c_concurrency.rst

C Concurrency cheatsheet

Table of Contents

• How to write a UNIX daemon
• Using daemon(nochdir, noclose)

How to write a UNIX daemon

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <syslog.h>
#include <sys/stat.h>

int main(int argc, char *argv[])
{
    int ret = -1;
    pid_t pid;

    /* become daemon */

    pid = fork();
    if (-1 == pid) {
        printf("Fork get error\n");
        goto Error;
    } else if (pid != 0) {
        ret = 0;
        goto Error;
    }
    /* Change the file mode mask */
    umask(0);

    /* set sid */
    if (-1 == setsid()) {
        printf("set sid failed\n");
        goto Error;
    }
    /* chdir to root "/" */
    if (-1 == chdir("/")) {
        printf("chdir(\"/\") failed\n");
        goto Error;
    }
    /* close stdin, stdout, stderr */
    close(STDIN_FILENO);
    close(STDOUT_FILENO);
    close(STDERR_FILENO);

    /* Do some task here */
    while (1) { sleep(3); syslog(LOG_ERR, "Hello"); }

    ret = 0;
Error:
    return ret;
}

Using daemon(nochdir, noclose)

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

int main(int argc, char *argv[])
{
    int ret = -1;
    /* make process as a daemon */
    if (-1 == daemon(0, 0)) {
        syslog(LOG_ERR, "create a daemon get error");
        goto Error;
    }
    /* do the daemon task */
    while(1) { sleep(3); syslog(LOG_ERR, "Hello"); }
    ret = 0;
Error:
    return ret;
}