I have the following c function.
/* returns (uint8_t *)outbuf */
uint8_t *func(uint8_t *inbuf, uint32_t inbuf_len, uint32_t *outbuf_len);
This function returns outbuf , the output length is unknown before calling the function so the function receives a pointer to the length as an argument outbuf_len , also the caller is responsible to free() outbuf .
I want to get the result of this function from python, so I started writing the following code:
import ctypes as ct
libb = ct.cdll.LoadLibrary('./a.so')
libb.func.restype = ct.c_char_p
inbuf = bytearray(inbuf_len)
inbuf = python_data
arr = ct.c_ubyte * inbuf_len
outbuf_len = ct.c_uint #there is no ct.c_uint_p...
outbuf = libb.func(arr.from_buffer_copy(inbuf), inbuf_len, outbuf_len)
print hexlify(outbuf) #prints only the first 4 bytes of outbuf
The problems i have is:
I have the source of the c function so it is possible to change how arguments are passed the the c function.
Thanks.
If you'll be passing Python byte strings as the input buffer, here's a way to do it. I made a minimal example of the C call:
test.c
#include <stdint.h>
#include <stdlib.h>
#define API __declspec(dllexport)
// double every character in the input buffer as an example
API uint8_t *func(uint8_t *inbuf, uint32_t inbuf_len, uint32_t *outbuf_len) {
*outbuf_len = inbuf_len * 2;
uint8_t* outbuf = malloc(*outbuf_len);
for(uint32_t i = 0; i < inbuf_len; ++i) {
outbuf[i*2] = inbuf[i];
outbuf[i*2+1] = inbuf[i];
}
return outbuf;
}
API void freebuf(uint8_t* buf) {
free(buf);
}
test.py
import ctypes as ct
dll = ct.CDLL('./test')
# c_char_p accepts Python byte strings and is compatible with C uint8_t*
# but don't use it for the output buffer because ctypes converts the pointer
# back to a Python byte string and you would lose access to the pointer for
# later freeing it. Use POINTER(ct.c_char) to get the actual pointer back.
dll.func.argtypes = ct.c_char_p, ct.c_uint32, ct.POINTER(ct.c_uint32)
dll.func.restype = ct.POINTER(ct.c_char)
dll.freebuf.argtypes = ct.POINTER(ct.c_char),
dll.freebuf.restype = None
def func(inbuf):
outlen = ct.c_uint32() # create storage for the output length and pass by reference
outbuf = dll.func(inbuf, len(inbuf), ct.byref(outlen))
# Slicing the returned POINTER(c_char) returns a Python byte string.
# If you used POINTER(c_uint8) for the return value instead,
# you'd get a list of integer byte values.
data = outbuf[:outlen.value]
# Can free the pointer now if you want, or return it for freeing later
dll.freebuf(outbuf)
return data
print(func(b'ABC'))
Output:
b'AABBCC'
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