Pointer argument passing in python ctypes

Question

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 didn't find pointer to uint in ctypes types, so how can I pass the outbuf_len pointer to the C function?
when printing the outbuf, only the first 4 bytes that are pointed by the pointer are printed.
How do I free() the outbuf buffer from python?

I have the source of the C function so it is possible to change how arguments are passed the the C function.

Thanks.

Mark Tolonen · Accepted Answer · 2022-01-09 19:57:37Z

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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