lavu/error: add av_make_error_string() and av_err2str() convenience utilities

These functions are modeled after the corresponding utilities in
libavutil/timestamp.h.

doc/APIchanges

@@ -15,6 +15,10 @@ libavutil:     2011-04-18
 
 API changes, most recent first:
 
+2012-04-11 - xxxxxxx - lavu 51.58.100 - error.h
+  Add av_make_error_string() and av_err2str() utilities to
+  libavutil/error.h.
+
 2012-06-05 - xxxxxxx - lavc 54.24.100
   Add pkt_duration field to AVFrame.
 

libavutil/avutil.h

@@ -153,7 +153,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR 51
-#define LIBAVUTIL_VERSION_MINOR 57
+#define LIBAVUTIL_VERSION_MINOR 58
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \

libavutil/error.c

@@ -79,17 +79,14 @@ int av_strerror(int errnum, char *errbuf, size_t errbuf_size)
 int main(void)
 {
     int i;
-    char errbuf[256];
 
     for (i = 0; i < FF_ARRAY_ELEMS(error_entries); i++) {
         struct error_entry *entry = &error_entries[i];
-        av_strerror(entry->num, errbuf, sizeof(errbuf));
-        printf("%d: %s [%s]\n", entry->num, errbuf, entry->tag);
+        printf("%d: %s [%s]\n", entry->num, av_err2str(entry->num), entry->tag);
     }
 
     for (i = 0; i < 256; i++) {
-        av_strerror(-i, errbuf, sizeof(errbuf));
-        printf("%d: %s\n", -i, errbuf);
+        printf("%d: %s\n", -i, av_err2str(-i));
     }
 
     return 0;

libavutil/error.h

@@ -68,6 +68,8 @@
 #define AVERROR_BUG2               FFERRTAG( 'B','U','G',' ')
 #define AVERROR_UNKNOWN            FFERRTAG( 'U','N','K','N') ///< Unknown error, typically from an external library
 
+#define AV_ERROR_MAX_STRING_SIZE 64
+
 /**
  * Put a description of the AVERROR code errnum in errbuf.
  * In case of failure the global variable errno is set to indicate the
@@ -82,6 +84,29 @@
  */
 int av_strerror(int errnum, char *errbuf, size_t errbuf_size);
 
+/**
+ * Fill the provided buffer with a string containing an error string
+ * corresponding to the AVERROR code errnum.
+ *
+ * @param errbuf         a buffer
+ * @param errbuf_size    size in bytes of errbuf
+ * @param errnum         error code to describe
+ * @return the buffer in input, filled with the error description
+ * @see av_strerror()
+ */
+static inline char *av_make_error_string(char *errbuf, size_t errbuf_size, int errnum)
+{
+    av_strerror(errnum, errbuf, errbuf_size);
+    return errbuf;
+}
+
+/**
+ * Convenience macro, the return value should be used only directly in
+ * function arguments but never stand-alone.
+ */
+#define av_err2str(errnum) \
+    av_make_error_string((char[AV_ERROR_MAX_STRING_SIZE]){0}, AV_ERROR_MAX_STRING_SIZE, errnum)
+
 /**
  * @}
  */