15.18 传递已打开的文件给C扩展

问题¶

You have an open file object in Python, but need to pass it to C extension code that will use the file.

解决方案¶

To convert a file to an integer file descriptor, use PyFile_FromFd(), as shown:

PyObject fobj; / File object (already obtained somehow) */ int fd = PyObject_AsFileDescriptor(fobj); if (fd < 0) {

return NULL;

}

The resulting file descriptor is obtained by calling the fileno() method on fobj. Thus, any object that exposes a descriptor in this manner should work (e.g., file, socket, etc.). Once you have the descriptor, it can be passed to various low-level C functions that expect to work with files. If you need to convert an integer file descriptor back into a Python object, use PyFile_FromFd() as follows:

int fd; /* Existing file descriptor (already open) */ PyObject *fobj = PyFile_FromFd(fd, “filename”,”r”,-1,NULL,NULL,NULL,1);

The arguments to PyFile_FromFd() mirror those of the built-in open() function. NULL values simply indicate that the default settings for the encoding, errors, and newline arguments are being used.

讨论¶

If you are passing file objects from Python to C, there are a few tricky issues to be concerned about. First, Python performs its own I/O buffering through the io module. Prior to passing any kind of file descriptor to C, you should first flush the I/O buffers on the associated file objects. Otherwise, you could get data appearing out of order on the file stream. Second, you need to pay careful attention to file ownership and the responsibility of closing the file in particular. If a file descriptor is passed to C, but still used in Python, you need to make sure C doesn’t accidentally close the file. Likewise, if a file descriptor is being turned into a Python file object, you need to be clear about who is responsible for closing it. The last argument to PyFile_FromFd() is set to 1 to indicate that Python should close the file. If you need to make a different kind of file object such as a FILE * object from the C standard I/O library using a function such as fdopen(), you’ll need to be especially careful. Doing so would introduce two completely different I/O buffering layers into the I/O stack (one from Python’s io module and one from C stdio). Operations such as fclose() in C could also inadvertently close the file for further use in Python. If given a choice, you should probably make extension code work with the low-level integer file descriptors as opposed to using a higher-level abstraction such as that provided by <stdio.h>.