谈谈snprintf

From:http://www.cppblog.com/sandy/archive/2006/09/29/13133.html

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;
}