Question

I have one question regarding design of my application.
Here’s the pseudo code:

char* buffer_to_be_filled = (char*) malloc(somesize);
fill_the_buffer(buffer_to_be_filled);
free(buffer_to_be_filled);

The problem is that I don’t know how much size fill_the_buffer requires.

I was thinking about the solution inside the fill_the_buffer function.
I could maybe realloc the space inside when needed; the problem is, is there any way to find out how much space I have available?

How is this usually solved? I think that the one who allocates the buffer should also realloc the buffer as well, right?

NOTE : I’m filling the buffer using fread function, so I don’t know how much space will I need.

Answer 1

Your function can’t realloc the pointer that was passed to it, because realloc isn’t guaranteed to return the same pointer it was passed (the new buffer might be too big to expand in-place). The typical solution is for the function to take a second argument specifying the size of the buffer, and to return an error code if the buffer is too small. Ideally, the error code will tell the user how big the buffer needs to be, so they can reallocate it themselves and recall the function. For example, from the man page for snprintf (which has this problem):

The functions snprintf() and vsnprintf() do not write more than size bytes (including the terminating null byte (‘\0’)). If the output was truncated due to this limit then the return value is the number of characters (excluding the terminating null byte) which would have been written to the final string if enough space had been available. Thus, a return value of size or more means that the output was truncated.