Re: How to pass String BYREF to C function?

Hi Andrew,

 

Thanks for the help on this, great stuff.

 

If I run the code from a C client I will get “half the first string”....

 

If I print out the “exitResult” like you specified below:

 

print ctypes.c_char_p(exitResult.value).value

 

I get the following output in WingIDE

 

helloö

 

So it works but what are those funny characters at the end of the string? It
is not serious though, I am just curious...

 

Do I need to change my C code to get rid of those funny characters?

 


int executeUserExit(char *buffer, char **userExitResultBuffer){

   int total_len;

 

   total_len = strlen(buffer) / 2;

   *userExitResultBuffer = (char*) malloc(total_len);

   strncpy(*userExitResultBuffer, buffer, total_len);

   return 0;

}

 

Thanks again

 

Lynton

 

From: [email protected] [mailto:[email protected]] On Behalf
Of Andrew Wilson
Sent: 18 August 2010 02:00 PM
To: Lynton Grice
Cc: [email protected]; [email protected]
Subject: Re: [ctypes-users] How to pass String BYREF to C function?

 

Lynton, 
  The following code is a hack; keep in mind that python strings are
immutable so your c_char_p variables can't be written to by the dll.
Instead you should be using "create_string_buffer" for writeable strings.  I
wasn't so sure about the char** variable so I just made it an int pointer
and then cast it back to a string once the result were in.


import ctypes 
import os

if __name__ == "__main__":
    if os.name=='nt':           
        userExit = ctypes.cdll.LoadLibrary("libUserExits.dll")
        executeUserExit = userExit.executeUserExit

        stream = ctypes.create_string_buffer("hello world")        
        exitResult = ctypes.c_uint(0)
        result = executeUserExit(stream, ctypes.byref(exitResult))
        print result, stream.value, ctypes.c_char_p(exitResult.value).value
    print "Finished"






















On Wed, Aug 18, 2010 at 4:24 AM, Lynton Grice <[email protected]>
wrote:

Hi there,

 

Thanks for the comments. Please note that this is just a test script so I am
not fussed about the “os.name”....I can take all that extra out later....I
just wanted to get it working for now.

 

I tried out something like what you said but still no luck:

 


import ctypes as c

       

userExit = c.cdll.LoadLibrary("libUserExits.dll")

executeUserExit = userExit.executeUserExit

executeUserExit.argtypes = [c.c_char_p, c.POINTER(c.c_char_p)]

executeUserExit.restype = c.c_int

stream = (c.c_char * 20)(*list("hello world"))

exitResult = c.c_char_p()

result = executeUserExit(c.byref(stream), c.byref(exitResult))

print result         

print "Finished"      

      

 

I will keep trying with a couple options...thanks for the push in the right
direction though, much appreciated

 

Lynton

 

From: [email protected] [mailto:[email protected]] 
Sent: 18 August 2010 09:46 AM


To: Lynton Grice
Subject: Re: [ctypes-users] How to pass String BYREF to C function?

 

 

On Aug 17, 2010, at 8:08 PM, Lynton Grice wrote:

 

Hi there,

 

I have the following example C function that basically take 2 arguments and
“halves the first string” and puts that result in the second argument.

 


int executeUserExit(char *buffer, char **userExitResultBuffer){

   int total_len;

 

   total_len = strlen(buffer) / 2;

   *userExitResultBuffer = (char*) malloc(total_len);

   strncpy(*userExitResultBuffer, buffer, total_len);

   return 0;

}

 

I have call this from C code happily, but am having trouble from Python.

 

I created a DLL from the code above

 


import ctypes as c

import os

 

if __name__ == "__main__":

      if os.name=='nt':           

            userExit = c.cdll.LoadLibrary("libUserExits.dll")

            executeUserExit = userExit.executeUserExit

            executeUserExit.argtypes = [c.c_char_p, c.c_char_p]

            executeUserExit.restype = c.c_int

            stream = c.c_char_p()

            stream.value = "hello world"           

            exitResult = c.c_char_p()

            result = executeUserExit(c.pointer(stream), c.byref(exitResult))

            print result

     

      print "Finished"     

            

 

 

There is a lot wrong with this on a pure python-level. Please think of why
you use __name__ == "__main__" and os.name for a script you clearly don't
use somewhere else.

 

But that's just a remark.

 

The important thing here is that you have to create *arrays* of characters,
the same as with C. Then you can create pointers or byref references for
this.

 

Something like this (untested):

 

  a = (c_char * 20)(*list("foobarbaz"))

  b = c_char_p()

  result = executeUserExit(byref(a), byref(b))

 

 

And of course the declaration of executeUserExit is wrong, you need argtypes
to be [c.c_char_p, POINTER(c.c_char_p)]

 

Diez


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