谈谈snprintf

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众所周知,sprintf不能检查目标字符串的长度，可能造成众多安全问题,所以都会推荐使用snprintf.

snprintf(_snprintf)的声明是这样的

int _snprintf(
   char *buffer,
   size_t count,
   const char *format [,
      argument] ...
);

If len < count, thenlen characters are stored in buffer, a null-terminator is appended, and len is returned.

If len = count, thenlen characters are stored in buffer, no null-terminator is appended, and len is returned.

If len > count, thencount characters are stored in buffer, no null-terminator is appended, and a negative value is returned.

最常见的错误用法有:

1.char sa[256]={0};_snprintf(sa,sizeof(sa),"%s",sb);//错误原因:当sb的长度>=256的时候,sa将没有'/0'结尾2.char sa[256];_snprintf(sa,sizeof(sa)-1,"%s",sb);//错误原因:当sb的长度>=255的时候,sa将没有'/0'结尾,忘记给sa初始化3.char sa[256];_snprintf(sa,sizeof(sa)-1,"%s",sb);sa[sizeof(sa)]=0;//错误原因:最后一行数组越界

正确的用法

1. //推荐用法char sa[256];sa[sizeof(sa)-1]=0;_snprintf(sa,sizeof(sa),"%s",sb);if(sa[sizeof(sa)-1]!=0){   printf("warning:string will be truncated");   sa[sizeof(sa)-1]=0;}2.char sa[256]={0};int result = _snprintf(sa,sizeof(sa),"%s",sb);if(result==sizeof(sa) || result<0){    printf("warning:sting will be truncated");   sa[sizeof(sa)-1]=0;}

========================

int snprintf(char *restrict buf, size_t n, const char * restrict format, ...);

函数说明:最多从源串中拷贝n－1个字符到目标串中，然后再在后面加一个0。所以如果目标串的大小为n

的话，将不会溢出。

函数返回值:若成功则返回欲写入的字符串长度，若出错则返回负值。

Result1(推荐的用法)

#include <stdio.h>#include <stdlib.h>int main(){     char str[10]={0,};     snprintf(str, sizeof(str), "0123456789012345678");     printf("str=%s/n", str);     return 0;} root] /root/lindatest$ ./test str=012345678

Result2:(不推荐使用)

#include <stdio.h>#include <stdlib.h>int main(){    char str[10]={0, };    snprintf(str, 18, "0123456789012345678");    printf("str=%s/n", str);    return 0;} root] /root/lindatest$ ./teststr=01234567890123456

snprintf函数返回值的测试：

#include <stdio.h>#include <stdlib.h>int main(){    char str1[10] ={0, };    char str2[10] ={0, };    int ret1=0,ret2=0;    ret1=snprintf(str1, sizeof(str1), "%s", "abc");    ret2=snprintf(str2, 4, "%s", "aaabbbccc");    printf("aaabbbccc length=%d/n", strlen("aaabbbccc"));    printf("str1=%s,ret1=%d/n", str1, ret1);    printf("str2=%s,ret2=%d/n", str2, ret2);    return 0;}[root] /root/lindatest$ ./test aaabbbccc length=9str1=abc,ret1=3str2=aaa,ret2=9