I just completed the kata using the same technique and it worked for me. If your solution crashes, it's probably caused by some other issue, and not by the way how your result matrix is allocated.

I see two problems :

1- Check upper bound of for loops

2- str1 does not exists outside of char* solve(), one way to solve might be to allocate some memory and return a pointer to that.

Please also read :
https://github.com/Codewars/codewars.com/wiki/Markdown-Formatting

The test is ok.

the upper & lower different is 32=0010 0000
'A' ^ 'a' wanna get this value
c|= 0010 0000 will let upper trans to lower
that's why next line only compare the lower vowel.

no idea either

Tests does not cover all the cases. Try to add test cases from "Instructions" to Sample tests, you will see what instruction you forgot about.

Since C strings end with a NUL character, and that character evals to false, it can be used to iterate through all chars in the string. Extra attention needed when used with a string obtained from stdin, since if it fails for some reason, it may not have it.