Question

I’m writing a straightforward C program on Linux and wish to use an existing library’s API which expects data from a file. I must feed it a file name as a const char*. But i have data, just like content of a file, already sitting in a buffer allocated on the heap. There is plenty of RAM and we want high performance. Wanting to avoid writing a temporary file to disk, what is a good way to feed the data to this API in a way that looks like a file?

Here’s a cheap pretend version of my code:

marvelouslibrary.h:

int marvelousfunction(const char *filename); 

normal-persons-usage.cpp, for which library was originally designed:

#include 'marvelouslibrary.h' int somefunction(char *somefilename) {     return marvelousfunction(somefilename); } 

myprogram.cpp:

#include 'marvelouslibrary.h' int one_of_my_routines()  {     byte* stuff = new byte[1000000];     // fill stuff[] with...stuff!     // stuff[] holds same bytes as might be found in a file      /* magic goes here: make filename referring to stuff[] */     return marvelousfunction( ??? ); } 

To be clear, the marvelouslibrary does not offer any API functions that accept data by pointer; it can only read a file.

I thought of pipes and mkfifo(), but seems meant for communicating between processes. I am no expert at these things. Does a named pipe work okay read and written in the same process? Is this a wise approach?

Or skip being clever, go with plan ‘B’ which is to shuddup and just write a temp file. However, i’d like to learn something new and find out what’s possible in this situation, beside getting high performance.

Answer 1

Given that you likely have a function like:

char *read_data(const char *fileName) 

I think you will need to ‘skip being clever, go with plan ‘B’ which is to shuddup and just write a temp file.’

If you can dig around and find out if the call you are making is calling another function that takes a File * or an int for the file descriptor then you can do something better.

One thought that does come to mind, can you cahnge your code to write to a memory mapped file instead of to the heap? That way you would have a file on disk already and you would avoid the copying (though it’ll still be on disk) and you can still give the function call the file name.