String append function written in C

c_string_append.c

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

#define MAX_BUF 128

int string_append(char * buffer, size_t buffer_size, const char * string) {
	size_t buffer_len = strlen(buffer);
	size_t free_size = buffer_size - buffer_len;
	size_t string_len = strlen(string);
	if(free_size > string_len) {
		strncpy(buffer + buffer_len, string, string_len);
		return string_len;
	} else {
		return -1;
	}
}

int main() {
	char string_all[MAX_BUF];
	memset(string_all, 0x00, sizeof(string_all));

	char string[64];
	int count = 0;
	int bytes_copied = 0;
	do {
		snprintf(string, sizeof(string), "%d: Hello World!\n", ++count);
		bytes_copied = string_append(string_all, sizeof(string_all), string);
	} while(bytes_copied > 0);

	printf("%s\n", string_all);

	return 0;
}