How can I marshal this character array parameter from C to a string in C#?

Question

I have the following function, written as part of another class in C:

int example(char *remoteServerName)
{
    if (doSomething(job))
        return getError(job);
    if (job->server != NULL) {
        int length = strlen(jobPtr->server->name); // name is a char * of length 1025
        remoteServerName = malloc (length * sizeof(char));
        strncpy(remoteServerName, jobPtr->server->name, length);
    }
    return 0;
}

How can I get the remoteServerName back from it? I have tried the following:

[DllImport("example.dll")]
public static extern int example(StringBuilder remoteServerName);

var x = new StringBuilder();
example(x);
Console.WriteLine(x.ToString());

But the string is always empty.

C does not support classes.

too honest for this site
– too honest for this site

2017-03-21 14:33:55 +00:00
Commented Mar 21, 2017 at 14:33 — too honest for this site
– too honest for this site, Commented Mar 21, 2017 at 14:33

David Heffernan · Accepted Answer · 2017-03-21 14:46:49Z

4

You need to allocate some space for the string to be returned in. Instead of:

var x = new StringBuilder();

provide a capacity value:

var x = new StringBuilder(1024);

You should also remove your call to malloc. The caller allocates the memory. That is the purpose of marshalling with StringBuilder.

You are not using strncpy correctly, and so fail to write a null terminator. You could pass the buffer length like this:

int example(char *remoteServerName)
{
    if (doSomething(job))
        return getError(job);
    if (job->server != NULL) {
        // note that new StringBuilder(N) means a buffer of length N+1 is marshaled
        strncpy(remoteServerName, jobPtr->server->name, 1025);
    }
    return 0;
}

But that would be a bit wasteful, with all the zero padding that is implied. Really, strncpy is next to useless and you should use a different function to copy, as has been discussed many times before here. I don't really want to get drawn into that because it's a little off to the side of the question.

It would be prudent to design your API to allow the caller to also pass the length of the character array so that the callee can make sure not to overrun the buffer, and so that you don't need to use magic constants as the code here does.

edited Mar 21, 2017 at 14:46

answered Mar 21, 2017 at 14:17

David Heffernan

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10 Comments

Alexandru Over a year ago

I am trying this but there is something else not working here, because its still empty. I can verify that the int length variable has a value of 9.

David Heffernan Over a year ago

Yes, there were other mistakes in your code that I did not see originally. Sorry. Now fixed, I hope.

Matthew Watson Over a year ago

@Alexandru For an example of how the Windows API manages buffer size reporting, see GetWindowText() and GetWindowTextLength()

Alexandru Over a year ago

David, I hate bringing this up but this code you posted has a bit of a small problem, but the solution to it is in the following link. Perhaps you could update your answer in order to take this into account? stackoverflow.com/questions/45103396/…

David Heffernan Over a year ago

@Alexandru Can you explain what is wrong with this answer to this question?

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