there was a 19 day gap in July 2015. I do take vacations, but I usually keep up with the most important curl development even during those.

On average it is one commit done by me every 12.1 hours. Every 15.9 hours since 2010.

I’ve been working full time on curl since early 2019, up until then it was a spare time project only for me. Development with pull-requests and CI and things that verify a lot of the work before merge is a recent thing so one explanation for a slightly higher commit frequency in the past is that we then needed more “oops” commits to rectify mistakes. These days, most of them are done in the PR branches that are squashed when subsequently merged into master. Fewer commits with higher quality.

curl committers

We have merged commits authored by over 833 authors into the curl master repository. Out of these, 537 landed only a single commit (so far).

We are 48 authors who ever wrote 10 or more commits within the same year. 20 of us committed that amount of commits during more than one year.

We are 9 authors who wrote more than 1% of the commits each.

We are 5 authors who ever wrote 10 or more commits within the same year in 10 or more years.

Our second-most committer (by commit count) has not merged a commit for over seven years.

To reach curl’s top-100 committers list right now, you only need to land 6 commits.

can I keep it up?

I intend to stick around in the curl project going forward as well. If things just are this great and life remains fine, I hope that I will be maintaining roughly this commit speed for years to come. My prediction is therefore that it will take longer than another twenty years to reach 30,000 commits.

I’ve worked on curl and its precursors for almost twenty-four years. In another twenty-four years I will be well into my retirement years. At some point I will probably not be fit to shoulder this job anymore!

I have never planned long ahead before and I won’t start now. I will instead keep focused on keeping curl top quality, an exemplary open source project and a welcoming environment for newcomers and oldies alike. I will continue to make sure the project is able to function totally independently if I’m present or not.

The 15,000th commit?

So what exactly did I change in the project when I merged my 15,000th ever change into the branch?

It was a pretty boring and non-spectacular one. I removed a document (RESOURCES) from the docs/ folder as that has been a bit forgotten and now is just completely outdated. There’s a much better page for this provided on the web site:


I of coursed asked my twitter friends a few days ago on how this occasion is best celebrated:

I showed these results to my wife. She approved.

store the curl output over there

tldr: --output-dir [directory] comes in curl 7.73.0

The curl options to store the contents of a URL into a local file, -o (--output) and -O (--remote-name) were part of curl 4.0, the first ever release, already in March 1998.

Even though we often get to hear from users that they can’t remember which of the letter O’s to use, they’ve worked exactly the same for over twenty years. I believe the biggest reason why they’re hard to keep apart is because of other tools that use similar options for maybe not identical functionality so a command line cowboy really needs to remember the exact combination of tool and -o type.

Later on, we also brought -J to further complicate things. See below.

Let’s take a look at what these options do before we get into the new stuff:

--output [file]

This tells curl to store the downloaded contents in that given file. You can specify the file as a local file name for the current directory or you can specify the full path. Example, store the the HTML from in "/tmp/foo":

curl -o /tmp/foo


This option is probably much better known as its short form: -O (upper case letter o).

This tells curl to store the downloaded contents in a file name name that is extracted from the given URL’s path part. For example, if you download the URL "" users often think that saving that using the local file name “pancakes.jpg” is a good idea. -O does that for you. Example:

curl -O

The name is extracted from the given URL. Even if you tell curl to follow redirects, which then may go to URLs using different file names, the selected local file name is the one in the original URL. This way you know before you invoke the command which file name it will get.


This option is commonly used as -J (upper case letter j) and needs to be set in combination with --remote-name.

This makes curl parse incoming HTTP response headers to check for a Content-Disposition: header, and if one is present attempt to parse a file name out of it and then use that file name when saving the content.

This then naturally makes it impossible for a user to be really sure what file name it will end up with. You leave the decision entirely to the server. curl will make an effort to not overwrite any existing local file when doing this, and to reduce risks curl will always cut off any provided directory path from that file name.

Example download of the pancake image again, but allow the server to set the local file name:

curl -OJ

(it has been said that “-OJ is a killer feature” but I can’t take any credit for having come up with that.)

Which directory

So in particular with -O, with or without -J, the file is download in the current working directory. If you want the download to be put somewhere special, you had to first ‘cd’ there.

When saving multiple URLs within a single curl invocation using -O, storing those in different directories would thus be impossible as you can only cd between curl invokes.

Introducing --output-dir

In curl 7.73.0, we introduce this new command line option --output-dir that goes well together with all these output options. It tells curl in which directory to create the file. If you want to download the pancake image, and put it in /tmp no matter which your current directory is:

curl -O --output-dir /tmp

And if you allow the server to select the file name but still want it in /tmp

curl -OJ --output-dir /tmp

Create the directory!

This new option also goes well in combination with --create-dirs, so you can specify a non-existing directory with --output-dir and have curl create it for the download and then store the file in there:

curl --create-dirs -O --output-dir /tmp/receipes

Ships in 7.73.0

This new option comes in curl 7.73.0. It is curl’s 233rd command line option.

You can always find the man page description of the option on the curl website.


I (Daniel) wrote the code, docs and tests for this feature.

Image by Alexas_Fotos from Pixabay