MrHooker

MrHooker is a command-line tool for hooking function calls in Cython, using LD_PRELOAD. Hooks are written in Cython (a dialect of Python), and compiled on the fly. MrHooker will take care of setting up the environment, initializing Python in the child process, and cleaning up afterwards.

MrHooker is currently only known to work on Linux. Feel free to send pull requests for additional platform support (presumably in src/common.c).

Installation

pip install mrhooker

Note: Pip doesn't seem to be honoring the "requires=cython" in setup.py. Until I figure out why, you'll need to also run:

pip install cython

(If you don't, expect to see "ImportError: No module named pyximport".)

Usage

$ mrhooker [options] <script.pyx> <command> [args]

Example Hook Script

# Import stuff from <dlfcn.h>
cdef extern from "dlfcn.h":
void* dlsym(void*, char*)
void* RTLD_NEXT

# Redefine "send". Must be defined "with gil", to ensure the Python GIL is
# held when the hook is invoked. This hook doesn't do anything particularly
# fancy -- it just prints out the arguments to 'send' and then calls the
# original function.
cdef extern ssize_t \
  send(int sockfd, char *buf, size_t len, int flags) with gil:
    print "====> send(%r, %r, %r, %r)" % (sockfd, buf[:len], len, flags)
    real_send = dlsym(RTLD_NEXT, "send")
    if real_send:
        with nogil:
            res = (<ssize_t(*)(int, void*, size_t, int) nogil>real_send)(
                sockfd, buf, len, flags)
        return res
    else:
        return -1