Ports and File Descriptors - Guile Reference Manual

File ports are implemented using low-level operating system I/O facilities, with optional buffering to improve efficiency see File Ports

Note that some procedures (e.g., recv!) will accept ports as arguments, but will actually operate directly on the file descriptor underlying the port. Any port buffering is ignored, including the buffer which implements peek-char and unread-char.

The force-output and drain-input procedures can be used to clear the buffers.

Each open file port has an associated operating system file descriptor. File descriptors are generally not useful in Scheme programs; however they may be needed when interfacing with foreign code and the Unix environment.

A file descriptor can be extracted from a port and a new port can be created from a file descriptor. However a file descriptor is just an integer and the garbage collector doesn't recognize it as a reference to the port. If all other references to the port were dropped, then it's likely that the garbage collector would free the port, with the side-effect of closing the file descriptor prematurely.

To assist the programmer in avoiding this problem, each port has an associated "revealed count" which can be used to keep track of how many times the underlying file descriptor has been stored in other places. If a port's revealed count is greater than zero, the file descriptor will not be closed when the port is garbage collected. A programmer can therefore ensure that the revealed count will be greater than zero if the file descriptor is needed elsewhere.

For the simple case where a file descriptor is "imported" once to become a port, it does not matter if the file descriptor is closed when the port is garbage collected. There is no need to maintain a revealed count. Likewise when "exporting" a file descriptor to the external environment, setting the revealed count is not required provided the port is kept open (i.e., is pointed to by a live Scheme binding) while the file descriptor is in use.

To correspond with traditional Unix behaviour, the three file descriptors (0, 1 and 2) are automatically imported when a program starts up and assigned to the initial values of the current input, output and error ports. The revealed count for each is initially set to one, so that dropping references to one of these ports will not result in its garbage collection: it could be retrieved with fdopen or fdes->ports.

— Scheme Procedure: port-revealed port
— C Function: scm_port_revealed (port)

Return the revealed count for port.

— Scheme Procedure: set-port-revealed! port rcount
— C Function: scm_set_port_revealed_x (port, rcount)

Set the revealed count for port to rcount, an integer.

— Scheme Procedure: fileno port
— C Function: scm_fileno (port)

Return the integer file descriptor underlying port. Do not change its revealed count.

— Scheme Procedure: port->fdes port

Return the integer file descriptor underlying port. As a side effect the revealed count of port is incremented.

— Scheme Procedure: fdopen fdes modes
— C Function: scm_fdopen (fdes, modes)

Return a new port based on the file descriptor fdes. Modes are given by the string modes. The revealed count of the port is initialized to zero. The modes string is the same as that accepted by open-file (see File Ports).

— Scheme Procedure: fdes->ports fdes
— C Function: scm_fdes_to_ports (fdes)

Return a list of existing ports which have fdes as an underlying file descriptor, without changing their revealed counts.

— Scheme Procedure: fdes->inport fdes

Return an existing input port which has fdes as its underlying file descriptor, if one exists, and increments its revealed count. Otherwise, returns a new input port with a revealed count of 1.

— Scheme Procedure: fdes->outport fdes

Return an existing output port which has fdes as its underlying file descriptor, if one exists, and increments its revealed count. Otherwise, returns a new output port with a revealed count of 1.

— Scheme Procedure: primitive-move->fdes port fdes
— C Function: scm_primitive_move_to_fdes (port, fdes)

Move the underlying file descriptor for port to the integer value fdes without changing the revealed count of port. Any other ports already using this descriptor are automatically shifted to new descriptors and their revealed counts reset to zero. The return value is #f if the file descriptor already had the required value or #t if it was moved.

— Scheme Procedure: move->fdes fd/port fd

Move the underlying file descriptor for port to the integer value fdes and sets its revealed count to one. Any other ports already using this descriptor will be automatically shifted to new descriptors and their revealed counts reset to zero. The return value is unspecified.

— Scheme Procedure: release-port-handle port

Decrement the revealed count for a port.

— Scheme Procedure: fsync port/fd
— C Function: scm_fsync (port/fd)

Copy any unwritten data for the specified output file descriptor to disk. If port/fd is a port, its buffer is flushed before the underlying file descriptor is fsync'd.

— Scheme Procedure: open path flags [mode]
— C Function: scm_open (path, flags, mode) |2 |1 |0

Open the file named by path for reading and/or writing. flags is an integer specifying how the file should be opened. mode is an integer specifying the permission bits of the file, if it needs to be created, before the umask is applied. The default is 0666 (Unix itself has no default).
flags can be constructed by combining variables using logior. Basic flags are:
— Variable: O_RDONLY

Open the file read-only.

— Variable: O_WRONLY

Open the file write-only.

— Variable: O_RDWR

Open the file read/write.

— Variable: O_APPEND

Append to the file instead of truncating.

— Variable: O_CREAT

Create the file if it does not already exist.

See the Unix documentation of the open system call for additional flags.

— Scheme Procedure: open-fdes path flags [mode]
— C Function: scm_open_fdes (path, flags, mode) |2 |1 |0

Similar to open but return a file descriptor instead of a port.

— Scheme Procedure: close fd_or_port
— C Function: scm_close (fd_or_port)

Similar to close-port (see Closing), but also take file descriptors. A side effect of closing a file descriptor is that any ports using that file descriptor are moved to a different file descriptor and have their revealed counts set to zero.

— Scheme Procedure: close-fdes fd
— C Function: scm_close_fdes (fd)

Close file descriptor fd, which must be an integer. This is a simple wrapper for the close system call. Unlike close (see close), the file descriptor will be closed even if a port is using it.

— Scheme Procedure: unread-char char [port]
— C Function: scm_unread_char (char, port)

Place char in port so that it will be read by the next read operation. If called multiple times, the unread characters will be read again in last-in first-out order. If port is not supplied, the current input port is used.

— Scheme Procedure: unread-string str line-buffering-input-port
— C Function: scm_unread_string (str, line-buffering-input-port)

Return string str to line-buffering-input-port. A subsequent call to read-string or read-char from this port will retrieve this string or its first character, respectively, before consulting the underlying port.

— Scheme Procedure: pipe
— C Function: scm_pipe ()

Return a newly created pipe: a pair of ports which are linked together on the local machine. The car is the input port and the cdr is the output port. Data written (and flushed) to the output port can be read from the input port. Pipes are commonly used for communication with a newly forked child process. The need to flush the output port can be avoided by making it unbuffered using setvbuf.
Writes occur atomically provided the size of the data in bytes is not greater than the value of PIPE_BUF. Note that the output port is likely to block if too much data (typically equal to PIPE_BUF) has been written but not yet read from the input port.

The next group of procedures perform a dup2 system call, if newfd (an integer) is supplied, otherwise a dup. The file descriptor to be duplicated can be supplied as an integer or contained in a port. The type of value returned varies depending on which procedure is used.

All procedures also have the side effect when performing dup2 that any ports using newfd are moved to a different file descriptor and have their revealed counts set to zero.

— Scheme Procedure: dup->fdes fd/port new-fd
— C Function: scm_dup_to_fdes (fd/port, new-fd)

Return a new integer file descriptor referring to the open file designated by fd/port, which must be either an open file port or a file descriptor. Optional second arg new-fd specifies the new fd to use (via dup2), otherwise one will be chosen by the system (via dup).

— Scheme Procedure: dup->inport port/fd [maybe-fd...]

Return a new input port using the new file descriptor.

— Scheme Procedure: dup->outport port/fd [maybe-fd...]

Return a new output port using the new file descriptor.

— Scheme Procedure: dup port/fd [maybe-fd...]

Return a new port if port/fd is a port, with the same mode as the supplied port, otherwise returns an integer file descriptor.

— Scheme Procedure: dup->port port/fd mode [maybe-fd...]

Return a new port using the new file descriptor. mode supplies a mode string for the port (see open-file).

— Scheme Procedure: duplicate-port port modes

Return a new port which is opened on a duplicate of the file descriptor underlying port, with mode string modes as for open-file. The two ports share a file position and file status flags.
Unexpected behaviour can result if both ports are subsequently used and the original and/or duplicate ports are buffered. The mode string can include 0 to obtain an unbuffered duplicate port.
This procedure is equivalent to (dup->port port modes).

— Scheme Procedure: redirect-port old-port new-port
— C Function: scm_redirect_port (old-port, new-port)

Duplicate the underlying file descriptor from old-port into new-port. The current file descriptor in new-port is closed. After the redirection the two ports share a file position and file status flags.
Unexpected behaviour can result if both ports are subsequently used and the original and/or duplicate ports are buffered.
This procedure does not have any side effects on other ports or revealed counts.

— Scheme Procedure: dup2 oldfd newfd
— C Function: scm_dup2 (oldfd, newfd)

A simple wrapper for the dup2 system call. Copy the file descriptor oldfd to descriptor number newfd, replacing the previous meaning of newfd. Both oldfd and newfd must be integers.
Unlike for dup->fdes or primitive-move->fdes, no attempt is made to move away ports which are using newfd.

— Scheme Procedure: port-mode port
— C Function: scm_port_mode (port)

Return the port modes associated with the open port port. These are necessarily identical to the modes used when the port was opened, since modes such as "append" which are used only during port creation are not retained.

— Scheme Procedure: setvbuf port mode [size]
— C Function: scm_setvbuf (port, mode, size) |2 |1 |0

Set the buffering mode for port. mode can be:

_IONBF
non-buffered
_IOLBF
line buffered
_IOFBF
block buffered, using a newly allocated buffer of size bytes. If size is omitted, a default size will be used.

— Scheme Procedure: fcntl fdes/port command [value]
— C Function: scm_fcntl (fdes/port, command, value)

Apply command to the specified file descriptor or the underlying file descriptor of the specified port. value is an optional integer argument.
Values for command are:

F_DUPFD
Duplicate a file descriptor.
F_GETFD
Get flags associated with the file descriptor.
F_SETFD
Set flags associated with the file descriptor to value.
F_GETFL
Get flags associated with the open file.
F_SETFL
Set flags associated with the open file to value.
F_GETOWN
Get the process ID of a socket's owner, for SIGIO signals.
F_SETOWN
Set the process that owns a socket to value, for SIGIO signals.
FD_CLOEXEC
The value used to indicate the "close on exec" flag with F_GETFL or F_SETFL.

— Scheme Procedure: select reads writes excepts [secs [usecs]]
— C Function: scm_select (reads, writes, excepts, secs, usecs) |3 |2 |0

This procedure has a variety of uses: waiting for the ability to provide input, accept output, or the existance of exceptional conditions on a collection of ports or file descriptors, or waiting for a timeout to occur. It also returns if interrupted by a signal.
reads, writes and excepts can be lists or vectors, with each member a port or a file descriptor. The value returned is a list of three corresponding lists or vectors containing only the members which meet the specified requirement. The ability of port buffers to provide input or accept output is taken into account. Ordering of the input lists or vectors is not preserved.
The optional arguments secs and usecs specify the timeout. Either secs can be specified alone, as either an integer or a real number, or both secs and usecs can be specified as integers, in which case usecs is an additional timeout expressed in microseconds. If secs is omitted or is #f then select will wait for as long as it takes for one of the other conditions to be satisfied.
The scsh version of select differs as follows: Only vectors are accepted for the first three arguments. The usecs argument is not supported. Multiple values are returned instead of a list. Duplicates in the input vectors appear only once in output. An additional select! interface is provided.

38.2 Ports and File Descriptors