C++ FILE without writing to disk

Beside the already mentioned GNU's fmemopen(), which is known in POSIX as open_memstream, similar solution can be obtained combining mmap() (using MAP_ANONYMOUS) or any other OS-specific function that returns a file descriptor to a block of memory, and fdopen().

EDIT: that was wrong, mmap doesn't create a file descriptor.

The GNU libc has, e.g., fmemopen which will give you a FILE * that writes to memory. Try man fmemopen on your Linux system for details.

I suspect (but do not know for sure) that fmemopen is a wrapper that orchestrates the mmap/fdopen approach mentioned by @Cubbi.

If you are on Mac OS X or iOS you don't have access to fmemopen. I've open sourced a solution here:

http://jverkoey.github.com/fmemopen/