c - Writing data to stderr makes the program to exit after the program becomes a daemon -

please @ code. runs in centos6 64bit.

#include<stdio.h>  int main(int argc, char **argv) {          fprintf(stderr, "output 1\n");          printf("output 2\n");          fflush(stdout);          system("echo abc");          fprintf(stderr, "output 3\n ");          printf("output 4\n");           fflush(stdout);           daemon(0, 1);           fprintf(stderr, "output 5\n");          printf("output 6\n");          fflush(stdout);          system("echo abc");          fprintf(stderr, "output 7\n");          printf("output 8\n");          fflush(stdout);  } 

if run it, see these messages:

output 1 output 2 abc output 3 output 4 output 5 output 6 abc output 7 output 8 

if use ssh login , run it, see same results.

however, if use binary name parameter of ssh , run it, program exit when writing data stderr after calling daemon(0, 1). suppose binary name myapp. run

ssh localhost myapp 

and see these messages:

output 1 output 2 abc output 3 output 4 output 5 output 6 abc 

does know why? according debugging, program exits after doing 3 things:

1. call daemon(0, 1).
2. call system run application or bash command.
3. write stderr.

thanks lot!

if run command shell, you're seeing new shell prompt between output 4 , output 5 (this more noticeable if there sleeps in between output lines).

that's because daemon() system call causes program split 2 independent processes. called "fork", , can controlled little more closely using fork() system call. after fork, both processes keep pointers open file descriptors: stdin, stdout , stderr. according "man 3 daemon", parent process calls exit() after fork.

when call executable ssh, ssh session run 1 process. forks off child, , main process exits. ssh sees command issued has finished, , closes ssh connection. closes stdout , stderr. unfortunately, child process still has work do, can't write shared stderr because file descriptor has been closed. if printed more debug info, return values printf() , fprintf() calls, you'd see not able write closed file descriptors.

if, instead of printing stderr, printed log file (which daemons do), you'll see child process continue run in background , write expected.

if had chosen use fork() instead of daemon(), have parent wait until child finishes. pid_t waitpid(pid_t pid, int *stat_loc, int options);.

you might want take @ signals sent between parent , child. when child process dies, send sigchild parent. if you'd reverse notification, can set 1 (on linux only) using prctl(pr_set_pdeathsig, sighup);, parent sends sighup child.