Why would you ever want to allocate memory on the heap rather than the stack? [duplicate]

Answer 1

Stack variables (often called 'automatic variables') is best used for things you want to always be the same, and always be small.

int x;
char foo[32];

Are all stack allocations, These are fixed at compile time too.

The best reason for heap allocation is that you cant always know how much space you need. You often only know this once the program is running. You might have an idea of limits but you would only want to use the exact amount of space required.

If you had to read in a file that could be anything from 1k to 50mb, you would not do this:-

int readdata ( FILE * f ) {
  char inputdata[50*1024*1025];
  ...
  return x;
}

That would try to allocate 50mb on the stack, which would usually fail as the stack is usually limited to 256k anyway.