Autotools

The Autotools are a collection of different tools that are used together to generate the configure script. The configure script is run by the user who wants to build curl and it does a whole bunch of things:

It checks for features and functions present in your system.
It offers command-line options so that you as a builder can decide what to enable and disable in the build. Features and protocols, etc., can be toggled on/off, even compiler warning levels and more.
It offers command-line options to let the builder point to specific installation paths for various third-party dependencies that curl can be built to use.
It specifies on which file path the generated installation should be placed when ultimately the build is made and make install is invoked.

In the most basic usage, just running ./configure in the source directory is enough. When the script completes, it outputs a summary of what options it has detected/enabled and what features that are still disabled, some of which possibly because it failed to detect the presence of necessary third-party dependencies that are needed for those functions to work. If the summary is not what you expected it to be, invoke configure again with new options or with the previously used options adjusted.

After configure has completed, you invoke make to build the entire thing and then finally make install to install curl, libcurl and associated things. make install requires that you have the correct rights in your system to create and write files in the installation directory or you get an error displayed.

Cross-compiling

Cross-compiling means that you build the source on one architecture but the output is created to be run on a different one. For example, you could build the source on a Linux machine but have the output work on a Windows machine.

For cross-compiling to work, you need a dedicated compiler and build system setup for the particular target system for which you want to build. How to get and install that system is not covered in this book.

Once you have a cross compiler, you can instruct configure to use that compiler instead of the native compiler when it builds curl so that the end result then can be moved over and used on the other machine.

Static linking

By default, configure setups the build files so that the following 'make' command creates both shared and static versions of libcurl. You can change that with the --disable-static or --disable-shared options to configure.

If you instead want to build with static versions of third party libraries instead of shared libraries, you need to prepare yourself for an uphill battle. curl's configure script is focused on setting up and building with shared libraries.

One of the differences between linking with a static library compared to linking with a shared one is in how shared libraries handle their own dependencies while static ones do not. In order to link with library xyz as a shared library, it is basically a matter of adding -lxyz to the linker command line no matter which other libraries xyz itself was built to use. But, if that xyz is instead a static library we also need to specify each dependency of xyz on the linker command line. curl's configure cannot keep up with or know all possible dependencies for all the libraries it can be made to build with, so users wanting to build with static libs mostly need to provide that list of libraries to link with.

Select TLS backend

The configure-based build offers the user to select from a wide variety of different TLS libraries when building. You select them by using the correct command line options. Before curl 7.77.0, the configure script would automatically check for OpenSSL, but modern versions do not.

AmiSSL: --with-amissl
AWS-LC: --with-openssl
BearSSL: --with-bearssl
BoringSSL: --with-openssl
GnuTLS: --with-gnutls
LibreSSL: --with-openssl
mbedTLS: --with-mbedtls
OpenSSL: --with-openssl
Rustls: --with-rustls (point to the rustls-ffi install path)
Schannel: --with-schannel
Secure Transport: --with-secure-transport
wolfSSL: --with-wolfssl

If you do not specify which TLS library to use, the configure script fails. If you want to build without TLS support, you must explicitly ask for that with --without-ssl.

These --with-* options also allow you to provide the install prefix so that configure searches for the specific library where you tell it to. Like this:

./configure --with-gnutls=/home/user/custom-gnutls

You can opt to build with support for multiple TLS libraries by specifying multiple --with-* options on the configure command line. Pick which one to make the default TLS backend with --with-default-ssl-backend=[NAME]. For example, build with support for both GnuTLS and OpenSSL and default to OpenSSL:

./configure --with-openssl --with-gnutls \
  --with-default-ssl-backend=openssl

Select SSH backend

The configure-based build offers the user to select from a variety of different SSH libraries when building. You select them by using the correct command-line options.

libssh2: --with-libssh2
libssh: --with-libssh
wolfSSH: --with-wolfssh