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pythoncctypes
Nick S
Nick S

Reputation: 621

C function name-dependent segfault with Python ctypes

I'm getting a really weird crash when using ctypes in Python, but I'm not sure if the problem comes from Python or C.

Here is the C source (in test.c):

#include <stdio.h>

void compress(char *a, int b) {
  printf("inside\n");
}

void run() {
  printf("before\n");
  compress("hi", 2);
  printf("after\n");
}

Then here's what happens when I call run() with ctypes:

$ python -c 'import ctypes; ctypes.cdll.LoadLibrary("./test.so").run()'
before
Segmentation fault (core dumped)

The weirdest thing is that the crash doesn't happen when I rename compress() to anything else.

Other things that prevent it from crashing:

I'm pretty new to C, so I'm assuming there's something I'm missing here. What could be causing this?

System info:
Python 2.7.6
gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04)
Ubuntu 14.04
uname -r: 3.13.0-58-generic

Upvotes: 4

Views: 1206

Answers (2)

Patrick Maupin
Patrick Maupin

Reputation: 8137

While @falsetru has diagnosed the problem, his solution won't work in the general case where you have a lot of files to statically link together (because the entire point of declaring things static is to not have them visible from other files).

And while @eryksun has posted a solution for when you want to declare a function the same name as another, in general, you may have a lot of C functions you don't want to export, and you don't want to have to worry about whether they collide with some random function in some library that Python happens to import, and you don't want to have to prefix every one of your internal functions with an attribute.

(GCC maintains documentation on function attributes, including this function visibility feature.)

A more general solution to avoiding namespace collisions is to tell the linker not to export any symbols by default, and then to mark only those functions you want exported, like run(), as visible.

There is probably a standard way to define the macro for this, but my C is so out-of-date I wouldn't know it. In any case, this will work:

#include <stdio.h>

#define EXPORT __attribute__((visibility("protected")))

void compress(char *a, int b) {
  printf("inside\n");
}

EXPORT void run() {
  printf("before\n");
  compress("hi", 2);
  printf("after\n");
}

You can link and run it like this:

$ gcc -x c test.c --shared -fvisibility=hidden -o test.so
$ python -c 'import ctypes; ctypes.cdll.LoadLibrary("./test.so").run()'
before
inside
after

Upvotes: 5

falsetru
falsetru

Reputation: 369374

According to the debugging, the program is trying to call compress in libz.so.1.

$ gdb python -c core
...
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `python -c import ctypes; ctypes.cdll.LoadLibrary("./test.so").run()'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007f9ddea18bff in compress2 () from /lib/x86_64-linux-gnu/libz.so.1

which accepts different parameters (zlib.h):

ZEXTERN int ZEXPORT compress OF((Bytef *dest,   uLongf *destLen,
                                 const Bytef *source, uLong sourceLen));

ZEXTERN int ZEXPORT compress2 OF((Bytef *dest,   uLongf *destLen,
                                  const Bytef *source, uLong sourceLen,
                                  int level));
/*

You can modify the compress function to be static to work around the issue:

static void compress(char *a, int b)
{
    printf("inside\n");
}

Upvotes: 6

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