Question

I was playing around with memory-mapped files in C and was wondering if there
is a way to replace the FILE * from fopen with a memory mapped file transparently.

Example:

FILE * fp = g_fopen(...);

//Program does things to this fp.

fclose();

But instead, is it possible to have FILE *fp = my_fopen(…)

Where my own function would open a file on disk mmap it, maybe change contents and then pass back the FILE * without the program seeing anything
different except the new my_fopen() and my_fclose().

Is this possible to do without having to rewrite the way the operations are done in the program?

Answer 1

The magic words are “library interposition”. Here’s a good tutorial: http://developers.sun.com/solaris/articles/lib_interposers.html

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When Oracle purchased sun a lot of articles seem to have disappeared and/or aren’t accessible through Oracle’s website.

Here’s some links that, for the time being, are functional:

• This is the original article as published in 2001: Building Library Interposers for Fun and Profit (Dr. Dobbs)
• The developers.sun.com link was a repost by the original author, possibly with changes from the 2001 article; this is the repost: Debugging and Performance Tuning with Library Interposers (Wayback Machine)
• The Linker & Libraries Guide for Sun Studio / Oracle Solaris Studio. It covers the topic of library interposition with more technical details.