LibSee

See where you use LibC the most.
Trace calls failing tests. Then - roast!

One-liner, to download and compile the script and run your vavorite query:

gcc -g -O2 -fno-builtin -fPIC -nostdlib -nostartfiles -shared -o libsee.so libsee.c

LibSee overrides LibC symbols using LD_PRELOAD, profiling the most commonly used functions, and, optionally, fuzzing their behaviour for testing. The library yields a few binaries when compiled:

libsee.so # Profiles LibC calls
libsee_and_knee.so # Correct LibC behaviour, but fuzzed!

Tricks Used

There are several things worth knowing, that came handy implementing this.

• One way to implement this library would be to override the _start symbols, but implementing correct loading sequence for a binary is tricky, so I use conventional dlsym to lookup the symbols on first function invocation.
• On x86_64 architecture, the rdtscp instruction yields both the CPU cycle and also the unique identifier of the core. Very handy if you are profiling a multi-threaded application.
• Once the unloading sequence reaches libsee.so, the STDOUT is already closed. So if you want to print to the console, you may want to reopen the /dev/tty device before printing usage stats.
• On MacOS the sprintf, vsprintf, snprintf, vsnprintf are macros. You have to #undef them.
• On Release builds compilers love replacing your code with memset and memcpy calls. As the symbol can't be found from inside LibSee, it will SEGFAULT so don't forget to disable such optimizations for built-ins -fno-builtin.
• Aarch64 doesn't seem to have an open system call, but it has the generalized openat number 56.

Coverage

LibC standard is surprisingly long, so not all of the functions are covered. Feel free to suggest PRs covering the rest:

• memory management
• byte-strings
• algorithms
• date and time
• input/output
• concurrency and atomics
• retrieving error numbers
• numerics
• wide-character strings
• multibyte strings
• wide-character IO
• localization
• anything newer than C 11

Program support utilities aren't intended.