Python ctype help: working with C unsigned char pointers

Question

My first post so please take at easy. :) I'm a bit new to Python as well, but I like what I see so far. What I'm trying to do is access a c library that allows me to print to a receipt printer via Python. I'm creating a wrapper in Python using ctypes, and everything is going great, EXCEPT for two functions. Here our their prototypes:

int C56_api_printer_write(int printer, unsigned char * data, int size, unsigned long timeout_ms);
int C56_api_printer_read(int printer, unsigned char * data, int size, unsigned long timeout_ms);

My issue is with writing to and reading from unsigned char pointers using ctypes. I have to read in a bitmap file in Python and pass the array to the write function, or in the case of read, I need to read that pointer in as an integer array.

I've been floundering on this one for the past few hours, so I was hoping an expert could help by posting a simple example of how this could be accomplished.

Thank you!

Answer 1

Ok, after help from Kyss I was able to finish this out. Here's my test code and result for completion to this problem:

My test.c code:

#include <stdio.h>
int test(unsigned char *test, int size){
    int i;
    for(i=0;i<size;i++){
        printf("item %d in test = %d\n",i, test[i]);
    }   
}
int testout(unsigned char *test, int *size){
   test[2]=237;
   test[3]=12;
   test[4]=222;
   *size = 5;
}
main () {
    test("hello", 5); 
    unsigned char hello[] = "hi";
    int size=0;
    int i;
    testout(hello,&size);
    for(i=0;i<size;i++){
        printf("item %d in hello = %d\n",i, hello[i]);
    }   
}

I created a main for testing my c function. Here's the output of the function test:

item 0 in test = 104
item 1 in test = 101
item 2 in test = 108
item 3 in test = 108
item 4 in test = 111
item 0 in hello = 104
item 1 in hello = 105
item 2 in hello = 237
item 3 in hello = 12
item 4 in hello = 222

Then I compiled for shared, so it could be used from python:

gcc -shared -o test.so test.c

And here's what I used for my python code:

from ctypes import *

lib = "test.so"
dll = cdll.LoadLibrary(lib)
testfunc = dll.test
print "Testing pointer input"
size = c_int(5)
param1 = (c_byte * 5)()
param1[3] = 235 
dll.test(param1, size)
print "Testing pointer output"
dll.testout.argtypes = [POINTER(c_ubyte), POINTER(c_int)]
sizeout = c_int(0)
mem = (c_ubyte * 20)() 
dll.testout(mem, byref(sizeout))
print "Sizeout = " + str(sizeout.value)
for i in range(0,sizeout.value):
    print "Item " + str(i) + " = " + str(mem[i])

And the output:

Testing pointer input
item 0 in test = 0
item 1 in test = 0
item 2 in test = 0
item 3 in test = 235
item 4 in test = 0
Testing pointer output
Sizeout = 5
Item 0 = 0
Item 1 = 0
Item 2 = 237
Item 3 = 12
Item 4 = 222

Works!

My only issue now resides with dynamic resizing of the c_ubyte array based on the size of the output. I have posted a separate question regarding that though.

Thanks for your help Kyss!

Answer 2

Does the following help you? Please let me know if it gives you errors or I misunderstood your question:

size =
printer =
timeout =

data = (ctypes.c_ubyte * size)() # line 5

C56_api_printer_read(printer, data, size, timeout)

# manipulate data eg
data[3] = 7

C56_api_printer_write(printer, data, size, timeout)

Edit:

About line 5: See also http://docs.python.org/library/ctypes.html sections 15.17.1.13 and 15.17.1.20.

(ctypes.c_ubyte * size)

gives a function that constructs a ctypes array of length size. Then in this line I call the function with no arguments, causing initialization with zeros.