daemonize: set stderr to /dev/null.

ccan/daemonize/daemonize.c

@@ -3,6 +3,7 @@
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 
 /* This code is based on Stevens Advanced Programming in the UNIX
  * Environment. */
@@ -21,6 +22,14 @@ bool daemonize(void)
 	close(STDOUT_FILENO);
 	close(STDERR_FILENO);
 
+	/* Many routines write to stderr; that can cause chaos if used
+	 * for something else, so set it here. */
+	if (open("/dev/null", O_WRONLY) != 0)
+		return false;
+	if (dup2(0, STDERR_FILENO) != STDERR_FILENO)
+		return false;
+	close(0);
+
 	/* Session leader so ^C doesn't whack us. */
 	setsid();
 	/* Move off any mount points we might be in. */

ccan/daemonize/daemonize.h

@@ -6,11 +6,12 @@
  * daemonize - turn this process into a daemon.
  *
  * This routine forks us off to become a daemon.  It returns false on failure
- * (ie. fork() failed) and sets errno.
+ * (eg. fork(), chdir or open failed) and sets errno.
  *
  * Side effects for programmers to be aware of:
  *  - PID changes (our parent exits, we become child of init)
- *  - stdin, stdout and stderr file descriptors are closed
+ *  - stdin and stdout file descriptors are closed
+ *  - stderr is reopened to /dev/null so you don't reuse it
  *  - Current working directory changes to /
  *  - Umask is set to 0.
  */

ccan/daemonize/test/run.c

@@ -43,8 +43,12 @@ int main(int argc, char *argv[])
 			= read(STDIN_FILENO, buffer, 1) == -1 ? errno : 0;
 		daemonized.write_to_stdout
 			= write(STDOUT_FILENO, buffer, 1) == -1 ? errno : 0;
-		daemonized.write_to_stderr
-			= write(STDERR_FILENO, buffer, 1) == -1 ? errno : 0;
+		if (write(STDERR_FILENO, buffer, 1) != 1) {
+			daemonized.write_to_stderr = errno;
+			if (daemonized.write_to_stderr == 0)
+				daemonized.write_to_stderr = -1;
+		} else
+			daemonized.write_to_stderr = 0;
 
 		/* Make sure parent exits. */
 		while (getppid() == pid)
@@ -64,7 +68,7 @@ int main(int argc, char *argv[])
 	ok1(daemonized.in_root_dir);
 	ok1(daemonized.read_from_stdin == EBADF);
 	ok1(daemonized.write_to_stdout == EBADF);
-	ok1(daemonized.write_to_stderr == EBADF);
+	ok1(daemonized.write_to_stderr == 0);
 
 	return exit_status();
 }