Add option for a custom formatter

README.md

@@ -10,7 +10,7 @@ Some features of uLog:
 * uLog is easy to incorporate into nearly any environment, comprising one header file and one source file, and is written in pure C.
 * uLog provides familiar severity levels (CRITICAL, ERROR, WARNING, INFO, DEBUG, TRACE).
 * uLog supports multiple user-defined outputs (console, log file, in-memory buffer, etc), each with its own reporting threshold level.
-* uLog is "aggressively standalone" with minimal dependencies, requiring only stdio.h, string.h and stdarg.h.  
+* uLog is "aggressively standalone" with minimal dependencies, requiring just string.h, stdarg.h and only additionally stdio.h if you don't provide a custom formatter.
 * uLog gets out of your way when you're not using it: if ULOG_ENABLED is undefined at compile time, no logging code is generated.
 * uLog is well tested.  See the accompanying ulog_test.c file for details.
 
@@ -66,6 +66,26 @@ int main() {
 }
 ```
 
+## A custom formatter for uLog
+
+If `-DULOG_FORMATTER` is defined, uLog then tries to use `ulog_formatter()` with
+the same signature as `vsnprintf()` to format the output:
+
+``` c
+int ulog_formatter(char*, size_t, const char*, va_list);
+```
+
+For cases you wish to handroll the formatter yourself, or just want to use
+another `vsnprintf()` implementation by simply forwarding the arguments to it:
+
+``` c
+int ulog_formatter(char* buffer, size_t count, const char* format, va_list va) {
+  return vsnprintf_(buffer, count, format, va); // another vsnprintf impl.
+}
+```
+
+Otherwise uLog falls back to `vsnprintf()` from `<stdio.h>`.
+
 ## Questions?  Comments?  Improvements?
 
 Comments and pull requests are welcome in https://github.com/rdpoor/ulog/issues

src/ulog.c

@@ -33,9 +33,13 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 #ifdef ULOG_ENABLED  // whole file...
 
-#include <stdio.h>
 #include <string.h>
 #include <stdarg.h>
+#ifdef ULOG_FORMATTER
+extern int ulog_formatter(char*, size_t, const char*, va_list);
+#else
+#include <stdio.h>
+#endif
 
 
 // =============================================================================
@@ -124,7 +128,11 @@ void ulog_message(ulog_level_t severity, const char *fmt, ...) {
   va_list ap;
   int i;
   va_start(ap, fmt);
+#ifdef ULOG_FORMATTER
+  ulog_formatter(s_message, ULOG_MAX_MESSAGE_LENGTH, fmt, ap);
+#else
   vsnprintf(s_message, ULOG_MAX_MESSAGE_LENGTH, fmt, ap);
+#endif
   va_end(ap);
 
   for (i=0; i<ULOG_MAX_SUBSCRIBERS; i++) {