linux c - get server hostname?

like gethostname() ?

That's the name of the machine on which your app is running.

Or read from

/proc/sys/kernel/hostname

Update

Simple example

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

int main(void) {

    char hostname[1024];
    gethostname(hostname, 1024);

    puts(hostname);

    return EXIT_SUCCESS;
}

Building on the answer from Alain Pannetier, you can spare a few bytes by using HOST_NAME_MAX:

#include <limits.h>
...
  char hostname[HOST_NAME_MAX+1];
  gethostname(hostname, HOST_NAME_MAX+1);
...