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

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.