如何让SWIG传递含NULL的C char*给Python回调时不截断？

I’ve dealt with this exact problem when wrapping binary protocol code for Python—SWIG’s default char* handling treats everything as a C-style string, so it stops at the first \0 byte. Let’s fix this so your Python callback gets the full binary buffer every time.

The Root Cause #

SWIG automatically converts char* to Python strings by looking for a null terminator. Since your payload is binary data (with valid \0 bytes), this truncates your buffer early. We need to tell SWIG to use the explicit payload_length parameter instead to determine how much data to pass.

Step-by-Step Solution #

1. Add a Custom SWIG Typemap #

Create or modify your SWIG interface file (e.g., packets.i) to define a multi-parameter input typemap that pairs the char* payload with its length. This tells SWIG to create a Python bytes object using the full buffer length, ignoring null bytes.

%module MyModule

// Include your C header
%{
#include "Packets.h"
%}

// Key typemap: handle (char* payload, uint8_t payload_length) pairs in callbacks
%typemap(in) (char *payload, uint8_t payload_length) {
    // Convert C buffer to Python bytes using the explicit length
    $result = PyBytes_FromStringAndSize((const char*)$1, $2);
}

// Expose your C functions/types to SWIG
%include "Packets.h"

2. Update Your Python Callback #

Now that SWIG passes a bytes object (instead of a truncated string), you can simplify your callback code—no need for ord() since bytes are already integer values:

def py_callback(buffer, buffer_length, destination, seq_no):
    print(f"Expected length: {buffer_length}")
    print(f"Received buffer type: {type(buffer)}")
    print(f"Received buffer length: {len(buffer)}")
    
    for idx, byte in enumerate(buffer):
        print(f"{idx:2d} | 0x{byte:02X}")

# Register the callback
MyModule.send_callback_set(py_callback)

3. Compile the SWIG Module #

Use these commands to build your wrapped module (adjust paths/flags for your setup):

# Generate the wrapper code
swig -python packets.i

# Compile the wrapper to object code
gcc -fPIC -c packets_wrap.c -I/usr/include/python3.x  # Replace with your Python header path

# Build the shared library
gcc -shared packets_wrap.o -o _MyModule.so

Why cstring.i Didn’t Work #

The %cstring_output_withsize and similar macros in cstring.i are designed for C functions that return strings and output their length (output direction, C→Python via return values). Your case is the opposite: a callback receiving data (input direction, C→Python via function parameters), so those macros don’t apply here.

Test It Out #

After compiling, run your Python code. You’ll see the buffer length matches payload_length, and all bytes (including \0 values) are present in the buffer argument. No more truncation!

内容的提问来源于stack exchange，提问作者Mudf4ce