Sleep system call ,What is the default sleep time?

Question

What is the default sleep time if we don't pass any arguments to sleep( ) function?

#include<stdio.h>
int main()
{
    int pid,dip,cpid;
    pid = fork();
    if (pid == 0)
    {
        printf("\n first child is created %d",getpid());
    }
    else
    {
        dip = fork();
        if (dip == 0)
        {
            printf("\n second process is creatred %d",getpid());
        }
        else
        {
            sleep();
            cpid = wait(0);
            printf("\n child with pid %d  ", cpid);
            cpid = wait(0);
            printf("\n child with pid %d  ",cpid);
            printf("\n I am parent \n");
        }
    }
}

What is the output of the above code?

There is no such thing. Not passing an argument is an error. — Mat, Aug 27 '18 at 08:04
Try to compile it? Does it compile? If not, then the book is wrong. — Lundin, Aug 27 '18 at 08:13
first child is created 4998 second process is creatred 4999 This is the output that I am getting — usr123, Aug 27 '18 at 08:14
I don't think that the `sleep` function would work in the case above as as far as I know there's no `sleep` function defined in `stdio.h`. — Nae, Aug 27 '18 at 08:15
BTW, take the habit to *end* (not start) `printf` format strings with `\n` — Basile Starynkevitch, Aug 27 '18 at 08:25
Take the habit to read the documentation of *every* function you are calling. — Basile Starynkevitch, Aug 27 '18 at 08:35
It looks like you forgot to enable a good set of warnings. For GCC, I recommend `-Wall -Wextra -Wwrite-strings` as a minimum (that will help identify the use of `sleep()` without a visible prototype); consider also `-Wpedantic -Warray-bounds` for identifying some other common mistakes. — Toby Speight, Aug 27 '18 at 11:12

Basile Starynkevitch · Accepted Answer · 2018-08-27T08:34:08.470

You should not call a function (including sleep) which has not been declared. According to the sleep(3) man page, you should #include <unistd.h>; on my Linux/Debian system /usr/include/unistd.h declares:

 extern unsigned int sleep (unsigned int __seconds);

If you don't declare a function (this is bad habit; Use gcc with -Wall -Wmissing-prototypes to be warned) it has unspecified arguments and int result.

If you did (like you should) #include <unistd.h> your code won't even compile (the call to sleep without arguments would be marked as error).

In practice, your sleep call is invoked with whatever garbage value is in the register used to pass the first argument. This is typical undefined behavior, and you cannot predict that value, and you should be scared.

So there is no default sleep time, and stricto sensu your question makes no sense. Check by reading the C11 standard n1570 and the POSIX standards.

Sleep system call ,What is the default sleep time?

1 Answers1