Correct way to use prctl()

Question

The prototype of prctl is

int prctl(int option, unsigned long arg2, unsigned long arg3,
          unsigned long arg4, unsigned long arg5);

in the man page whereas in the header it is declared as a variadic function:

extern int prctl (int __option, ...) __THROW;

Do I have to call it with 5 arguments when I only need 2?
Do I need to cast args to unsigned long?

4pie0 · Answer 1 · 2016-04-11T16:09:01.160

3

Just pass what you have to pass and write 0 casted to unsigned long in the rest of arguments or skip them entirely. As prctl is declared as variadic function it will handle this situation accordingly.

const char* name = "The user";
if (prctl(PR_SET_NAME, (unsigned long) name,
         (unsigned long)0, (unsigned long)0, (unsigned long)0) == -1)
{
    // handle error
    perror("prctl failed");
    return -1;
}

or

const char* name = "The user";
if (prctl(PR_SET_NAME, (unsigned long) name) == -1)
{
    // handle error
    perror("prctl failed");
    return -1;
}

edited Apr 11 '16 at 16:09

answered Apr 11 '16 at 14:43

4pie0

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My concern is with the zeroes, they are promoted to `int` but the expected type is `unsigned long`. Shouldn't this be undefined behavior? – sigsegv Apr 11 '16 at 15:35
@sigsegv Yes, either cast them or skip them. – 4pie0 Apr 11 '16 at 16:07

Correct way to use prctl()

1 Answers1