if pidfile turned out to be !regular file, do not unlink it.

It's most probably the /dev/null.

Config.in

@@ -144,7 +144,7 @@ config FEATURE_PIDFILE
 	bool "Support writing pidfiles"
 	default n
 	help
-	  This option makes some applets (crond, syslogd and inetd) write
+	  This option makes some applets (e.g. crond, syslogd, inetd) write
 	  a pidfile in /var/run. Some applications rely on them.
 
 config FEATURE_SUID

include/libbb.h

@@ -622,12 +622,14 @@ llist_t *llist_rev(llist_t *list);
 /* start_stop_daemon and udhcpc are special - they want
  * to create pidfiles regardless of FEATURE_PIDFILE */
 #if ENABLE_FEATURE_PIDFILE || defined(WANT_PIDFILE)
-int write_pidfile(const char *path);
-#define remove_pidfile(f) ((void)unlink(f))
+/* True only if we created pidfile which is *file*, not /dev/null etc */
+extern smallint wrote_pidfile;
+void write_pidfile(const char *path);
+#define remove_pidfile(path) do { if (wrote_pidfile) unlink(path); } while (0)
 #else
-/* Why? #defining it to 1 gives "warning: statement with no effect"... */
-static ALWAYS_INLINE int write_pidfile(const char *path) { return 1; }
-#define remove_pidfile(f) ((void)0)
+enum { wrote_pidfile = 0 };
+#define write_pidfile(path)  ((void)0)
+#define remove_pidfile(path) ((void)0)
 #endif
 
 enum {

libbb/pidfile.c

@@ -11,22 +11,30 @@
 #define WANT_PIDFILE 1
 #include "libbb.h"
 
-int write_pidfile(const char *path)
+smallint wrote_pidfile;
+
+void write_pidfile(const char *path)
 {
 	int pid_fd;
 	char *end;
 	char buf[sizeof(int)*3 + 2];
+	struct stat sb;
 
 	if (!path)
-		return 1;
+		return;
 	/* we will overwrite stale pidfile */
 	pid_fd = open(path, O_WRONLY|O_CREAT|O_TRUNC, 0666);
 	if (pid_fd < 0)
-		return 0;
-	/* few bytes larger, but doesn't use stdio */
-	end = utoa_to_buf(getpid(), buf, sizeof(buf));
-	*end = '\n';
-	full_write(pid_fd, buf, end - buf + 1);
+		return;
+
+	/* path can be "/dev/null"! Test for such cases */
+	wrote_pidfile = (fstat(pid_fd, &sb) == 0) && S_ISREG(sb.st_mode);
+
+	if (wrote_pidfile) {
+		/* few bytes larger, but doesn't use stdio */
+		end = utoa_to_buf(getpid(), buf, sizeof(buf));
+		*end = '\n';
+		full_write(pid_fd, buf, end - buf + 1);
+	}
 	close(pid_fd);
-	return 1;
 }

networking/udhcp/dhcpc.c

@@ -546,7 +546,7 @@ int udhcpc_main(int argc, char **argv)
  ret0:
 	retval = 0;
  ret:
-	if (client_config.pidfile)
+	/*if (client_config.pidfile) - remove_pidfile has it's own check */
 		remove_pidfile(client_config.pidfile);
 	return retval;
 }