#include <sodium.h>
​
int main(void)
{
    if (sodium_init() == -1) {
        return 1;
    }
    ...
}

sodium.h is the only header that has to be included.

The library is called sodium (use -lsodium to link it), and proper compilation/linker flags can be obtained using pkg-config on systems where it is available:

CFLAGS=$(pkg-config --cflags libsodium)
LDFLAGS=$(pkg-config --libs libsodium)

For static linking, Visual Studio users should define SODIUM_STATIC=1 and SODIUM_EXPORT=. This is not required on other platforms.

Projects using CMake can include the Fidsodium.cmake file from the Facebook Fizz project in order to detect and link the library.

sodium_init() initializes the library and should be called before any other function provided by Sodium. It is safe to call this function more than once and from different threads -- subsequent calls won't have any effects.

After this function returns, all of the other functions provided by Sodium will be thread-safe.

sodium_init() doesn't perform any memory allocations. However, on Unix systems, it may open /dev/urandom and keep the descriptor open, so that the device remains accessible after a chroot() call.

Multiple calls to sodium_init() do not cause additional descriptors to be opened.

sodium_init() returns 0 on success, -1 on failure, and 1 if the library had already been initialized.

Before returning, the function ensures that the system's random number generator has been properly seeded.

sodium_init() stalling on Linux

On some Linux systems, this may take some time, especially when called right after a reboot of the system. That issue has been reported on Digital Ocean virtual machines as well as on Scaleway ARM instances.

This can be confirmed with the following command:

cat /proc/sys/kernel/random/entropy_avail

If the command returns 0 or a very low number (< 160), and you are not running an obsolete kernel, this is very likely to be the case.

In a virtualized environment, make sure that the virtio-rng interface is available. If this is a cloud service and the hypervisor settings are out of your reach, consider switching to a different service.

On a bare-metal host such as Scaleway instances, a possible workaround is to install the rng-tools package:

apt-get install rng-tools

And check the value of /proc/sys/kernel/random/entropy_avail again. If the value didn't go any higher, install haveged:

apt-get install haveged

Haveged should only be used as a very last resort. It hasn't received any updates for 10+ years, and shouldn't be trusted as a single entropy source, especially on virtualized environments.

​Jitterentropy is a better alternative, but most Linux distributions don't offer it as an installable package yet.

Applications can warn users about the Linux RNG not being seeded before calling sodium_init() using code similar to the following:

#if defined(__linux__)
# include <fcntl.h>
# include <unistd.h>
# include <sys/ioctl.h>
# include <linux/random.h>
#endif
// ...
#if defined(__linux__) && defined(RNDGETENTCNT)
int fd;
int c;
​
if ((fd = open("/dev/random", O_RDONLY)) != -1) {
    if (ioctl(fd, RNDGETENTCNT, &c) == 0 && c < 160) {
        fputs("This system doesn't provide enough entropy to quickly generate high-quality random numbers.\n"
              "Installing the rng-utils/rng-tools, jitterentropy or haveged packages may help.\n"
              "On virtualized Linux environments, also consider using virtio-rng.\n"
              "The service will not start until enough entropy has been collected.\n", stderr);
    }
    (void) close(fd);
}
#endif

Congrats, you're all set up!

A good documentation page to read next might be Quickstart and FAQ.