谈谈snprintf

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, then len characters are stored in buffer, a null-terminator is appended, and len is returned.

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

If len > count, then count 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
$ ./test
str=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=9
str1=abc,ret1=3
str2=aaa,ret2=9