killer.c

#define _POSIX_SOURCE

#include <stdio.h>
#include <sys/types.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>

#define CMDLINE "NEJAKY_PRIKAZ"

int main (int argc, char **argv)
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s PID\n", argv[0]);
        return 1;
    }

    char *endptr = NULL;
    int pid = strtol(argv[1], &endptr, 10);
    if (argv[1] == endptr) {
        fprintf(stderr, "PID is not a number\n");
        return 2;
    }

    char filename[128];
    snprintf(filename, 128, "/proc/%d/cmdline", pid);

    FILE *fh = fopen(filename, "r");
    if (!fh) {
        fprintf(stderr, "Error opening '%s': ", filename);
        perror("");
        return 3;
    }

    char buffer[128];
    fgets(buffer, 128, fh);

    if (strcmp(buffer, CMDLINE) != 0) {
        fprintf(stderr, "You can not kill this process\n");
        return 4;
    }

    fclose(fh);

    int retval = kill(pid, SIGTERM);
    if (retval != 0) {
        fprintf(stderr, "Failed to kill PID %d: ", pid);
        perror("");
        return 5;
    }

    return 0;
}