Update pidfile() documentation and add unit test for __pidfile_name

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

README.md

@@ -177,8 +177,10 @@ Generic Functions
 - `pidfile(basename)`
 
   Create a daemon PID file in `_PATH_VARRUN` using either `basename` or,
-  if `basename` is `NULL`, `__progname`.  The file name has the form
-  `/var/run/basename.pid`.
+  if `basename` is `NULL`, `__progname`.  The resulting file name is
+  available to the user as a read-only pointer:
+
+        extern char *__pidfile_name;
 
   Use this function to create a PID file for your daemon when it is
   ready to receive signals.  A client application may poll for the
@@ -242,7 +244,9 @@ The following are the popular OpenBSD string functions.
   http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man3/pidfile.3
 
   **Note:** this version of `pidfile()` has been extended to handle it
-    being called multiple times.  See previous section for details.
+  being called multiple times, and also to export the path to the PID
+  file `__pidfile_name`, similar to `__progname`.  See previous section
+  for details.
 
 - `strlcpy(dst, src, len)`
 

pidfilefn.c

@@ -21,6 +21,7 @@
 #include <signal.h>
 #include <unistd.h>
 
+extern char *__pidfile_name;
 extern char *chomp(char *str);
 
 /**
@@ -126,12 +127,12 @@ int pidfile_signal(const char *pidfile, int signal)
 extern char *__progname;
 static char PIDFILE[42];
 
-static void sigterm_handler(int signo)
+static void sigterm_handler(int UNUSED(signo))
 {
 	printf("Exiting ...\n");
 }
 
-static void sigalrm_handler(int signo)
+static void sigalrm_handler(int UNUSED(signo))
 {
 	printf("Testing pidfile() ...\n");
 	pidfile(NULL);
@@ -159,6 +160,10 @@ int main(void)
 	printf("Reading pid file, should be %d ...\n", getpid());
         printf("=> %d\n", pidfile_read(PIDFILE));
 
+	printf("\nComparing __pidfile_name with our guessed PID filename ...\n");
+	printf("strcmp(\"%s\",\n       \"%s\") => %s\n\n", __pidfile_name, PIDFILE,
+	       !strcmp(__pidfile_name, PIDFILE) ? "OK" : "FAIL");
+
 	/* Occular verification that calling pidfile() again updates mtime */
 	snprintf(cmd, sizeof(cmd), "ls -l --full-time %s", PIDFILE);
 	system(cmd);

pidfilefn.mk

@@ -1,3 +1,4 @@
+CFLAGS      = -W -Wall -Wextra -Werror
 OBJS        = pidfile.o chomp.o fexist.o
 ASROOT      = 1
 TARGET      = clean $(OBJS) pidfilefn.test