passing arrays with ctypes

Question

I have a C function

void read_FIFO_AI0(int16_t** input, size_t size, NiFpga_Session* session, NiFpga_Status* status)
{
  *input = (int16_t*) malloc (size*siz         


        

Answer 1

The first argument's type is POINTER(POINTER(c_int16)) not POINTER(ARRAY(c_int16,size)).

Here's a short example:

x.c (compiled with cl /LD x.c:

#include <stdlib.h>
#include <stdint.h>
__declspec(dllexport) void read(int16_t** input, size_t size)
{
  int i;
  int16_t* p = (int16_t*) malloc (size*sizeof(int16_t));
  for(i=0;i<size;i++)
    p[i] = i;
  *input = p;
}
__declspec(dllexport) void release(int16_t* input)
{
    free(input);
}

x.py

from ctypes import *
x = CDLL('x')
x.read.argtypes = POINTER(POINTER(c_int16)),c_size_t
x.read.restype = None
x.release.argtypes = [POINTER(c_int16)]
x.release.restype = None
p = POINTER(c_int16)()
x.read(p,5)
for i in range(5):
    print(p[i])
x.release(p)

Output:

0
1
2
3
4

Note this leaves you with potential memory leak if you don't remember to free the malloc. A better way would be to allocate the buffer in Python and tell the C function the size:

x.c

#include <stdlib.h>
#include <stdint.h>
__declspec(dllexport) void read(int16_t* input, size_t size)
{
  int i;
  for(i=0;i<size;i++)
    input[i] = i;
}

x.py

from ctypes import *
x = CDLL('x')
x.read.argtypes = POINTER(c_int16),c_size_t
x.read.restype = None
p = (c_int16*5)()
x.read(p,len(p))
print(list(p))

Output

[0, 1, 2, 3, 4]