usock: add usock_port() for convenient use of numeric ports

Add a new helper function usock_port() which converts the given numeric port number
into a string using a private static buffer. This way a calling application can
conveniently use numeric port arguments without having to convert them before:

  int fd = usock(USOCK_UDP, "example.org", usock_port(80));

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

usock.c

@@ -26,6 +26,7 @@
 #include <errno.h>
 #include <string.h>
 #include <stdbool.h>
+#include <stdio.h>
 
 #include "usock.h"
 
@@ -101,6 +102,18 @@ static int usock_inet(int type, const char *host, const char *service, int sockt
 	return sock;
 }
 
+const char *usock_port(int port)
+{
+	static char buffer[sizeof("65535\0")];
+
+	if (port < 0 || port > 65535)
+		return NULL;
+
+	snprintf(buffer, sizeof(buffer), "%u", port);
+
+	return buffer;
+}
+
 int usock(int type, const char *host, const char *service) {
 	int socktype = ((type & 0xff) == USOCK_TCP) ? SOCK_STREAM : SOCK_DGRAM;
 	bool server = !!(type & USOCK_SERVER);

usock.h

@@ -30,6 +30,7 @@
 #define USOCK_IPV4ONLY		0x4000
 #define USOCK_UNIX		0x8000
 
+const char *usock_port(int port);
 int usock(int type, const char *host, const char *service);
 
 #endif /* USOCK_H_ */