How to scp from a C program?

Question

The following C program compiles and spits out a a directory listing of "/Users/home/tempdir" directory.

#include <stdio.h>
#include <unistd.h>
int main(int argc, const char * argv[])
{
        execl("/bin/ls", "ls", "/Users/home/tempdir");
        // execl("/usr/bin/scp", "scp", "myfile.txt [email protected]:/Users/home/scpdir");
    return 0;
}

This shows it is possible to execute a binary file from a C program. I am trying to figure out how to execute the "scp" binary file and input the required passphrase. How can I modify the above code to achieve a successful file transfer via scp ?

I attempted the line that is commented out above however I get the following message when it is uncommented:

usage: scp [-1246BCEpqrv] [-c cipher] [-F ssh_config] [-i identity_file]
           [-l limit] [-o ssh_option] [-P port] [-S program]
           [[user@]host1:]file1 ... [[user@]host2:]file2

So it's obviously trying to perform the scp without the password. How do I get around this?

Answer 1

While you could call scp from the command line using expect or something, you are much better off using a library if at all possible.

The general rule when developing is to prefer libraries to system executions whenever possible. It keeps your code cleaner and more portable. The exception is if you are writing a shell script.

http://www.libssh2.org/

libssh2 is a free C library that supports password authentication as well as SCP/SFTP and supports OS X.

http://curl.haxx.se/libcurl/

libcurl supports pretty much every file transfer type you could want, works on just about every platform you could want, and has bindings for pretty much every language you could want.

Answer 2

Short answer, I know, but you'd typically use something like Expect for automating interactive applications in this way.

Answer 3

It appears to me that the output you are printing is from incorrect command syntax. According to the manual page for execl (at least on Arch Linux):

The const char *arg and subsequent ellipses in the execl(), execlp(), and execle() functions can be thought of as arg0, arg1, ..., argn.

So it would seem the correct way to call it is:

execl("/usr/bin/scp", "scp", "myfile.txt", "[email protected]:/Users/home/scpdir");

Also, if you want to avoid having to put in the password completely, SSH (which scp is built on top of) can be configured with an "authorized key," which bypasses the password. It may still ask about unknown key fingerprints, though. An easy way to set up password-less scp access is to run (from the computer initiating the scp):

ssh-copy-id [email protected]

The above command will ask for a password once, they you shouldn't need to enter it again on subsequent ssh operations.

Answer 4

The following code works for me:

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

int main(int argc, char const *argv[])
{
    execl("/usr/bin/scp", "scp", "myfile.txt", "user@IP:path/to/file", (char *)0);
    return 0;
}

Answer 5

The following code work for me:

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

int main()
{

execl("/usr/bin/sshpass","sshpass","-p","typeYourPasswd","/usr/bin/scp", "scp", "/home/harekrishna/Downloads/LoRaModule/ArduinoWithMeterCode/dlmsLoraV1.ino", "[email protected]:/home/harekrishna",(char *)0);
 
    return 0;
}

Answer 6

but this work for me:

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

int main()
{
    execl("/usr/bin/sshpass","sshpass","-p","typeYourPasswd", "scp", "/home/harekrishna/Downloads/LoRaModule/ArduinoWithMeterCode/dlmsLoraV1.ino", "[email protected]:/home/harekrishna",(char *)0);
 
    return 0;
}