Returning strings from DLL

Hi all,

I am hoping someone can help solve this one: How do I return a string to MT4 from a dll function?

You cannot allocate memory space in MQL so I can easily crash MT4 by sending back a long string. If you allocate memory in the dll the result is a memory leak?

I'd be really grateful for some assistance and a small sample function.


TIA edit: Could someone also show how to return a single string and an array of strings? Thanks.

I know it's not the cleanest of setups, or most efficient, but you could have the dll output to a log file and have the MT4 code check the log file. Like I said, not the cleanest setup, but still doable.

search for 4xcoder's post on internal files - pipes. Good example. you have to convert it to an integer array and he shows you how to do that.
ps: look here
http://www.forexfactory.com/showthread.php?t=125117

Thanks for that link... but it doesn't show the whole answer, is it possible to efficiently pass strings (arrays of char) and arrays of strings to/from a DLL for MT4?

convert the string to an int array with CopyToBuffer,
send it to MT4, where
you convert it back to a string with StringFromBuffer
he gives you the code. it works.

You dont say what language you're using for your DLL so a direct example isnt easy. The following applies to all though :

Mt4 will quite happily allocate up to 64Kb for a Null terminated string so all your DLL has to do is pass back a pointer to (the address of) the first character of your string.

The string can contain any data as long as it abides by the Null terminated rule ... IE the first zero byte will determine the end of the string as far as MT4 is concerned.

so simple statements like

#import "your.dll"
string getdllstring();

string tstr = getdllstring();

will have mt4 locally allocate memory and copy the string.

More important is how your DLL recieves strings from Mt4

Again Mt4 will send your DLL a pointer to the first character of a string ...
so to pass a 20Kb string to your DLL

#import "your.dll"
void sendstrtodll(string s);

string tstr;

for (int x=0;x<20*1024;x++)
{ tstr = tstr +"X"

sendstrtodll(tstr);

your DLL MUST now allocate the (unknown amount of) memory
in which to copy to the string to. The size can obtained using standard Null terminted string routines in your DLLs native language EG


len = strlen(s);

or by copying the pointer passed from Mt4 and iterating till a zero byte is found.

As far as arrays of string goes simply do something like :

String manystrings[];
int totstrings;

string tstr;

totstrings = 0;
tstr = getdllstring();
while (tstr !="")
{
totstrings++;
arrayresize(manystrings,totstrings);
manystrings[totstrings-1]= tstr;
tstr = getdllstring();
}

of course you could add parameters to the getdllstring function and add other functions if your dll uses an array of strings rather than a linked list.

eg.

int totstrings = getdllnumofstrings();
arrayresize(manystrings,totstrings-1);

for (int y=0;y<totstrings;y++)
{
manystrings[y] = getdllstring(y);
}

Merci rangebound, I must have forgotten to mention a C/C++ dll.

This would be so great ! but where to find those function : CopyToBuffer ??

THIS IS INTERESTING AND CONSISTENT !

do you have any complete sample code ? (I know it is asking a lot !)

But I will just try this, and post the code if it works !

Thanks a lot !

PS : DELPHIIIIIIIIIIIIIIIIIi

I have a question - I am trying to return an array of strings. I believe I saw somewhere that an array of strings is returned as a defined MqlStr object, not a null terminated address. Is this correct? Or any examples for returning an array of strings.

Secndly, looking at the conversion of a string to an int - it looks like that would allow for a better memory allocation (in case the length of the string is unknown when initializing the return in MT4) - is this correct? Is this a preferred way to return a string (instead of null terminated since that requires an initialization to a set length)?

This thread is very old but it appears on the first page of Google search result when searching about getting string from DLL. So I'd like to post my solution here to make it be easy for others. It's very simple.

* C++:

#define MT4EXPORT extern "C" __declspec(dllexport)

MT4EXPORT void getText(char* buffer)
{
char* text = "ABCD";
strcpy(buffer, text);
}

* MQL:

#import "MT4 DLL.dll"
void getText(char&[]);
#import

int OnInit()
{
char buffer[1024];
getText(buffer);
string text=CharArrayToString(buffer);
Print(text); // ABCD

return(INIT_SUCCEEDED);
}