Add socket99_snprintf, for constructing nicer error messages.

socket99.c

@@ -36,6 +36,7 @@ static bool set_defaults_and_check_cfg(socket99_config *cfg);
 static bool make_tcp_udp(socket99_config *cfg, socket99_result *out);
 static bool make_unixdomain(socket99_config *cfg, socket99_result *out);
 static bool set_nonblocking(socket99_result *out);
+static const char *status_key(enum socket99_status s);
 
 /* Attempt to open a socket, according to the configuration stored in
  * CFG. Returns whether the the socket opened; further details will be
@@ -62,6 +63,28 @@ bool socket99_open(socket99_config *cfg, socket99_result *res) {
     return true;
 }
 
+/* Construct an error message in BUF, based on the status codes
+ * in *RES. This has the same return value and general behavior
+ * as snprintf -- if the return value is >= buf_size, the string
+ * has been truncated. Returns -1 if either BUF or RES are NULL. */
+int socket99_snprintf(char *buf, size_t buf_size, socket99_result *res) {
+    if (buf == NULL || res == NULL) { return 0; }
+    return snprintf(buf, buf_size, "%s: %s",
+        status_key(res->status),
+        (res->status == SOCKET99_ERROR_GETADDRINFO
+            ? gai_strerror(res->getaddrinfo_error)
+            : strerror(res->saved_errno)));
+}
+
+/* Print an error message based on the status contained in *RES. */
+void socket99_fprintf(FILE *f, socket99_result *res) {
+    if (f == NULL || res == NULL) { return; }
+    fprintf(f, "%s: %s\n", status_key(res->status),
+        (res->status == SOCKET99_ERROR_GETADDRINFO
+            ? gai_strerror(res->getaddrinfo_error)
+            : strerror(res->saved_errno)));
+}
+
 /* Set "hints" in an addrinfo struct, to be passed to getaddrinfo. */
 void socket99_set_hints(socket99_config *cfg, struct addrinfo *hints) {
     if (cfg == NULL || hints == NULL) { return; }
@@ -212,6 +235,8 @@ static bool make_tcp_udp(socket99_config *cfg, socket99_result *out) {
         if (out->status == SOCKET99_OK) {
             return fail_with_errno(out, SOCKET99_ERROR_UNKNOWN);
         } else {
+            out->saved_errno = errno;
+            errno = 0;
             return false;
         }
     }
@@ -233,3 +258,29 @@ static bool set_nonblocking(socket99_result *out) {
     }
     return true;
 }
+
+static const char *status_key(enum socket99_status s) {
+    switch (s) {
+    case SOCKET99_OK:
+        return "ok";
+    case SOCKET99_ERROR_GETADDRINFO:
+        return "getaddrinfo";
+    case SOCKET99_ERROR_SOCKET:
+        return "socket";
+    case SOCKET99_ERROR_BIND:
+        return "bind";
+    case SOCKET99_ERROR_LISTEN:
+        return "listen";
+    case SOCKET99_ERROR_CONNECT:
+        return "connect";
+    case SOCKET99_ERROR_FCNTL:
+        return "fcntl";
+    case SOCKET99_ERROR_SNPRINTF:
+        return "snprintf";
+    case SOCKET99_ERROR_CONFIGURATION:
+        return "configuration";
+    case SOCKET99_ERROR_UNKNOWN:
+    default:
+        return "unknown";
+    }
+}

socket99.h

@@ -2,7 +2,7 @@
 #define SOCKET99_H
 
 #define SOCKET99_VERSION_MAJOR 0
-#define SOCKET99_VERSION_MINOR 1
+#define SOCKET99_VERSION_MINOR 1 // dev, pre 0.2.0
 #define SOCKET99_VERSION_PATCH 0
 
 #include <stdint.h>
@@ -69,6 +69,15 @@ typedef struct {
  * stored in RES. */
 bool socket99_open(socket99_config *cfg, socket99_result *res);
 
+/* Construct an error message in BUF, based on the status codes
+ * in *RES. This has the same return value and general behavior
+ * as snprintf -- if the return value is >= buf_size, the string
+ * has been truncated. */
+int socket99_snprintf(char *buf, size_t buf_size, socket99_result *res);
+
+/* Print an error message based on the status contained in *RES. */
+void socket99_fprintf(FILE *f, socket99_result *res);
+
 /* Set "hints" in an addrinfo struct, to be passed to getaddrinfo. */
 void socket99_set_hints(socket99_config *cfg, struct addrinfo *hints);
 

test_socket99.c

@@ -94,7 +94,7 @@ int main(int argc, char **argv) {
                 printf("pass %s\n", tc->name);
                 return 0;
             } else {
-                printf("FAIL %s\n", tc->name);
+                socket99_fprintf(stderr, res);
                 return 1;
             }
         } else {