实现一个scnprinf

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

/* 该函数ret = min(size, 实际写入长度) - 1，即ret永远小于size
 * scnprintf(NULL, ...) -> segment err
 * scnprintf(str, 0, ...) -> -1
 * scnprintf(str, 1, ...) -> 0
 * scnprintf(str, size, "%s", NULL) <==> scnprintf(str, size, "%s", "(null)");
 */
static inline int scnprintf(char *str, size_t size, const char *format, ...)
{
    va_list ap;
    int len;

    va_start(ap, format);
    len = vsnprintf(str, size, format, ap);
    va_end(ap);

    if(len > -1 && len >= size)
        len = size - 1;

    return len;
}

int main(int argc, char **argv)
{
    int write_len = 0, tmp_len;
    char argbuf[16] = {0};
    /*
    for(int i = 1, left_len = sizeof(argbuf); i < argc; i++)
        if((tmp_len = scnprintf(argbuf + write_len, sizeof(argbuf) - write_len, "%s", argv[i])) > 0){
            write_len += tmp_len;
        }else{
            break; // 一般情况下，不会进入这里， 除非snprintf转换失败
        }
    */

    for(int i = 1, left_len = sizeof(argbuf); i < argc; i++)
        write_len += scnprintf(argbuf + write_len, sizeof(argbuf) - write_len, "%s", argv[i]);
    /*  argv很长时，必有sizeof(argbuf) - write_len == 1，
     *  此时，下次调用scnprintf返回0，之后的argv[i]空转，不会再拼装任何字符到argbuf
     */

    printf("write_len =%d, argbug=%s\n", write_len, argbuf);

    return 0;
}