实施strnstr [英] Implementing strnstr

问题描述

我正试图在C中实现一个strnstr函数(strstr，但它检查长度)，由于某种原因它不起作用(输出始终为no):

I am trying to implement a strnstr function into C (strstr but it checks the length), for some reason it doesn't work (output is always no):

#include <stdio.h>

char *searchingFor = "stackdummy";
char *in = "la da\ndoo a da\nnow here comes the stack\nok there it was.\n";

char *strnstr(char *s1, char *s2, int length) {
    if(s1 == NULL || s2 == NULL) return NULL;
    printf("searching \n\n\"%s\"\n for %.*s\n", s1, length, s2);
    char *ss1 = malloc(strlen(s1) + 1);
    strcpy(ss1, s1);
    char *ss2 = malloc(length + 1);
    strncpy(ss2, s2, length);
    char *result = strstr(ss1, ss2);
    free(ss1);
    free(ss2);
    return result;
}

int main(void) {
    printf("found: %s\n", strnstr(in, searchingFor, 5) ? "yes" : "no");
    printf("found: %s\n", strnstr(in, searchingFor, 5) ? "yes" : "no");
    printf("found: %s\n", strnstr(in, searchingFor, 5) ? "yes" : "no");
    return 0;
}

推荐答案

Chris Dodd 提供的实现具有有以下缺点:

The implementation provided by Chris Dodd has the following disadvantages:

1. 它违反了strnstr的目的，因为while条件使用了无界字符串函数strchr
2. 这取决于haystack被NULL终止，这与strnstr的常规实现有所不同，例如，

1. It defeats the purpose of strnstr in that the while condition uses the unbounded string function strchr
2. It depends on haystack being NULL terminated, which is a deviation from the usual implementation of strnstr, for example as provided by GNU-Darwin
3. The call to strchr is an unnecessary function call when strchar is not inlined
4. Returns haystack instead of NULL when len is zero, a deviation from the accepted strstr semantics
5. Returns an empty string instead of haystack when needle has length of zero

以下实现可以解决上述问题，而不会像GNU-Darwin实现那样难以理解，并且已获得知识共享许可:

The following implementation remedies the above problems without becoming as difficult to read as the GNU-Darwin implementation, and is Creative Commons licensed:

#include <string.h>

char *strnstr(const char *haystack, const char *needle, size_t len)
{
        int i;
        size_t needle_len;

        if (0 == (needle_len = strnlen(needle, len)))
                return (char *)haystack;

        for (i=0; i<=(int)(len-needle_len); i++)
        {
                if ((haystack[0] == needle[0]) &&
                        (0 == strncmp(haystack, needle, needle_len)))
                        return (char *)haystack;

                haystack++;
        }
        return NULL;
}

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