How can I search for substring in a buffer that contains null?
Using C, I need to find a substring inside a buffer that may contain nulls.
hay开发者_Go百科stack = "Some text\0\0\0\0 that has embedded nulls".
needle = "has embedded"r
I need to return the start of the substring, or null, similat to strstr():
request_segment_end = mystrstr(request_segment_start, boundary);
Are there any existing implementations that you know of?
Update
I found implementations of memove on google's codesearch, which I've 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;
}
You can use memmem if you are on a system that has it, like linux (it is a GNU extension). Just like strstr, but works on bytes and requires lengths of both "strings" since it doesn't check for null terminated strings.
#include <string.h>
void *memmem(const void *haystack, size_t haystacklen, const void *needle, size_t needlelen);
It doesn't make sense to me for a "string" to contain null characters. Strings are null-terminated so the first occurrence marks the end of the string. Besides, what's to say that the null-terminator after the word "nulls"
doesn't have any more characters after it.
If you mean to search in a buffer, then that would make more sense to me. You'd just have to search the buffer ignoring null characters and just relying on the lengths. I don't know of any existing implementations but it should be easy to whip up a simple naive implementation. Of course use a better search algorithm here as needed.
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);
精彩评论