The use of strncmp and memcmp

Question

Does

if(strncmp(buf, buf2, 7) == 0)

do the same thing as

if(memcmp(buf, buf2, 7) == 0)

buf and buf2 are char* arrays or similar.

I was going to append this to another question but then decided perhaps it was better to post it separately. Presumably the answer is either a trivial "yes" or if not then what is the difference?

(I found these functions from online documentation, but wasn't sure about strncmp because the documentation was slightly unclear.)

score 8 · Answer 1 · answered Jul 31 '16 at 19:34

Like strcmp(), strncmp() is for comparing strings, therefore it stops comparing when it finds a string terminator in at least one argument. Any differences past that point have no effect on the result. strncmp() differs in that it will also stop comparing after the specified number of bytes if it does not encounter a terminator before then.

memcmp(), on the other hand, is for comparing blocks of random memory. It compares up to the specified number of bytes from each block until it finds a difference, regardless of the values of the bytes. That is, it does not stop at string terminators.

score 2 · Answer 2 · answered Jul 31 '16 at 19:31

2

In C and C++ the end of a string is indicated by a byte with value 0. The function memcmp does not care about the end of a strig but will in any case compare exactly the number of bytes specified.

In contrast to that, the function strncmp will stop at a byte with value 0 even though the passed number of bytes to compare is not yet reached.

answered Jul 31 '16 at 19:31

Georg Schnabel

631
4
8

Ah yes of course - should have noted that in my question – FreelanceConsultant Jul 31 '16 at 19:35
2

In addition to the functionality of the method, the implementation of memcmp() may in fact be different in that it may be designed to work in terms of whole words while strncmp() is likely to be implemented with byte reads only. If performance is an issue, memcmp() may be faster. – MoDJ Jul 31 '16 at 19:36
Thanks that's also useful to know – FreelanceConsultant Jul 31 '16 at 19:38

score 2 · Answer 3 · answered Jul 31 '16 at 19:36

The main difference between strncmp() and memcmp() is that the first is sensible to (stops at) '\0' where the latest is not. If the first 7 bytes of memory from buf and buf2 do not contain a '\0' in it, then the behaviour is the same.

Consider the following example:

#include <stdio.h>
#include <string.h>

int main(void) {
    char buf[]  = "123\0 12";
    char buf2[] = "123\0 34";

    printf("strncmp(): %d\n", strncmp(buf, buf2, 7));
    printf("memcmp(): %d\n", memcmp(buf, buf2, 7));

    return 0;
}

It will output:

strncmp(): 0
memcmp(): -2

Because strncmp() will stop at buf[3], where it'll find a '\0', where memcmp() will continue until all 7 bytes are compared.

The use of strncmp and memcmp

3 Answers3