Implement gebs_fmt() as a function

gebs.h

@@ -24,6 +24,7 @@
 #include <stddef.h>
 #include <stdlib.h>
 #include <inttypes.h>
+#include <stdarg.h>
 
 #if GEBS_PLATFORM == GEBS_PLATFORM_POSIX
 #   include <unistd.h>
@@ -262,13 +263,7 @@ void gebs_rebuild_self1_alloc(Gebs_Allocator *alloc, int argc, char ** argv,
 
 extern Gebs_Arena *gebs_scratch_arena;
 
-#define gebs_fmt(fmt, ...) \
-({ \
-    size_t __size = snprintf(NULL, 0, (fmt), ##__VA_ARGS__); \
-    char *__buf = gebs_malloc(gebs_scratch_arena, __size+1); \
-    sprintf(__buf, (fmt), ##__VA_ARGS__); \
-    __buf; \
-})
+char *gebs_fmt(const char *fmt, ...);
 
 #ifdef GEBS_IMPLEMENTATION
 
@@ -468,6 +463,22 @@ void gebs_scratch_areana_free(void)
     gebs_arena_destroy(gebs_scratch_arena);
 }
 
+char *gebs_fmt(const char *fmt, ...)
+{
+    va_list list;
+    va_start(list, fmt);
+
+    va_list list_copy;
+    va_copy(list_copy, list);
+    size_t size = vsnprintf(NULL, 0, fmt, list_copy);
+    va_end(list_copy);
+
+    char *buf = gebs_malloc(gebs_scratch_arena, size+1);
+    vsprintf(buf, fmt, list);
+    va_end(list);
+    return buf;
+}
+
 // ----------------------------------------------------------------------------
 // Filesystem operations
 // ----------------------------------------------------------------------------