Allocate formatted strings without worrying about size.

makestr.c

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>

size_t counter(char *str) {
	size_t commas = 0;
	int string = 0;
	
	while (*str) {
		//escape commas inside string;
		if (*str == '"' && *(str - 1) != '\\') string = !string;
		
		//count commas
		if (string == 0)
			if (*str == ',') commas++;
		str++;
	}
	
	return commas+1;
}

char* __countstr__(char *format, size_t count, ...) {
	int i;
	char *str;
	va_list args;
	size_t length;
	va_start(args, count);
	
	for (i=0,length=0; i<count; i++)
		length += strlen(va_arg(args, char*));
		
	str = (char*)malloc((sizeof(char) * length)
					  + (sizeof(char) * strlen(format))
					  + 1);
	
	return str;
}

char* makestr(char *ptr, char *format, ...) {}
#define makestr(str, format, ...) \
	str = __countstr__(format, counter(#__VA_ARGS__), __VA_ARGS__); \
	sprintf(str, format, __VA_ARGS__);

/*///////////////////////////////////
	
		Testing:
		
///////////////////////////////////*/

int main() {
	char *ptr;
	
	makestr(ptr, "{\"foo\":%s}", "123");
	
	printf("%s\n", ptr);
	
	free(ptr);
}