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When was random() introduced in C and why does standard 89 not recognise it?

December 1, 2022

I’m working on a Operating Systems project with requirements being the usage of C89 and pedantic flags.

Since I am on macOS, I’ve encountered a fair amount of issues with the rand() function during a few exercises. In fact, its macOS man page calls it a “bad number generator”. So, to avoid issues with it I had switched to using random(). Now, since I am compiling with -std=c89, -pedantic, wall and werror, it refuses to function correctly due to having a warning about an implicit declaration for random(). If I remove werror, it does generate the warning, but it compiles (as expected) but more interestingly, it works perfectly fine. It generates a number as expected. What am I doing wrong? Does C89 support random or not? What am I supposed to include that’ll make the warning go away? The man page mentions nothing other than stdlib.h which is included.

A viable alternative as mentioned by the manpage, would be arc4random(), however I am pretty sure it doesn’t exist cross-platform.

My test snippet is

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

int main()
{
int test;
srandom(5);

    test = random() % 190;
    printf("Hello World %d\n", test);
    
    return 0;

}

And it generates the following output:

main.c:15:5: warning: implicit declaration of function ‘srandom’; did you mean ‘srand’? [-Wimplicit-function-declaration]
   15 |     srandom(5);
      |     ^~~~~~~
      |     srand
main.c:17:12: warning: implicit declaration of function ‘random’; did you mean ‘rand’? [-Wimplicit-function-declaration]
   17 |     test = random() % 190;
      |            ^~~~~~
      |            rand
Hello World 115

>Solution :

Both random and srandom are not part of the C standard. They are POSIX functions, so compiling
with -std=c89 doesn’t make their prototypes available when you compile.

You can define one of the following at the top of your source file to make their prototypes available: