Subject: Receiving intermediate results from an application

Receiving intermediate results from an application

From: Bob Alexander <>
Date: Mon, 23 Mar 2009 12:52:27 -0400


I've started using libssh2, but I'm having a problem with receiving
intermediate results from an application. I use libssh2 on a Windows
client, connect to a Linux or AIX server (the problem occurs on either),
run an application on the server by sending a command line with
libssh2_channel_write, then start reading both stdout and stderr with
libssh2_channel_read and libssh2_channel_read_stderr (non-blocking on both).

The app on the server dribbles out information on stderr over a period
of 45 seconds, then exits. But my calls to libssh2_channel_read_stderr
return nothing until the server app exits - and then I quickly get all
the data that had been sent.

I've tried setting the socket's TCP_NODELAY but the problem persists. Is
there something I'm missing?

Basically, my simplified code is:

WSAStartup(WINSOCK_VERSION, &wsadata);
addrinfo aiHints = {0};
aiHints.ai_family = AF_INET;
aiHints.ai_socktype = SOCK_STREAM;
aiHints.ai_protocol = IPPROTO_TCP;

// Call getaddrinfo(). If the call succeeds, paiList will hold a linked list
// of addrinfo structures containing response information about the host.
addrinfo *paiList = NULL;
if (getaddrinfo(host, "22", &aiHints, &paiList) == 0)
  // Create socket and establish connection
  m_socket = socket(AF_INET, SOCK_STREAM, 0);
  BOOL on = TRUE;
  setsockopt(m_socket, IPPROTO_TCP, TCP_NODELAY, (const char *) &on,
  connect(m_socket, paiList->ai_addr,

  m_session = libssh2_session_init_ex(&Malloc, &Free, &Realloc, NULL);
  libssh2_session_startup(m_session, 0);
  libssh2_userauth_keyboard_interactive_ex(m_session, userName,
(unsigned int) strlen(userName), &SKeyboardAuthCallback);

  m_channel = libssh2_channel_open_session(session);
  libssh2_channel_set_blocking(m_channel, 1);
  libssh2_channel_write(m_channel, "testprogram\n", 12);
  while (1)
    char buf[1000];
    int len = libssh2_channel_read(m_channel, buf, 999);
    buf[len] = '\0';
    fputs(buf, stdout);
    len = libssh2_channel_read_stderr(m_channel, buf, 999);
    buf[len] = '\0';
    fputs(buf, stderr);

I'd appreciate any help you can give.

Thank you,

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Received on 2009-03-23