libssh2_channel_read_ex man page
libssh2_channel_read_ex — read data from a channel stream
ssize_t libssh2_channel_read_ex(LIBSSH2_CHANNEL *channel, int stream_id, char *buf, size_t buflen);
ssize_t libssh2_channel_read(LIBSSH2_CHANNEL *channel, char *buf, size_t buflen);
ssize_t libssh2_channel_read_stderr(LIBSSH2_CHANNEL *channel, char *buf, size_t buflen);
Attempt to read data from an active channel stream. All channel streams have one standard I/O substream (stream_id == 0), and may have up to 2^32 extended data streams as identified by the selected stream_id. The SSH2 protocol currently defines a stream ID of 1 to be the stderr substream.
channel - active channel stream to read from.
stream_id - substream ID number (e.g. 0 or SSH_EXTENDED_DATA_STDERR)
buf - pointer to storage buffer to read data into
buflen - size of the buf storage
libssh2_channel_read(3) and libssh2_channel_read_stderr(3) are macros.
Actual number of bytes read or negative on failure. It returns LIBSSH2_ERROR_EAGAIN when it would otherwise block. While LIBSSH2_ERROR_EAGAIN is a negative number, it isn't really a failure per se.
Note that a return value of zero (0) can in fact be a legitimate value and only signals that no payload data was read. It is not an error.
LIBSSH2_ERROR_SOCKET_SEND - Unable to send data on socket.
LIBSSH2_ERROR_CHANNEL_CLOSED - The channel has been closed.
libssh2_channel_handle_extended_data(3), libssh2_channel_handle_extended_data2(3), libssh2_channel_read(3), libssh2_channel_read_stderr(3), libssh2_channel_set_blocking(3), libssh2_channel_write_ex(3).