Add format_str() and vformat_str()

Simple helpers to allocate and format a string.

common/stringop.c

@@ -1,5 +1,6 @@
 #define _POSIX_C_SOURCE 200809L
 #include <ctype.h>
+#include <stdarg.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -328,3 +329,35 @@ bool expand_path(char **path) {
 	wordfree(&p);
 	return true;
 }
+
+char *vformat_str(const char *fmt, va_list args) {
+	char *str = NULL;
+	va_list args_copy;
+	va_copy(args_copy, args);
+
+	int len = vsnprintf(NULL, 0, fmt, args);
+	if (len < 0) {
+		sway_log_errno(SWAY_ERROR, "vsnprintf(\"%s\") failed", fmt);
+		goto out;
+	}
+
+	str = malloc(len + 1);
+	if (str == NULL) {
+		sway_log_errno(SWAY_ERROR, "malloc() failed");
+		goto out;
+	}
+
+	vsnprintf(str, len + 1, fmt, args_copy);
+
+out:
+	va_end(args_copy);
+	return str;
+}
+
+char *format_str(const char *fmt, ...) {
+	va_list args;
+	va_start(args, fmt);
+	char *str = vformat_str(fmt, args);
+	va_end(args);
+	return str;
+}

include/stringop.h

@@ -5,6 +5,12 @@
 #include <stddef.h>
 #include "list.h"
 
+#ifdef __GNUC__
+#define _SWAY_ATTRIB_PRINTF(start, end) __attribute__((format(printf, start, end)))
+#else
+#define _SWAY_ATTRIB_PRINTF(start, end)
+#endif
+
 void strip_whitespace(char *str);
 void strip_quotes(char *str);
 
@@ -31,4 +37,7 @@ char *argsep(char **stringp, const char *delim, char *matched_delim);
 // Expand a path using shell replacements such as $HOME and ~
 bool expand_path(char **path);
 
+char *vformat_str(const char *fmt, va_list args) _SWAY_ATTRIB_PRINTF(1, 0);
+char *format_str(const char *fmt, ...) _SWAY_ATTRIB_PRINTF(1, 2);
+
 #endif