reading from ssh channel and writing to a buffer - c++

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;
}