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based on below statement, you cannot pass directly S2 to index of buffer storate. You need to pass *S2. So that it will take first position of S2 Address then increment +1 every time.

Actual: buffer[++i]=s2; s2++
Expected : buffer[++i]=s2; s2++

- creating a local storage and passing it back.
Should have allocated the memory from heap.

- 1024 size limitation but no check made of the sizes of s1, s2
can overrun the buffer.

- buffer[++i] => buffer[i++]

- not terminating the newly created string with '\0'

- function parameters should be const char

etc etc