1

I have a DLL with a function of the following form:

void Foo ( int * i, char ** s )
{
   if ( *i > (int)(strlen(date_string) + strlen(time_string) + 2) )
       sprintf ( *s, "%s %s", time_string, date_string );
}

where

char date_string[] = { __DATE__ };
char time_string[] = { __TIME__ };

But adapting the method used here:

Python and ctypes: how to correctly pass "pointer-to-pointer" into DLL?

by using:

i = c_int(22)
s = POINTER(c_byte)()
Foo(byref(i),byref(s))

just results in the interpreter not producing any output. Does anyone know what I'm doing wrong?

Improve this question
5
  • What is the size of s? Commented Feb 26, 2015 at 10:25
  • Check the ctypes documentation on create_string_buffer: docs.python.org/2/library/ctypes.html. That's what you should be using for passing a mutable **char. Commented Feb 26, 2015 at 13:14
  • As you can see in the post you linked the dll alloc the space for the string s: * ppMem = malloc( * pSize ); Commented Feb 26, 2015 at 13:33
  • @sebastian How do I go about passing s=create_string_buffer(22)? Using a variety of combinations of byref and pointer either results in an write access violation, or a warning that I've passed too many arguments (8 bytes excess). Commented Feb 26, 2015 at 13:45
  • Pass a reference to a pointer to the first element of array s, i.e. byref(POINTER(c_char)(s)). This uses POINTER(c_char)(s) to match a declaration of Foo.argtypes = (POINTER(c_int), POINTER(POINTER(c_char))). Given the latter, using pointer(s) instead would raise an exception because it creates a pointer to a c_char array instead of to a single c_char. Commented Feb 26, 2015 at 16:06
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