f-transcript.txt

$ gcc f.c -o f -Wall -Wextra -pedantic -Wno-missing-field-initializers -g3 -O0 && echo OK
f.c: 関数 ‘main’ 内:
f.c:5:14: 警告: 仮引数 ‘argc’ が未使用です [-Wunused-parameter]
OK
$ nc -l 33333 &                                                                          
[1] 25653                            
$ cd /proc/25653 
$ killall nc                                                                             
[1]  + terminated  nc -l 33333              
$ /tmp/f 25653
I am also 25653!!

f.c

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

int main(int argc, char *argv[]) {
    char *p;
    long checked_pid;
    int i;

    if ( ! argv[1])
        abort();

    checked_pid = strtol(argv[1], &p, 0);
    if (*p != '\0')
        abort();

    for (i = 0; i < 1000 * 1000; ++i) {
        pid_t pid = fork();
        if (0 == pid) {
            return 0;
        } else if (0 < pid) {
            if (checked_pid == pid) {
                printf("I am also %d!!\n", (int)pid);
                return 0;
            }
        } else {
            perror("fork(2)");
            return 10;
        }

        if (0 == i % 1000)
            usleep(0);
    }

    return 0;
}