avutil/log: added av_log_format_line2 which returns buffer length

The new function behaves the same as av_log_format_line, but also forwards
the return value from the underlying snprintf call. This will allow
callers to accurately determine the size requirements for the line buffer.

Signed-off-by: Andreas Weis <github@ghulbus-inc.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/APIchanges

@@ -15,6 +15,10 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2016-04-27 - xxxxxxx - lavu 55.23.100 - log.h
+  Add a new function av_log_format_line2() which returns number of bytes
+  written to the target buffer.
+
 2016-xx-xx - xxxxxxx - lavc 57.37.100 - avcodec.h
   Add a new audio/video encoding and decoding API with decoupled input
   and output -- avcodec_send_packet(), avcodec_receive_frame(),

libavutil/log.c

@@ -283,11 +283,20 @@ static void format_line(void *avcl, int level, const char *fmt, va_list vl,
 
 void av_log_format_line(void *ptr, int level, const char *fmt, va_list vl,
                         char *line, int line_size, int *print_prefix)
+{
+    av_log_format_line2(ptr, level, fmt, vl, line, line_size, print_prefix);
+}
+
+int av_log_format_line2(void *ptr, int level, const char *fmt, va_list vl,
+                        char *line, int line_size, int *print_prefix)
 {
     AVBPrint part[4];
+    int ret;
+
     format_line(ptr, level, fmt, vl, part, print_prefix, NULL);
-    snprintf(line, line_size, "%s%s%s%s", part[0].str, part[1].str, part[2].str, part[3].str);
+    ret = snprintf(line, line_size, "%s%s%s%s", part[0].str, part[1].str, part[2].str, part[3].str);
     av_bprint_finalize(part+3, NULL);
+    return ret;
 }
 
 void av_log_default_callback(void* ptr, int level, const char* fmt, va_list vl)

libavutil/log.h

@@ -317,6 +317,23 @@ AVClassCategory av_default_get_category(void *ptr);
 void av_log_format_line(void *ptr, int level, const char *fmt, va_list vl,
                         char *line, int line_size, int *print_prefix);
 
+/**
+ * Format a line of log the same way as the default callback.
+ * @param line          buffer to receive the formatted line;
+ *                      may be NULL if line_size is 0
+ * @param line_size     size of the buffer; at most line_size-1 characters will
+ *                      be written to the buffer, plus one null terminator
+ * @param print_prefix  used to store whether the prefix must be printed;
+ *                      must point to a persistent integer initially set to 1
+ * @return Returns a negative value if an error occured, otherwise returns
+ *         the number of characters that would have been written for a
+ *         sufficiently large buffer, not including the terminating null
+ *         character. If the return value is not less than line_size, it means
+ *         that the log message was truncated to fit the buffer.
+ */
+int av_log_format_line2(void *ptr, int level, const char *fmt, va_list vl,
+                        char *line, int line_size, int *print_prefix);
+
 #if FF_API_DLOG
 /**
  * av_dlog macros

libavutil/version.h

@@ -64,8 +64,8 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  55
-#define LIBAVUTIL_VERSION_MINOR  22
-#define LIBAVUTIL_VERSION_MICRO 101
+#define LIBAVUTIL_VERSION_MINOR  23
+#define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
                                                LIBAVUTIL_VERSION_MINOR, \