Seeding the PRNG in a C program

The POSIX specification provides a pseudo-random number generator via the random(3) and srandom(3) functions. srandom() should be called at program start to seed the generator. (If no seed is provided, random() acts as though srandom(1); was called. This provides reproducible behaviour for testing, but an explicit seed might be preferred.) How should we get a random seed for the call to srandom()?

On x86_64 Linux, the loader for ELF files places a collection of values known as the auxilary vector just below the bottom of the stack. These values can be accessed through the getauxval(3) function, found in sys/auxv.h. One of the entries in this vector is AT_RANDOM, a pointer to 16 bytes of random data.

We can therefore seed the generator via

#include <stdlib.h> // random(), srandom()
#include <sys/auxv.h> // getauxval(), AT_RANDOM

void random_seed() {
    int* seed = getauxval(AT_RANDOM);
    srandom(*seed);
}

This should not be used in a situation where the seed must be hidden from the user, as the auxilary vector can be displayed by setting the LD_SHOW_AUXV environment variable prior to calling the program.