C++ FILE without writing to disk

Question

I'm using a library that has quite a few functions that write to a FILE but none that seem to conveniently dump the same data to an object in memory. Is there any way to create a FILE object (or override it) that stores the data in memory instead of writing to disk -- I'd like to avoid the performance hit of opening/writing/reading from files over and over again.

UPDATE: per Rob's suggestion, trying stringstream:

ss.put(c);

std::string myval = ss.str();

printf("Value: %s\n after writing: %i length %lu\n",myval.c_str(),c, myval.length());

But, now trying to get the data (binary) out of the stringstream has me stuck -- how do I grab the binary data I've been adding?

Answer 1

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.

Answer 2

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.

Answer 3

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/

Answer 4

If you have the option of modifying your library, you could use C++ streams instead of C FILE streams.

If your old library function looked like this:

void SomeFun(int this, int that, FILE* logger) {
  ... other code ...
  fprintf(logger, "%d, %d\n", this, that);
  fputs("Warning Message!", logger);
  char c = '\n';
  fputc(c, logger);
}

you might replace that code with:

void SomeFun(int this, int that, std::ostream& logger) {
  ... other code ...
  logger << this << ", " << that << "\n";
  // or: logger << boost::format("%d %d\n") %this %that;
  logger << "Warning Message!";
  char c = '\n';
  logger.put(c);
  // or: logger << c;
}

Then, in your non-library code, do something like:

#include <sstream>    
std::stringstream logStream;
SomeFun(42, 56, logStream);
DisplayCStringOnGui(logStream.str().c_str());

Answer 5

Consider mounting a tmpfs and have the application write to it. Of course this is *nix only.

Answer 6

https://github.com/Snaipe/fmem appears to be a portable fmemopen in C. It gives you FILE you can write to and when you done you get a void* that points to the memory where you data is.