random() not working in C

Question

I am currently learning C though "C all-in-one Desk Reference for Dummies" and came to the point where it starts teaching how to get random numbers. However, the sample code it gives doesn't work (the compiler comes up with an error "undefined reference to 'random'"). My code is below, copied from the book.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int hat;

    hat = random();
    printf("%d is a random number.\n",hat);
    return(0);
}

Answer 1

random and srandom are non-standard C functions. They are included in glibc on many platforms (linux, BSD etc). However, since they aren't part of the C Standard, they are not required to be available on all compilers.

All standard C compilers however come with rand and srand, so just change your program to call rand instead of random. If other sample programs in your book call srandom, then use srand instead.

Change your program to

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int hat;

    hat = rand();
    printf("%d is a random number.\n",hat);
    return(0);
}

The next program in your book probably adds a call to srandom to make the psuedo random numbers generated to be really random. If so, use srand instead.

If a beginner book on C actually uses random and srandom, then it should probably be thrown out. And any book on C which uses random and srandom without telling you that they are non-standard should be thrown out.

Answer 2

The name of the function to create pseudo-random numbers, defined by the ISO C standard, is rand().

To actually get a pseudo-random number that looks random, you need to seed the function rand with srand(), like the following:

#include <time.h>
#include <stdlib.h>

srand(time(NULL));
int r = rand();

Here, time time() function (declared in the header time.h) gets the current calendar time as a value of type time_t. By doing that, it gets a different value to seed the rand() function everytime.

EDIT: as pointed by vanza out in comments below, there is a random() function, but it is not defined in the C Standards.