How to read from pointer address in Python?

I want to read in a Python script a number of bytes starting from a specific address. E.g., I want to read 40000 bytes starting from 0x561124456.

The pointer is given from a C# app. I want to use this method to pass data between the app and script. I've used a TCP socket via localhost, but I want to try this method also.

How can I do this?

Why do you want to do this? Python does not have pointers. What is responsible for putting those bytes at that address in the first place?
– Daniel Roseman
Feb 15 at 13:57

@DanielRoseman I've updated the question.
– Cristian M
Feb 15 at 14:02

I wonder if ctypes.<some c type>.from_address would be useful here? Having just tried it twice, I got OverflowError and a hard crash with no exception, but maybe there's some way to avoid those.
– Kevin
Feb 15 at 14:03

ctypes.<some c type>.from_address

OverflowError

Yeah, putting data into a memory location isn't a good way of passing between applications. That is what sockets are for.
– Daniel Roseman
Feb 15 at 14:55

@DanielRoseman Why isn't it a good way? I can make the the buffer fixed, so it isn't deallocated and I pass the pointer to the script. I think this method is faster than using a socket through localhost whose performance is limited by the CPU and TCP/IP stack performance.
– Cristian M
Feb 16 at 6:13

2 Answers
2

If you really want to, enjoy:

import ctypes
g = (ctypes.c_char*40000).from_address(0x561124456)


Looks like segfault fun. There are good socket-connection libraries on both languages (sockets, RPC etc...), so I would think about this again if this is for some large project.

I happened to work on the similar issue. My python script load .so library to get an image buffer address from c++ .so. After I got the buffer address, I need to be able to read each byte in the buffer. I used "from_address" to create a list object:

imageBytes = list(c_ubyte * dataSize).from_address(pointer)


The following shows the details how to get memory address passed from c++ to pyth and how to access the memory data on python side too. In c++ code frameprovider.cpp:

dataPackPtr = new DataPack();

DataPack * getFrame(){
uint32_t width = 1920;
uint32_t height = 1208;
const size_t buffersize = width * height * 4;//rgba, each color is one byte
unsigned char* rgbaImage = (unsigned char * )malloc(buffersize);
memset(rgbaImage, 0, buffersize); // set all the buffer data to 0.
dataPackPtr->width = width;
dataPackPtr->height = height;
dataPackPtr->buffersize = buffersize;
dataPackPtr->bufferPtr = rgbaImage;
return dataPackPtr;
}

extern "C" {
DataPack* getFrame_wrapper(){
return getFrame();
}
}


My python:

import ctypes
import binascii
lib = ctypes.cdll.LoadLibrary('/libpath/frameprovider.so')
print vars(lib)

class dataPack(ctypes.Structure):
_fields_ = [("width",ctypes.c_int),
("height",ctypes.c_int),
("buffersize",ctypes.c_int),
("bufferAddress", ctypes.c_void_p)]

lib.getFrame_wrapper.restype = ctypes.POINTER(dataPack)
data = lib.getFrame_wrapper()
print "in python the w= ", data.contents.width, "h=",data.contents.height
print "the buffersize=",data.contents.height
imageBytes = list(
(data.contents.buffersize * ctypes.c_ubyte).
from_address(data.contents.bufferAddress))

print "the len of imageBytes are ", len(imageBytes)
print imageBytes[data.contents.buffersize -1] #print the last byte in the buffer
print "in python, the hex value of element 12 is ", hex(imageBytes[12])


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