The curl source code tree is neither large nor complicated. A key thing to remember is, perhaps, that libcurl is the library and that library is the biggest component of the curl command-line tool.
We try to keep the number of files in the source tree root to a minimum. You will see a slight difference in files if you check a release archive compared to what is stored in the git repository as several files are generated by the release scripts.
Some of the more notable ones include:
buildconf: used to build configure and more when
building curl from source out of the git repository.
buildconf.bat: the Windows version of buildconf. Run this after having
checked out the full source code from git.
CHANGES: generated at release and put into the release archive. It
contains the 1000 latest changes to the source repository.
configure: a generated script that is used on Unix-like systems to
generate a setup when building curl.
COPYING: the license detailing the rules for your using the code.
GIT-INFO: only present in git and contains information about how to
build curl after having checked out the code from git.
maketgz: the script used to produce release archives and daily snapshots
README: a short summary of what curl and libcurl are.
RELEASE-NOTES: contains the changes done for the latest release; when
found in git it contains the changes done since the previous release that
are destined to end up in the coming release.
This directory contains the full source code for libcurl. It is the same source code for all platforms—over one hundred C source files and a few more private header files. The header files used when building applications against libcurl are not stored in this directory; see include/curl for those.
Depending on what features are enabled in your own build and what functions your platform provides, some of the source files or portions of the source files may contain code that is not used in your particular build.
The VTLS sub section within libcurl is the home of all the TLS backends libcurl can be built to support. The "virtual" TLS internal API is a common API that is used within libcurl to access TLS and crypto functions without the main code knowing exactly which TLS library that is used. This allows the person who builds libcurl to select from a wide variety TLS libraries to build with.
We also maintain a SSL comparison table on the web site to aid users.
OpenSSL: the (by far) most popular TLS library.
BoringSSL: an OpenSSL fork maintained by Google. It will make libcurl disable a
few features due to lacking some functionality in the library.
LibreSSL: an OpenSSL fork maintained by the OpenBSD team.
NSS: a full-blown TLS library perhaps most known for being used by the
Firefox web browser. This was the default TLS backend for curl on Fedora and
Redhat systems for a while in the past.
GnuTLS: a full-blown TLS library used by default by the Debian packaged curl.
mbedTLS: (formerly known as PolarSSL) is a TLS library more targeted
towards the embedded market.
WolfSSL: (formerly known as cyaSSL) is a TLS library more targeted
towards the embedded market.
MesaLink: a TLS library written in rust
Schannel: the native TLS library on Windows.
SecureTransport: the native TLS library on Mac OS X.
GSKit: the native TLS library on OS/400.
This directory holds the source code for the curl command-line tool. It is the same source code for all platforms that run the tool.
Most of what the command-line tool does is to convert given command line options into the corresponding libcurl options or set of options and then makes sure to issue them correctly to drive the network transfer according to the user's wishes.
This code uses libcurl just as any other application would.
Here are the public header files that are provided for libcurl-using applications. Some of them are generated at configure or release time so they do not look identical in the git repository as they do in a release archive.
With modern libcurl, all an application is expected to include in its C source code is
The main documentation location. Text files in this directory are typically plain text files. We have slowly started to move towards Markdown format so a few (but growing number of) files use the .md extension to signify that.
Most of these documents are also shown on the curl web site automatically converted from text to a web friendly format/look.
BINDINGS: lists all known libcurl language bindings and where to find them
BUGS: how to report bugs and where
CODE_OF_CONDUCT.md: how we expect people to behave in this project
CONTRIBUTE: what to think about when contributing to the project
curl.1: the curl command-line tool man page, in nroff format
curl-config.1: the curl-config man page, in nroff format
FAQ: frequently asked questions about various curl-related subjects
FEATURES: an incomplete list of curl features
HISTORY: describes how the project started and has evolved over the years
HTTP2.md: how to use HTTP/2 with curl and libcurl
HTTP-COOKIES: how curl supports and works with HTTP cookies
index.html: a basic HTML page as a documentation index page
INSTALL: how to build and install curl and libcurl from source
INSTALL.cmake: how to build curl and libcurl with CMake
INSTALL.devcpp: how to build curl and libcurl with devcpp
INTERNALS: details curl and libcurl internal structures
KNOWN_BUGS: list of known bugs and problems
LICENSE-MIXING: describes how to combine different third party modules and
their individual licenses
MAIL-ETIQUETTE: this is how to communicate on our mailing lists
MANUAL: a tutorial-like guide on how to use curl
mk-ca-bundle.1: the mk-ca-bundle tool man page, in nroff format
README.cmake: CMake details
README.netware: Netware details
README.win32: win32 details
RELEASE-PROCEDURE: how to do a curl and libcurl release
RESOURCES: further resources for further reading on what, why and how curl
ROADMAP.md: what we want to work on in the future
SECURITY: how we work on security vulnerabilities
SSLCERTS: TLS certificate handling documented
SSL-PROBLEMS: common SSL problems and their causes
THANKS: thanks to this extensive list of friendly people, curl exists today!
TheArtOfHttpScripting: a tutorial into HTTP scripting with curl
TODO: things we or you can work on implementing
VERSIONS: how the version numbering of libcurl works
All libcurl functions have their own man pages in individual files with .3 extensions, using nroff format, in this directory. There are also a few other files that are described below.
This directory contains the man pages for the individual options for three different libcurl functions.
curl_easy_setopt() options start with
curl_multi_setopt() options start with
curl_easy_getinfo() options start with
Contains around 100 stand-alone examples that are meant to help readers understand how libcurl can be used.
See also the libcurl examples section of this book.
contributors.sh: extracts all contributors from the git repository since a
given hash/tag. The purpose is to generate a list for the RELEASE-NOTES file
and to allow manually added names to remain in there even on updates. The
script uses the 'THANKS-filter` file to rewrite some names.
contrithanks.sh: extracts contributors from the git repository since a
given hash/tag, filters out all the names that are already mentioned in
THANKS, and then outputs
THANKS to stdout with the list of new
contributors appended at the end; it's meant to allow easier updates of the THANKS
document. The script uses the 'THANKS-filter` file to rewrite some names.
log2changes.pl: generates the
CHANGES file for releases, as used by the
release script. It simply converts git log output.
zsh.pl: helper script to provide curl command-line completions to users of
the zsh shell.