What is the difference between memcmp, strcmp and strncmp in C?

I wrote this small piece of code in C to test memcmp() strncmp() strcmp() functions in C.

Here is the code that I wrote:

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

int main() {
        char *word1="apple",*word2="atoms";

        if (strncmp(word1,word2,5)==0)
                printf("strncmp result.\n");
        if (memcmp(word1,word2,5)==0)
                printf("memcmp result.\n");
        if (strcmp(word1,word2)==0)
                printf("strcmp result.\n");
}

Can somebody explain me the differences because I am confused with these three functions?

My main problem is that I have a file in which I tokenize its line of it,the problem is that when I tokenize the word "atoms" in the file I have to stop the process of tokenizing.

I first tried strcmp() but unfortunately when it reached to the point where the word "atoms" were placed in the file it didn't stop and it continued,but when I used either the memcmp() or the strncmp() it stopped and I was happy.

But then I thought,what if there will be a case in which there is one string in which the first 5 letters are a,t,o,m,s and these are being followed by other letters.

Unfortunately,my thoughts were right as I tested it using the above code by initializing word1 to "atomsaaaaa" and word2 to atoms and memcmp() and strncmp() in the if statements returned 0.On the other hand strcmp() it didn't. It seems that I must use strcmp().

In short:

• strcmp compares null-terminated C strings
• strncmp compares at most N characters of null-terminated C strings
• memcmp compares binary byte buffers of N bytes

So, if you have these strings:

const char s1[] = "atoms\0\0\0\0";  // extra null bytes at end
const char s2[] = "atoms\0abc";     // embedded null byte
const char s3[] = "atomsaaa";

Then these results hold true:

strcmp(s1, s2) == 0      // strcmp stops at null terminator
strcmp(s1, s3) != 0      // Strings are different
strncmp(s1, s3, 5) == 0  // First 5 characters of strings are the same
memcmp(s1, s3, 5) == 0   // First 5 bytes are the same
strncmp(s1, s2, 8) == 0  // Strings are the same up through the null terminator
memcmp(s1, s2, 8) != 0   // First 8 bytes are different

memcmp compares a number of bytes. strcmp and the like compare strings.

You kind of cheat in your example because you know that both strings are 5 characters long (plus the null terminator). However, what if you don't know the length of the strings, which is often the case? Well, you use strcmp because it knows how to deal with strings, memcmp does not.

memcmp is all about comparing byte sequences. If you know how long each string is then yeah, you could use memcmp to compare them, but how often is that the case? Rarely. You often need string comparison functions because, well... they know what a string is and how to compare them.

As for any other issues you are experiencing it is unclear from your question and code. Rest assured though that strcmp is better equipped in the general case for string comparisons than memcmp is.

strcmp():

• It is used to compare the two string stored in two variable, It takes some time to compare them. And so it slows down the process.

strncmp():

• It is very much similar to the previous one, but in this one, it compares the first n number of characters alone. This also slows down the process.

memcmp():

• This function is used compare two variables using their memory. It doesn't compare them one by one, It compares four characters at one time. If your program is too concerned about speed, I recommend using memcmp().

strncmp and memcmp are same except the fact that former takes care of NULL terminated string.

For strcmp you'll want to be only comparing what you know are going to be strings however sometimes this is not always the case such as reading lines of binary files and there for you would want to use memcmp to compare certain lines of input that contain NUL characters but match and you may want to continue checking further lengths of input.

To summarize:

• strncmp() and strcmp() treat a 0 byte as the end of a string, and don't compare beyond it

• to memcmp(), a 0 byte has no special meaning