CODEWITHSUNDEEP
cwindowssshcygwin
lesthaeghet
lesthaeghet

Reputation: 116

Simple C Wrapper Around OpenSSH (with Cygwin on Windows)

I am packaging a program on Windows that expects to be able to externally call OpenSSH. So, I need to package ssh.exe with it and I need to force ssh.exe to always be called with a custom command line parameter (specifically -F to specify a config file it should use). There is no way to force the calling program to do this, and there are no simple ways to do this otherwise in Windows (that I can think of anyway - symlinks or cmd scripts won't work) so I was just going to write a simple wrapper in C to do it.

This is the code I put together:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>


int main(int argc, char *argv[])
{
    int ret;
    char **newv = malloc((argc + 2) * sizeof(*newv));
    memmove(newv, argv, sizeof(*newv) * argc);

    newv[argc] = "-F ssh.conf";
    newv[argc+1] = 0;

    ret = execv("ssh.orig.exe", newv);

    printf("execv failed with return value of %i", ret);

    return ret;
}

I then compile this code using GCC 4.6.3 in Cygwin and it runs without error; however, there is a strange behavior with regards to input and output. When you go to type at the console (confirming the authenticity of the host and entering in a password, etc) only part of the input appears on the console. For example, if I type in the word 'yes' and press enter, only the 'e' will appear on the console and SSH will display an error about needing to type 'yes' or 'no'. Doing this from the Windows command prompt will result in your input going back to the command propmt, so when you type 'yes' and press enter, you get the ''yes' is not recognized as an internal or external command...' message as if the input were being typed at the command prompt. Eventually SSH will time out after that.

So, I'm obviously missing something here, and I'm assuming it has something to do with the way execv works (at least the POSIX Cygwin version of it).

Is there something I'm missing here or are there any alternatives? I was wondering if maybe I need to fork it and redirect the I/O to the fork (although fork() doesn't seem to work - but there are other issues there on Windows). I tried using _execv from process.h but I was having issues getting the code right for that (also could have been related to trying to use gcc).

It's also possible that there may be a non-programming way to do this that I haven't thought of, but all of the possibilities I've tried don't seem to work.

Thoughts?

Upvotes: 1

Views: 231

Answers (1)

lesthaeghet
lesthaeghet

Reputation: 116

I ended up finding a solution to this problem. I'm sure there were other ways to do this, but this seems to fix the issue and works well. I've replaced the execv line with the following code:

ret = spawnv(P_WAIT, "ssh.orig.exe", newv);

You have to use 'P_WAIT' otherwise the parent process completes and exits and you still have the same problem as before. This causes the parent process to wait, but still transfers input and output to the child process.

Upvotes: 1

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