Geek asks and answers

How to search for a substring in a buffer that contains null?

With C, I need to find a substring inside a buffer that can contain zeros.

haystack = "Some text\0\0\0\0 that has embedded nulls".
needle   = "has embedded"r

I need to return the beginning of a substring, or null, similat in strstr ():

request_segment_end = mystrstr(request_segment_start, boundary);

Are there any existing implementations that you know?

Update

I found implementations of memove on google codes that I copied here verbatim, untested,

 /*
 * memmem.c
 *
 * Find a byte string inside a longer byte string
 *
 * This uses the "Not So Naive" algorithm, a very simple but
 * usually effective algorithm, see:
 *
 * http://www-igm.univ-mlv.fr/~lecroq/string/
 */

#include <string.h>

void *memmem(const void *haystack, size_t n, const void *needle, size_t m)
{
        const unsigned char *y = (const unsigned char *)haystack;
        const unsigned char *x = (const unsigned char *)needle;

        size_t j, k, l;

        if (m > n || !m || !n)
                return NULL;

        if (1 != m) {
                if (x[0] == x[1]) {
                        k = 2;
                        l = 1;
                } else {
                        k = 1;
                        l = 2;
                }

                j = 0;
                while (j <= n - m) {
                        if (x[1] != y[j + 1]) {
                                j += k;
                        } else {
                                if (!memcmp(x + 2, y + j + 2, m - 2)
                                    && x[0] == y[j])
                                        return (void *)&y[j];
                                j += l;
                        }
                }
        } else
                do {
                        if (*y == *x)
                                return (void *)y;
                        y++;
                } while (--n);

        return NULL;
}
+3
source share
2 answers

It doesn't make sense to me that the string contains null characters. Lines have a zero end, so the first occurrence marks the end of the line. Also, what to say that the null delimiter after a word "nulls"has no more characters after it.

, . , . - , . , .

char *search_buffer(char *haystack, size_t haystacklen, char *needle, size_t needlelen)
{   /* warning: O(n^2) */
    int searchlen = haystacklen - needlelen + 1;
    for ( ; searchlen-- > 0; haystack++)
        if (!memcmp(haystack, needle, needlelen))
            return haystack;
    return NULL;
}

char haystack[] = "Some text\0\0\0\0 that has embedded nulls";
size_t haylen = sizeof(haystack)-1; /* exclude null terminator from length */
char needle[] = "has embedded";
size_t needlen = sizeof(needle)-1; /* exclude null terminator from length */
char *res = search_buffer(haystack, haylen, needle, needlen);
+4

memmem, , , linux ( GNU). , strstr, "", .

#include <string.h>

void *memmem(const void *haystack, size_t haystacklen, const void *needle, size_t needlelen);
+7

More articles:


All Articles