ctypes: decomposing/casting pointer to char into array of pointer to char

Question

I'm working with a closed source shared library with example code in C++ like so:

// Header
#define MAX_PARAM_NAME             10
int foo(..., char **ParNameList, ...);

// Main
char      *ParNameList = (char *)NULL;
ret = foo(..., &ParNameList,...);
par = (char (*)[MAX_PARAM_NAME])ParNameList;

How should it be handled in ctypes?

The problematic part is that in function declaration foo(..., char **ParNameList, ...); a char ** is expected, but a reference to char * is actually given in the function call.

So far I have:

from ctypes import *
so = cdll.LoadLibrary(...)  
so.foo.argtypes = [                                                                  
...
POINTER(POINTER(c_char)), # ParNameList                       
...
]                                                                                                        
so.foo.restype = c_int                                                               

#...
ParNameList = POINTER(c_char)()
so.foo(..., ParNameList ,...)

which gives me a garbage string, where I see the required output is interleaved with random changing RAM bits.

But how does (char (*)[MAX_PARAM_NAME]) cast work in ctypes?

If there is a more straight forward way for the whole thing, I'd appreciate to hear it.

Possible duplicate of Python Ctypes passing pointer for data — CristiFati
– CristiFati, Commented Jun 22, 2018 at 12:08
I think this is a slightly different case, the problem is in ambiguity of char ** as per function definition and char * in C-Code. In the C-Example the array of char pointers needs to be decomposed after the function call. — IljaBek
– IljaBek, Commented Jun 22, 2018 at 12:27

101 · Accepted Answer · 2018-06-22 12:53:46Z

1

You'd need to pass the address of the c_char pointer using byref:

foo.argtypes = [..., POINTER(POINTER(c_char)), ...]
foo.restype = c_int

par_name_list = POINTER(c_char)()

# pass by reference
foo(byref(par_name_list))

par = cast(par_name_list, POINTER(c_char * MAX_PARAM_NAME))

You may need to free the memory afterwards.

edited Jun 22, 2018 at 12:53

answered Jun 22, 2018 at 12:34

101

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6 Comments

IljaBek Over a year ago

ctypes-relevant and strange part is the cast of pointer to const length array: (char (*)[MAX_PARAM_NAME])ParNameList;

101 Over a year ago

I think it'd just be par = cast(par_name_list, POINTER(c_char) * MAX_PARAM_NAME).

IljaBek Over a year ago

actually POINTER(c_char *MAX_PARAM_NAME) works! full line: print [(cast(par_name_list, POINTER(c_char*MAX_PARAM_NAME) ))[i].value for i in range(ParNumber)]

101 Over a year ago

Well there you go :)

101 Over a year ago

Also, on second thought you'd probably want to use create_string_buffer() instead of POINTER(c_char)() to create a mutable string to pass to foo as it's more idiomatic.

|

IljaBek · Accepted Answer · 2018-06-22 10:36:57Z

There is pythonic workaround, which I'd like to share for completeness, it's not nice, but it works.

# ParNumber is known, ParNameList is a string
pars = []                                                                                          
for i in range(ParNumber):                                                                        
    q = ParNameList[i*10:i*10+10]                                                                      
    q = q[:q.index('\0')]                                                                          
    print q                                                                                        
    pars.append(q)

still would like to know, what is the better way

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