How to pass Bytes buffer from Python to C

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");
    }
}