python 在python中将二进制缓冲区写入文件
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Writing a binary buffer to a file in python
提问by Jamie Love
I have some python code that:
我有一些 python 代码：
- Takes a BLOB from a database which is compressed.
- Calls an uncompression routine in C that uncompresses the data.
- Writes the uncompressed data to a file.
- 从压缩的数据库中获取 BLOB。
- 在 C 中调用解压缩例程来解压缩数据。
- 将未压缩的数据写入文件。
It uses ctypesto call the C routine, which is in a shared library.
它使用ctypes调用共享库中的 C 例程。
This mostly works, except for the actual writing to the file. To uncompress, I get the data uncompressed into a python buffer, created using the ctypes create_string_buffermethod:
这主要是有效的，除了实际写入文件。要解压缩，我将解压缩的数据放入使用 ctypescreate_string_buffer方法创建的 python 缓冲区中：
c_uncompData_p = create_string_buffer(64000)
c_uncompData_p = create_string_buffer(64000)
so the uncompression call is like this:
所以解压调用是这样的：
c_uncompSize = mylib.explodeCharBuffer (c_data_p, c_data_len, c_uncompData_p)
c_uncompSize = mylib.explodeCharBuffer (c_data_p, c_data_len, c_uncompData_p)
The size of the resulting uncompressed data is returned as the return value.
生成的未压缩数据的大小作为返回值返回。
But... I have no idea how to force python on only write c_uncompSizebytes out - if I do:
但是......我不知道如何强制 python 只写c_uncompSize字节 - 如果我这样做：
myfile.write (c_uncompData_p.raw)
myfile.write (c_uncompData_p.raw)
it writes the whole 64k buffer out (the data is binary - so it is not null terminated).
它写出整个 64k 缓冲区（数据是二进制的 - 所以它不是空终止）。
So, my question is - using Python 2.5 how do I get c_uncompSize bytes printed out, rather than the whole 64k?
所以，我的问题是 - 使用 Python 2.5 如何打印出 c_uncompSize 字节，而不是整个 64k？
Thanks Jamie
谢谢杰米
采纳答案by elo80ka
Slicing works for c_char_Arrays too:
切片也适用于 c_char_Arrays：
myfile.write(c_uncompData_p[:c_uncompSize])
回答by jfs
buffer()might help to avoid unnecessary copying (caused by slicing as in @elo80ka's answer):
buffer()可能有助于避免不必要的复制（由@elo80ka 的回答中的切片引起）：
myfile.write(buffer(c_uncompData_p.raw, 0, c_uncompSize))
In your example it doesn't matter (due to c_uncompData_pis written only once and it is small) but in general it could be useful.
在您的示例中，这无关紧要（因为c_uncompData_p只写了一次并且很小）但总的来说它可能很有用。
Just for the sake of exercise here's the answer that uses C stdio's fwrite():
为了锻炼的缘故这里是一个使用C中回答stdio的fwrite()：
from ctypes import *
# load C library
try: libc = cdll.msvcrt # Windows
except AttributeError:
libc = CDLL("libc.so.6") # Linux
# fopen()
libc.fopen.restype = c_void_p
def errcheck(res, func, args):
if not res: raise IOError
return res
libc.fopen.errcheck = errcheck
# errcheck() could be similarly defined for `fwrite`, `fclose`
# write data
file_p = libc.fopen("output.bin", "wb")
sizeof_item = 1 # bytes
nitems = libc.fwrite(c_uncompData_p, sizeof_item, c_uncompSize, file_p)
retcode = libc.fclose(file_p)
if nitems != c_uncompSize: # not all data were written
pass
if retcode != 0: # the file was NOT successfully closed
pass
