ctypes - Python -Why cannot change the value in the C callback function? -
i try using python ctype call c library (.so) , , c library have callback function. c source code:
int showhelloword(int *result) { *result = 1025; return 55; } void bsp_show(int time,int (*callback)(int *result)) { int ret = 44; int boo = 4; while(time > 0){ --time; if(time == 0){ if(callback) { printf("boo %d\n",boo); // expect boo=4 ret = callback(&boo); printf("outfunc=%d\n",boo); // expect outfunc=1025 printf("ret=%d\n",ret); // expect ret=55 } callback = 0; } sleep(1); } } void main(){ bsp_show( 2, &showhelloword); } the print rseult :
boo=4 outfunc=1025 ret=55 and seem okay.
makefile source :
gcc -c -fpic -wno-format-security -g main.c gcc -shared -o libcall.so main.o gcc main.o -o callback -ldl -lpthread -lrt then, try using python ctype call "bsp_show" function , callback function. python source :
python version : 2.6.6
import sys import os import time ctypes import * import struct queue import * class callback_api(object): def __init__(self): cdll("/usr/lib64/librt.so",mode=rtld_global) path = "/root/workspace/callback/libcall.so" `c library path` self.dll = cdll.loadlibrary(path) def bsp_show(self,sec, cb_func): self.dll.bsp_show.restype = none self.dll.bsp_show.argtypes = [c_int, qa_cb_prototype] self.dll.bsp_show(c_int(sec),cb_func) def qa_callback(result): result = 1025 #it should change value. result not.... return 88 qa_cb_prototype = cfunctype( c_int, pointer(c_int)) qa_cache_cb_func = qa_cb_prototype(qa_callback) if __name__ == '__main__': api = callback_api() api.bsp_show(2,qa_cache_cb_func) `got callback` the print result is.
cb=4 outfunc=4 <---i expect should 1025,but not! ret=88 <---return value have been change python callback. have know python ctype limitation or use wrong method? thanks!
the = operator reassigns variable in python. in other words, change variable means.
you need change variable pointing to. since it's ctypes pointer, need index 0 this.
def qa_callback(result): result[0] = 1025 return 88
note [0] after result.
see also: https://docs.python.org/2/library/ctypes.html#pointers