如何将子进程的输出重定向到syslog？

Question

I'm developing in C under freebsd. I'm using cron job to invoke a program A which uses fork()/exec() to call another program B. In A, I use syslog() to write logs to /var/log/messages, but B outputs to stdout so B's output is not written to /var/log/messages.

How can I redirect B's output to syslog also without modifying B's code? I guess there should be some mechanism to redirect B's output to some fd in A, but since I'm using syslog() instead of opening a file directly in A, I'm not sure how this would work.

Thanks for suggestions!

Answer 1

I can't recall if freebsd includes the logger program. If so, your program A might be able to invoke B as some form of "B | logger". For example, instead of

execl("/my/path/to/B", "B", NULL);

Leverage the shell:

execl("/bin/sh", "sh", "-c", "/my/path/to/B | logger", NULL);

The logger program has additional options to direct its messages to a particular syslog facility, which may be needed to match your A application's behavior.

Lacking logger , you may need to do traditional Posix pipe-fitting: use pipe(2) to create a pipe, dup the output side of the pipe to B's stdout, and read (either in A, or in a separate child process) that pipe's input side for messages to send to syslog(3)