I'm experimenting with native C extensions in Python. What I'm trying to accomplish now is to pass Bytes buffer from Python to C.
I need to load a binary file from disk and pass that buffer to C extension however I have no clue what types should I use. What I have now is:
Python part:
import ctypes
lib = ctypes.cdll.LoadLibrary("lib.so")
f = open("file.png", "rb")
buf = f.read()
f.close()
lib.func(buf)
C part:
#include <stdio.h>
void func(int buf) {
// do something with buf
}
Example solution passing binary data and length to C function which dumps it.
Python part:
import ctypes
lib = ctypes.cdll.LoadLibrary("./lib.so")
f = open("file.png", "rb")
buf = f.read()
f.close()
lib.func.argtypes = [ctypes.c_void_p, ctypes.c_uint]
lib.func(ctypes.cast(buf, ctypes.c_void_p), len(buf))
C part:
#include <stdio.h>
void func(unsigned char *buf, unsigned int len) {
if (buf) {
for (unsigned int i=0; i<len; i++) {
if (i%16 == 0) {
printf("\n");
}
printf("0x%02x ", buf[i]);
}
printf("\n");
}
}
Alternative solution:
import ctypes
p_data: bytes = b'\x01\x02\x03'
c_p_data_len = ctypes.c_uint(len(p_data))
c_p_data = (ctypes.c_ubyte * len(p_data))(*p_data)
dll_file = ctypes.CDLL("my_lib.dll")
result = dll_file.func(c_p_data, c_p_data_len)
Your C functions should be one of:
void func1(const char* buf, size_t len); # for const data
void func2(char* buf, size_t len); # for non-const data
And Python is:
import ctypes as ct
lib = ct.CDLL("./lib.so")
# While this case will work without it, always explicitly declare the argument types
# and return type to allow ctypes to type-check. It will make assumptions otherwise.
lib.func1.argtypes = ct.c_char_p, ct.c_size_t
lib.func1.restype = None
lib.func2.argtypes = ct.c_char_p, ct.c_size_t
lib.func2.restype = None
with open("file.png", "rb") as f:
buf1 = f.read() # for const data
buf2 = ct.create_string_buffer(f.read()) # for non-const data
lib.func1(buf1, len(buf1))
lib.func2(buf2, len(buf2))
Note there is no protection for calling a non-const function with const data and you'll likely corrupt or crash Python. Make sure to pass writeable buffers to functions that edit the buffer in-place.
int buf? It sounds like you might be in over your head trying to interface Python with C here - you might want to get more familiar with C on its own first.