Usage

#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 += $$(pkg-config --libs-only-L libsodium)
LDLIBS += $$(pkg-config --libs-only-l 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 Findsodium.cmake file from the Facebook Fizz project 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. This issue has been reported on Digital Ocean virtual machines, Scaleway ARM instances, and AWS Nitro Enclaves.

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.

Current Linux kernels (>= 5.4) include the haveged algorithm in order to mitigate that problem. So, before trying the last-resort solutions below, try using a recent kernel.

If you have to use a kernel before version 5.4, a possible workaround is to install haveged:

apt-get install haveged

An alternative is rng-tools:

apt-get install rng-tools

In some environments, setting the -O jitter:timeout option to 20 might be necessary.

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

After installing these tools, check the value of /proc/sys/kernel/random/entropy_avail again.

On AWS Nitro Enclaves, workarounds include:

• Calling the aws_nitro_enclaves_library_seed_entropy() function before sodium_init(), and occasionally afterwards.

• Using the RDSEED CPU instruction to seed the kernel RNG (not recommended as a unique entropy source).

• Setting random.trust_cpu=on in the kernel command line (requires Linux kernel > 4.19).

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"
              "Upgrading the kernel, installing the rng-utils/rng-tools, jitterentropy-rngd 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.