test_daemon.c

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

void write_pid_file(const char* pid_file)
{
    // getcwd
    FILE* fp = fopen(pid_file, "w");
    if(fp)
    {
        fprintf(fp, "file(%d):%d\n", __LINE__, (int)getpid());
        fclose(fp);
    }
}

int main(int argc, char *argv[])
{
    char pid_file[PATH_MAX];
    strncat(getcwd(pid_file, PATH_MAX), "/pid", strlen("/pid"));

    fprintf(stdout, "stdout(%d):%d\n", __LINE__, (int)getpid());
    fprintf(stderr, "stderr(%d):%d\n", __LINE__, (int)getpid());

    daemon(0, 0);

    fprintf(stdout, "stdout(%d):%d\n", __LINE__, (int)getpid());
    fprintf(stderr, "stderr(%d):%d\n", __LINE__, (int)getpid());

    write_pid_file(pid_file);

    return 0;
}