How does SWIG wrap buffer(char array) and pointer from C to Python?

Question

Swig's manual is kinda confusing to me. I am wrapping my C library to python so that I can test my C code in python. Now I want to know how I can access the C pointer address in Python, for example, this is the code I have

typedef struct _buffer_t {
char buf[1024];
char *next_ptr;
} buffer_t;



void parse_buffer(buffer_t * buf_p) {
    buf_p -> next_ptr ++;
}

What I wanted to do is below, in C code

buffer_t my_buf;
my_buf.next_ptr = my_buf.buf;
parse_buffer(&my_buf);
expect_equal(&(my_buf.buf)+1, my_buf.next_ptr);

How do I do the same thing in Python? After import the SWIG wrapped module, I have the buffer_t class in python.

Flexo - Save the data dump · Accepted Answer · 2013-02-20 22:12:39Z

1

The problem is that SWIG is trying to wrap the buffer as a String in Python, which is not just a pointer. The synthesised assignment for next_ptr will allocate memory and make a string copy, not just do a pointer assignment. You can work around this in several ways.

The simplest is to use %extend to add a "reset buffer" method in Python:

%extend {
  void resetPtr() {
    $self->next_ptr=$self->buf;
  }
}

which can then be called in Python to make the assignment you want.

Alternatively if you want to force the buffer to be treated as just another pointer you could force SWIG to treat both members as void* instead of char* and char[]. I had hoped that would be as simple as a %apply per type, but it seems not to work correctly for memberin and memberout in my testing:

%apply void * { char *next };
%apply void * { char buf[ANY] };

Given that the memberin/memberout typemaps are critical to making this work I think %extend is by far the cleanest solution.

answered Feb 20, 2013 at 22:12

Flexo - Save the data dump♦

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2 Comments

wei Over a year ago

Thanks for point out the %extend option. It works, I also used it for getting the pointer address out.

wei Over a year ago

A follow up question, I have an function like void my_func(void * ptr); How can I pass in my buf_p->next_ptr to it? I have already used %apply void * {char * next_ptr} to replace the type. Thanks!

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