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In the first case, you are trying to append to a constant string. The details depend on your compiler but maybe the string was statically allocated in non-writable memory, or maybe the memory after the first string is either invalid or in use by something else.
In the second case you are using a C-string which has not been initialized. There is no guarentee that malloc will return memory with a zero in the first byte.
I highly recommend that you use the c++ stl strings instead. They will handle the buffer allocation for you.
And don't cast the return value of malloc. It's not necessary in C for the code to compile, but doing so may hide a diagnostic if you've failed to include the proper header for malloc (or another that includes it), namely <stdlib.h>. In C++ there's no automatic conversion from void*, so a cast is necessary but you should never use malloc()/free() with C++ because it doesn't know about constructors/destructors.