Scroll to navigation

gensio_control(3) Library Functions Manual gensio_control(3)


gensio_control - Perform gensio-specific actions


#include <gensio/gensio.h>
int gensio_control(struct gensio *io, int depth, bool get,

unsigned int option,
char *data, gensiods *datalen);


gensio_control performs a gensio-specific operation on the gensio (if depth is 0) or one of its children ( depth > 0). If depth is GENSIO_CONTROL_DEPTH_ALL, then call all the children with the data. GE_NOTSUP error returns from individual gensios are ignored in that case, but it will stop at the first error besides that. If depth is GENSIO_CONTROL_DEPTH_FIRST, it will return on the first gensio that doesn't return GE_NOTSUP. It returns GE_NOTFOUND if nothing handled it.

If you specify a depth >= 0, and depth is greater than the number of gensios in the stack, this will return GE_NOTFOUND. This way you can know if you reached the bottom of the stack.

Most controls use normal strings for configuration, a control will be this way unless othersise specified. Some controls allow binary information to be passed.

If get is true, attempt to fetch the option. You cannot use GENSIO_CONTROL_DEPTH_ALL with get==true. To fetch an option, you must pass in data long enough to hold the output and set datalen to the number of bytes available in data for the output. It will return the length of the string (like strlen, not including the terminating nil) or binary data in datalen.

An operation with get false is a set operation, it will set values or controls in the gensio. For string values, datalen is not used in a put operation or for determining the length of the input string in data, it must be a nil terminated string. For binary values, datalen must be provided.

A get operation is alway indepotent (it won't change anything, so multiple calls will not have any effect on the state of the system). A get operation may or may not use information passed in data, and returns information in the data field.

If the output does not fit in a get operation, datalen is updated to where it would have been if it had enough bytes (one less than the total number of bytes required for string controls), but the output in data is truncated (and nil terminated if possible for string controls). This can be used to probe to see how long a buffer is required by passing in a zero *datalen, and then allocating *datalen (+ 1 for string gensios) and calling the function again with that data.

gensio control operations in option depend on the particular gensio. Below some are documented, but there may be other controls available. See the gensio documentation in gensio(5) for details.


Set the enable/disable for any NAGLE type algorithms. For put the data should be a string "1" to disable delay, or "0" to enable delay. Return value from a get is a string "1" or "0".


Return information about incoming and outgoing streams for the gensio. This is read(get)-only and returns the value in the data in the form "instream=<n>,ostream=<n>". Used by SCTP.


Request that a break be sent over the line (primarily for telnet).


Return the object from the certificate from the remote end. This is primarily for SSL and certauth so the application can validate the certificate's common name, but it can fetch any object from the certificate.

There are two ways to use this interface: fetch by index or fetch by object type.

To fetch by index, just pass in a number in the data, like "0" and it will fetch the value at that index.

To fetch by object type, pass in a number and the object type separated by a comma. The object type to fetch is SN (short name) or LN (long name) descriptor per /usr/include/openssl/object.h. Like "CN" or "commonName". The index should be one less than the start search, you should use -1, for instance to fetch the first index.

The data returned is in the form: "<index>,<sn>,<value>". Where sn is the short name.

In fetch by object type mode, there may be more than one of an object in a certificate, so this interface can handle that. just pass in the index returned by the first into the second call and it will start after that index. For instance, to fetch the first common name, do (with error checking removed for clarity):

strcpy(data, "-1,CN");
gensio_control(io, 0, true, data, &len)

Say it returns "3,CN," You would use

strcpy(data, "3,CN");
gensio_control(io, 0, true, data, &len)

to get the next common name, which might be "4,CN,". You get an GE_NOTFOUND at the end.

Returns GE_NOCERT if there is no remote certificate, GE_CERTINVAL if the passed in object name is not valid, and GE_NOTFOUND if the object was not available in the certificate.


Set the certificate authority file to the string in data. If it ends in '/', it is assumed to be a directory, otherwise it is assumed to be a file. This generally must be done before authorization is done, generally before open or in the GENSIO_EVENT_PRECERT_VERIFY event (see gensio_event(3) for details).


Get the username for the gensio, generally the username sent from the remote end or a certauth gensio.


On a client, set the service data passed by the gensio to the server. On a server, et the service sent from the gensio client, generally available on a certauth server. Returns GE_DATAMISSING if a service was not sent.

This is a binary control, so arbitrary data can be passed in the service.


Get the full certificate in text form sent from the other end.


Get the fingerprint for the certificate from the other end.


Set the environment pointer for an exec. For pty and stdio connecter gensios. The data is a pointer to an argv array (char * const envp[])


Set the arguments for an exec. For pty and stdio connecter gensios. The data is a pointer to an argv array (char * const argv[])


On a packet gensio, return the maximum packet size that can be sent. Any write of this amount or less will be sent as a single message that will be delivered as one read on the other end, or it will not be sent at all (zero-byte send count).


On a stdio connectors and pty gensios, the exit code of the process that ran. This is only valid after close has completed. An integer string is returned.


On a stdio connectors and pty gensios, do a waitpid on the process. If it has closed, this will return success and the exit code in the string. Otherwise it will return GE_NOTREADY.


On UDP connections, add a multicast address that the socket will receive packets on.


On UDP connections, delete a multicast address that the socket will receive packets on.


Return the local address for the connection. Only for network connections. Since a single gensio may have more than one local address, this control provides a means to tell which one. The data string passed in should be the string representation of a the number (like created with snprintf()) for the particular index you want to fetch. If you specify a number larger than the number of open listen sockets, GE_NOTFOUND is returned. The return data is a string holding the address.

Note that a single fetched string may contain more than one address. These will be separated by semicolons. In some cases addresses may change dynamically (like with SCTP), so you get a single set of addresses.


Like GENSIO_CONTROL_LADDR but gets the remote addresses on a gensio. The gensio may need to be open. This is only implemented on bottom-level gensios, like serialdev, network interfaces, echo, file, ipmisol, etc.


Return the binary remote address for the given gensio. Only implemented for network gensios and pty.


Return the local port for the connection. Only for network connections. This is useful if you pass in "0" for the port to let the OS chose; you can get the actual port chosen.


Close writing to the gensio, but leave reading along. This is only for stdio gensios; it lets you close stdin to the subprogram without affecting the subprogram's stdout.


Return the address the connection was made to. For SCTP. gensio_raddr_to_str() returns all the remote addresses in SCTP's current state. This will return the addresses that the original connectx was done to.


Return some sort of remote id for what is on the other end of the connection. Not implemented for most gensios, only for getting the pid on a pty and stdio and the file descriptor on serialdev.


Zero is returned on success, or a gensio error on failure.


gensio_err(3), gensio(5)
27 Feb 2019