reading from ssh channel and writing to a buffer

Question

I have this function which if you connect to a system with ssh, you can call it to execute your given command on that system.

std::string sshconnection::exec_ssh_command(ssh_session session, char *command) {
    string receive = "";
    int rc, nbytes;
    char buffer[256];
    ssh_channel channel = ssh_channel_new(session);
    if( channel == NULL )
            return NULL;

    rc = ssh_channel_open_session(channel);
    if( rc != SSH_OK ) {
            ssh_channel_free(channel);
            return NULL;
    }

    rc = ssh_channel_request_exec(channel, command);
    if( rc != SSH_OK ) {
            ssh_channel_close(channel);
            ssh_channel_free(channel);
            cout << "Error";
            return NULL;
    }
    nbytes = ssh_channel_read(channel, buffer, sizeof(buffer), 0);
    while (nbytes > 0)
    {
            if (write(1, buffer, nbytes) != (unsigned int) nbytes)
            {
                    ssh_channel_close(channel);
                    ssh_channel_free(channel);
                    return NULL;
            }
            nbytes = ssh_channel_read(channel, buffer, sizeof(buffer), 0);
    }

    if( nbytes < 0 )
            return NULL;

    ssh_channel_send_eof(channel);
    ssh_channel_close(channel);
    ssh_channel_free(channel);

    return receive;
}

this function works great. I just don't understand that part which is about to write from buffer into a file descriptor=1 . we haven't filled receive anywhere but it is the return value. if we call this function like below:

s = exec_ssh_command(my_ssh_session, "cat /proc/stat" );

the s won't have any value, but if we do this:

std::cout<<s;

this will print s value. and of course we can't save s in a file.
can someone explain to me how is this happening?

EDIT:
function to connect to ssh:

int sshconnection::sshConnection()
{
    if( my_ssh_session == NULL ) {
            cout << "Error creating ssh session" << endl;
            return 1;
    }
    ssh_options_set(my_ssh_session, SSH_OPTIONS_HOST, "yourip");
    ssh_options_set(my_ssh_session, SSH_OPTIONS_USER, "username");
    int rc = ssh_connect(my_ssh_session);
    if( rc != SSH_OK ) {
            cout << "Error with connecting" << endl;
            ssh_free(my_ssh_session);
            return -1;
    }

    rc = ssh_userauth_password(my_ssh_session, NULL, "yourpassword");
    if( rc != SSH_AUTH_SUCCESS) {
            cout << "Error with authorization  " << ssh_get_error(my_ssh_session) << endl;
            ssh_disconnect(my_ssh_session);
            ssh_free(my_ssh_session);
            return -1;
    }
//    ssh_disconnect(my_ssh_session);
    //    ssh_free(my_ssh_session);
}

Answer 1

I know this is old, but I had the same issue. I came up with the following solution.

Use std::string::append like so receive.append(buffer, nbytes).

std::string sshconnection::exec_ssh_command(ssh_session session, char *command) {
    string receive = "";
    int rc, nbytes;
    char buffer[256];
    ssh_channel channel = ssh_channel_new(session);
    if( channel == NULL )
            return NULL;

    rc = ssh_channel_open_session(channel);
    if( rc != SSH_OK ) {
            ssh_channel_free(channel);
            return NULL;
    }

    rc = ssh_channel_request_exec(channel, command);
    if( rc != SSH_OK ) {
            ssh_channel_close(channel);
            ssh_channel_free(channel);
            cout << "Error";
            return NULL;
    }
    nbytes = ssh_channel_read(channel, buffer, sizeof(buffer), 0);
    while (nbytes > 0)
    {
            receive.append(buffer, nbytes);
            nbytes = ssh_channel_read(channel, buffer, sizeof(buffer), 0);
    }

    if( nbytes < 0 )
            return NULL;

    ssh_channel_send_eof(channel);
    ssh_channel_close(channel);
    ssh_channel_free(channel);

    return receive;
}