Search for a string variable within another string variable

Your return -1; is within the loop. It'll return if the first if (B[i+c]== C) comparison you do fails. Move it out of the loop.

You compare the wrong things. B[i+c] is a char and C is a char*. If you want to compare strings, use strcmp, strncmp or memcmp depending on the situation and supply the correct arguments.

• strcmp for arbitrary strings that you want to compare until the null terminator is found in both.
• strncmp if you want to limit the comparison to n number of characters but still be stopping at a null terminator. Seems like a good fit for a substring comparison.
• memcmp if you are sure that the length you supply will not make it pass a null terminator since it doesn't check for a null terminator. It just compare bytes in memory where you tell it to, for as long as you tell it to.

I'm using memcmp in this example:

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

#define SS_NOTFOUND ((size_t)-1)
size_t strsearch(const char *B, const char *C) {
    size_t Blen = strlen(B);
    size_t Clen = strlen(C);

    // If Blen < Clen, there's no way C can
    // can be a substring of B:
    if(Blen < Clen) return SS_NOTFOUND;

    // Subtract the length of C from the length of B. There is no need to
    // search in B beyond the point where C can't fit:
    Blen -= Clen;

    for (size_t i = 0; i <= Blen; i++) {
        // memcmp is fine here since no null terminators will be passed
        // due to the check (Blen < Clen) at the start and the subtraction
        // of Clen from Blen above.

        if (memcmp(B + i,   C,  Clen) == 0) return i;
        //         char*  char*
    }
    return SS_NOTFOUND;
}

int main() {
    char B[] = "thisatest";
    char C[] = "tes";
    
    size_t pos = strsearch(B, C);

    if(pos == SS_NOTFOUND) puts("sorry, not found");
    else printf("Found at index %zu\n", pos);
}

Output:

Found at index 5

If you for some reason can't use memcmp, create your own and use that instead in the example above:

int MemCmp(const void *s1, const void *s2, size_t len) {
    const unsigned char* a = s1;
    const unsigned char* b = s2;
    
    for(;len; --len, ++a, ++b) {
        if(*a < *b) return -1;
        else if(*a > *b) return 1;
    }
    return 0;
}