Canonicalize symlinks in portnames

This allows users to supply symlinks created e.g. by udev rules instead
of the actual device names.

configure.ac

@@ -126,6 +126,9 @@ AC_CHECK_DECLS([BOTHER],,, [[#include <linux/termios.h>]])
 # Check for serial_struct.
 AC_CHECK_TYPES([struct serial_struct],,, [[#include <linux/serial.h>]])
 
+# Check for realpath().
+AC_CHECK_FUNC([realpath], [AC_DEFINE(HAVE_REALPATH, 1, [realpath is available.])], [])
+
 AC_CACHE_CHECK([for visibility control], [sp_cv_visibility_control], [
 	sp_saved_CFLAGS=$CFLAGS
 	CFLAGS="$CFLAGS -Werror"

libserialport_internal.h

@@ -23,7 +23,7 @@
 
 
 #ifdef __linux__
-/* For timeradd, timersub, timercmp. */
+/* For timeradd, timersub, timercmp, realpath. */
 #define _BSD_SOURCE 1 /* for glibc < 2.19 */
 #define _DEFAULT_SOURCE 1 /* for glibc >= 2.20 */
 #endif

serialport.c

@@ -75,6 +75,20 @@ SP_API enum sp_return sp_get_port_by_name(const char *portname, struct sp_port *
 
 	DEBUG_FMT("Building structure for port %s", portname);
 
+#if !defined(_WIN32) && defined(HAVE_REALPATH)
+	/*
+	 * get_port_details() below tries to be too smart and figure out
+	 * some transport properties from the port name which breaks with
+	 * symlinks. Therefore we canonicalize the portname first.
+	 */
+	char pathbuf[PATH_MAX + 1];
+	char *res = realpath(portname, pathbuf);
+	if (!res)
+		RETURN_ERROR(SP_ERR_ARG, "Could not retrieve realpath behind port name");
+
+	portname = pathbuf;
+#endif
+
 	if (!(port = malloc(sizeof(struct sp_port))))
 		RETURN_ERROR(SP_ERR_MEM, "Port structure malloc failed");