I needed this to work cross-platform and with non-standard character types (something I defined as XChar8). You need to pass the lengths of the source string and the substring you are searching for.

/*
 Substring search when the string is not null-terminsted XChar8 array.

Arguments:

    source - The source string to be searched
    sourceLen - Length of the source string
    find - the substring to be found
     findLen - Length of the substring

Return value:

Pointer to the substring in the source, or NULL if not found.
 */

XChar8 *FindSubstring(XChar8 *source, size_t sourceLen, XChar8 *find, size_t findLen)
 {
     int found = 0;
     size_t i, j;
     XChar8 *s = NULL, *f = NULL;

    if (sourceLen < findLen)
         return NULL;

    for (i = 0; i < sourceLen - findLen; i++)
     {
         s = source++;
         f = find;
         if (*f++ == *s++)
         {
             for (j = 0; j < findLen - 1; j++)
              {
                 if (*f++ != *s++)
                 {
                     break;
                 }
             }

            if (j == findLen - 1)
             {
                 found = 1;
                 break;
             }
         }
     }

    if (!found)
     return NULL;

    return s - findLen;
 }

In case you need this to work for both: single and double byte encodings, you can call the function again, with a wide char string. Something like this:

XChar8 *findStr = NULL;
 XChar8 targetStr8[] = "someString";
 wchar_t targetStr16[] = L"someString";
 XChar8 *targetStr8 = targetStr8;
 XChar8 *targetStr16 = (XChar8 *) targetStr16;
 
findStr = FindSubstring(buffer, bufferLen, targetStr8, sizeof(targetStr8) - 1);

if (!findStr)
{
   findStr = FindSubstring(buffer, bufferLen, targetStr16, sizeof(targetStr16) - 2);
}