Why can C compile time() without its library?

Question

When I use the time() function (i.e., just randomize seed for rand() ) but not include the header file time.h, it works for C. For example:

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

int main()
{
  int i;
  srand(time(NULL));

  for(i=0;i<10;i++){
    printf("\t%d",rand()%10);
  }
  printf("\n");
  return 0;
}

When I try to compile the code above, g++ cannot compile it since time.h isn't included. But gcc can.

$gcc ra.c 
$./a.out 
    4       5       2       4       8       7       3       8       9       3
$g++ ra.c 
ra.c: In function ‘int main()’:
ra.c:8:20: error: ‘time’ was not declared in this scope
 srand(time(NULL));
                ^

Is it related with version of gcc or just a difference between C/C++ ?

Answer 1

You should include <time.h> for time(2) and turn on the warnings. In C, a function with no visible prototype is assumed to return int (which has been deprecated since C99). So compiling with gcc seems fine while g++ doesn't.

Compile with:

gcc -Wall -Wextra -std=c99 -pedantic-errors file.c

and you'll see gcc also complains about it.