How to get PID_MAX macro? - c

Question

I would like to get the value of the PID_MAX macro present in the file /sys/sys/proc.h.
My current code (main.c) :
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h> /* type pid_t */
#include <sys/proc.h> /* macro PID_MAX */
#include <sys/unistd.h> /* function getpid, getppid */
/*
gcc -Wall -Wextra main.c -o main
./main
*/
int main ()
{
pid_t pidmax = PID_MAX;
printf ( "Value = %d\n", pidmax );
return 0;
}
Return the following error :
error: 'PID_MAX' undeclared (first use in this function); did you mean 'UID_MAX'?
How is it possible?
Another method to recover the PID_MAX?

Answer 1

There is no platform-independent method for retrieving the maximum pid value.
Under Linux, for example, you can determine the value through the /proc/sys interface
$ sysctl kernel/pid_max
32768
Under FreeBSD, the value is 99999. If you look closely in sys/proc.h, you will notice that the macro PID_MAX is guarded by
#ifdef _KERNEL
...
#define PID_MAX 99999
...
#endif
This is, why you cannot (and should not) use it in userspace programs.

Answer 2

The PID_MAX macro defines the hard limit of PID values by BSD kernels (such as kFreeBSD); it shouldn't be used by user space programs because the actual maximum PID value could be further lowered in the runtime, controlled by kern.pid_max sysctl.
To get such runtime limitation in an user space program, use sysctlbyname(3); for example:
#include <sys/types.h>
#include <sys/sysctl.h>
#include <stdio.h>
int main() {
int pid_max;
size_t len = sizeof pid_max;
if(sysctlbyname("kern.pid_max", &pid_max, &len, NULL, 0) < 0) {
perror("sysctlbyname: kern.pid_max");
return 1;
}
printf("kern.pid_max=%d\n", pid_max);
return 0;
}