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7.2.2 Ports and File Descriptors
Conventions generally follow those of scsh, The Scheme shell (scsh).
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
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, three file descriptors
(0, 1, and 2) are automatically imported when a program starts up and
assigned to the initial values of the current/standard input, output,
and error ports, respectively. 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
- 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)
Sets the revealed count for a port to rcount. The return value is unspecified.
- Scheme Procedure: fileno port
- C Function: scm_fileno (port)
Return the integer file descriptor underlying port. Does not change its revealed count.
- Scheme Procedure: port->fdes port
Returns 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 section open-file).
- Scheme Procedure: fdes->ports fd
- C Function: scm_fdes_to_ports (fd)
Return a list of existing ports which have fdes as an underlying file descriptor, without changing their revealed counts.
- Scheme Procedure: fdes->inport fdes
Returns 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
Returns 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 fd
- C Function: scm_primitive_move_to_fdes (port, fd)
Moves 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 will be automatically shifted to new descriptors and their revealed counts reset to zero. The return value is
#fif the file descriptor already had the required value or
#tif it was moved.
- Scheme Procedure: move->fdes port fdes
Moves 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: fsync object
- C Function: scm_fsync (object)
Copies 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. The return value is unspecified.
- Scheme Procedure: open path flags [mode]
- C Function: scm_open (path, flags, mode)
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 (see section Processes) is applied. The default is 666 (Unix itself has no default).
flags can be constructed by combining variables using
logior. Basic flags are:
See File Status Flags in The GNU C Library Reference Manual, for additional flags.
- Scheme Procedure: open-fdes path flags [mode]
- C Function: scm_open_fdes (path, flags, mode)
openbut return a file descriptor instead of a port.
- Scheme Procedure: close fd_or_port
- C Function: scm_close (fd_or_port)
close-port(see section close-port), but also works on 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)
A simple wrapper for the
closesystem call. Close file descriptor fd, which must be an integer. Unlike
close, the file descriptor will be closed even if a port is using it. The return value is unspecified.
- 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 on that port. If called multiple times, the unread characters will be read again in “last-in, first-out” order (i.e. a stack). If port is not supplied, the current input port is used.
- Scheme Procedure: unread-string str port
Place the string str in port so that its characters will be read in subsequent read operations. 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: 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
- Variable: PIPE_BUF
A write of up to
PIPE_BUFmany bytes to a pipe is atomic, meaning when done it goes into the pipe instantaneously and as a contiguous block (see Atomicity of Pipe I/O in The GNU C Library Reference Manual).
Note that the output port is likely to block if too much data has been written but not yet read from the input port. Typically the capacity is
The next group of procedures perform a
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_or_port [fd]
- C Function: scm_dup_to_fdes (fd_or_port, fd)
Return a new integer file descriptor referring to the open file designated by fd_or_port, which must be either an open file port or a file descriptor.
- Scheme Procedure: dup->inport port/fd [newfd]
Returns a new input port using the new file descriptor.
- Scheme Procedure: dup->outport port/fd [newfd]
Returns a new output port using the new file descriptor.
- Scheme Procedure: dup port/fd [newfd]
Returns 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 [newfd]
Returns a new port using the new file descriptor. mode supplies a mode string for the port (see section open-file).
- Scheme Procedure: duplicate-port port modes
Returns 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 will 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
0to obtain an unbuffered duplicate port.
This procedure is equivalent to
(dup->port port modes).
- Scheme Procedure: redirect-port old new
- C Function: scm_redirect_port (old, new)
This procedure takes two ports and duplicates the underlying file descriptor from old-port into new-port. The current file descriptor in new-port will be closed. After the redirection the two ports will share a file position and file status flags.
The return value is unspecified.
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
dup2system call. Copies the file descriptor oldfd to descriptor number newfd, replacing the previous meaning of newfd. Both oldfd and newfd must be integers. Unlike for
primitive-move->fdes, no attempt is made to move away ports which are using newfd. The return value is unspecified.
- Scheme Procedure: port-mode port
Return the port modes associated with the open port port. These will not necessarily be 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: port-for-each proc
- C Function: scm_port_for_each (SCM proc)
- C Function: scm_c_port_for_each (void (*proc)(void *, SCM), void *data)
Apply proc to each port in the Guile port table (FIXME: what is the Guile port table?) in turn. The return value is unspecified. More specifically, proc is applied exactly once to every port that exists in the system at the time
port-for-eachis invoked. Changes to the port table while
port-for-eachis running have no effect as far as
The C function
scm_port_for_eachtakes a Scheme procedure encoded as a
scm_c_port_for_eachtakes a pointer to a C function and passes along a arbitrary data cookie.
- Scheme Procedure: setvbuf port mode [size]
- C Function: scm_setvbuf (port, mode, size)
Set the buffering mode for port. mode can be:
- Scheme Procedure: fcntl port/fd cmd [value]
- C Function: scm_fcntl (object, cmd, value)
Apply cmd on port/fd, either a port or file descriptor. The value argument is used by the
SETcommands described below, it’s an integer value.
Values for cmd are:
- Variable: F_GETFD
- Variable: F_SETFD
Get or set flags associated with the file descriptor. The only flag is the following,
- Variable: FD_CLOEXEC
“Close on exec”, meaning the file descriptor will be closed on an
execcall (a successful such call). For example to set that flag,
(fcntl port F_SETFD FD_CLOEXEC)
Or better, set it but leave any other possible future flags unchanged,
(fcntl port F_SETFD (logior FD_CLOEXEC (fcntl port F_GETFD)))
- Variable: F_GETFL
- Variable: F_SETFL
Get or set flags associated with the open file. These flags are
O_RDONLYetc described under
A common use is to set
O_NONBLOCKon a network socket. The following sets that flag, and leaves other flags unchanged.
(fcntl sock F_SETFL (logior O_NONBLOCK (fcntl sock F_GETFL)))
- Scheme Procedure: flock file operation
- C Function: scm_flock (file, operation)
Apply or remove an advisory lock on an open file. operation specifies the action to be done:
- Variable: LOCK_SH
Shared lock. More than one process may hold a shared lock for a given file at a given time.
- Variable: LOCK_EX
Exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.
- Variable: LOCK_NB
Don’t block when locking. This is combined with one of the other operations using
logior(see section Bitwise Operations). If
flockwould block an
EWOULDBLOCKerror is thrown (see section POSIX Interface Conventions).
The return value is not specified. file may be an open file descriptor or an open file descriptor port.
flockdoes not lock files across NFS.
- Scheme Procedure: select reads writes excepts [secs [usecs]]
- C Function: scm_select (reads, writes, excepts, secs, usecs)
This procedure has a variety of uses: waiting for the ability to provide input, accept output, or the existence 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
#fthen select will wait for as long as it takes for one of the other conditions to be satisfied.
The scsh version of
selectdiffers 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.
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