将MATLAB中的指针参数传递给C-DLL函数foo（char **）

Question

I am writing a C-DLL to be called from MATLAB. Is it possible to call a function with const char ** parameter? eg

void myGetVersion( const char ** );

The C code would be:

const char *version=0;
myGetVersion( &version );

What would be corresponding MATLAB-Code (if it is possible at all)?

Thank you very much!

PS: I think this is the help page , but I couldn't find my case :-(

Answer 1

The answer is to build a pointer to a c-string using the LIBPOINTER function as @JonasHeidelberg suggested. Let me expand his solution with a working example..

First lets build a simple DLL:

version.h

#ifndef VERSION_H
#define VERSION_H

#ifdef __cplusplus
extern "C" {
#endif

#ifdef _WIN32
#   ifdef BUILDING_DLL
#       define DLL_IMPORT_EXPORT __declspec(dllexport)
#   else
#       define DLL_IMPORT_EXPORT __declspec(dllimport)
#   endif
#else
#   define DLL_IMPORT_EXPORT
#endif

DLL_IMPORT_EXPORT void myGetVersion(char**str);

#ifdef __cplusplus
}
#endif

#endif

version.c

#include "version.h"
#include <string.h>

DLL_IMPORT_EXPORT void myGetVersion(char **str)
{
    *str = strdup("1.0.0");
}

You can use your preferred compiler to build the DLL library (Visual Studio, MinGW GCC, ..). I am using MinGW to compile the above, here is the makefile I am using:

Makefile

CC = gcc

all: main

libversion.dll: version.c
    $(CC) -DBUILDING_DLL version.c -I. -shared -o libversion.dll

main: libversion.dll main.c
    $(CC) main.c -o main -L. -lversion

clean:
    rm -rf *.o *.dll *.exe

Before we move to MATLAB, lets test the library with a C program:

main.c

#include <stdio.h>
#include <stdlib.h>
#include "version.h"

int main()
{
    char *str = NULL;
    myGetVersion(&str);
    printf("%s\n", str);
    free(str);

    return 0;
}

Now that all is working, here is how to use this library from MATLAB:

testDLL.m

%# load DLL and check exported functions
loadlibrary libversion.dll version.h
assert( libisloaded('libversion') )
libfunctions libversion -full

%# pass c-string by reference
pstr = libpointer('stringPtrPtr',{''}); %# we use a cellarray of strings
get(pstr)
calllib('libversion','myGetVersion',pstr)
dllVersion = pstr.Value{1}

%# unload DLL
unloadlibrary libversion

The output with the string returned:

Functions in library libversion:
stringPtrPtr myGetVersion(stringPtrPtr)

       Value: {''}
    DataType: 'stringPtrPtr'

dllVersion =
1.0.0

Answer 2

Looking at Working with pointers , section Passing an Array of Strings in the MATLAB documentation (linked from the help page you linked in your question), it seems you need to construct a libpointer object in MATLAB, something like

version = libpointer('stringPtrPtr',{''}); %# corrected according to Amro's comment
calllib('yourCdll', 'myGetVersion', version)

(I don't have a DLL to test this with right now, and I'm not very firm on C pointers, so no guarantee... hope this is a step in the right direction)

Answer 3

I could not get @Amro's solution to work (old version of MATLAB?). So, I had to do something a little more creative:

pstr = libpointer('voidPtr');
ret = calllib('libversion', 'myGetVersion', pstr);
% establish the length
n = 0;
while n==0 || pstr.Value(n) ~= 0
    n=n+1;
    pstr.setdatatype('uint8Ptr', 1, n);
end
% truncate to exclude the NULL character
pstr.setdatatype('uint8Ptr', 1, n-1);
% convert to string
display(char(pstr.Value))