是否有标准的C函数来计算字符串长度达到某个限制？

Question

考虑一种情况：我有一个已知长度的缓冲区，可能存储一个以null结尾的字符串，我需要知道字符串的长度。

问题是如果我使用strlen() 并且字符串结果不是以null结尾，则程序在读取超出缓冲区结束时会运行到未定义的行为。 所以我想要一个像下面这样的函数：

 size_t strlenUpTo( char* buffer, size_t limit )
 {
     size_t count = 0;
     while( count < limit && *buffer != 0 ) {
        count++;
        buffer++;
     }
     return count;
 }

所以它返回字符串的长度，但从不尝试读取超出缓冲区的结束。

C库中是否已有这样的功能，还是我必须使用自己的功能？

Answer 1

使用memchr(string, 0, limit) ，如：

size_t strlenUpTo(char *s, size_t n)
{
    char *z = memchr(s, 0, n);
    return z ? z-s : n;
}

Answer 2

根据我的文档，POSIX有size_t strnlen(const char *src, size_t maxlen);

n = strnlen("foobar", 7); // n = 6;
n = strnlen("foobar", 6); // n = 6;
n = strnlen("foobar", 5); // n = 5;

Answer 3

你可以使用strnlen 。 这是它的手册页：

NAME
       strnlen - determine the length of a fixed-size string

SYNOPSIS
       #include <string.h>

       size_t strnlen(const char *s, size_t maxlen);

DESCRIPTION
       The  strnlen  function returns the number of characters in
       the string pointed to by s, not including the  terminating
       '\0' character, but at most maxlen. In doing this, strnlen
       looks only at the first maxlen characters at s  and  never
       beyond s+maxlen.

RETURN VALUE
       The  strnlen  function  returns strlen(s), if that is less
       than maxlen, or maxlen if there is no '\0' character among
       the first maxlen characters pointed to by s.

CONFORMING TO
       This function is a GNU extension.

SEE ALSO
       strlen(3)