有没有办法确定sprintf将写入多少个字符？

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

int main() {
    int len;
    char *str;
    char buf[1];

    len = snprintf(buf, sizeof(buf), "%s", "Hello, world!");
    if (len < 0) {
        printf("Error: snprintf failed\n");
        return 1;
    }

    str = (char *)malloc(len + 1);
    if (str == NULL) {
        printf("Error: malloc failed\n");
        return 1;
    }

    snprintf(str, len + 1, "%s", "Hello, world!");
    printf("String: %s\n", str);

    free(str);
    return 0;
}