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vt start a program on a new virtual terminal (VT).

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1. open.1.man

Manpage of OPENVT

OPENVT

Section: Linux 1.x (1)
Updated: V1.4
Index Return to Main Contents

NAME

openvt - start a program on a new virtual terminal (VT).

SYNOPSIS

openvt [-c vtnumber] [-s] [-u] [-l] [-v] [--] command command_options

DESCRIPTION

openvt will find the first available VT, and run on it the given command with the given command options, standard input, output and error are directed to that terminal. The current search path ($PATH) is used to find the requested command. If no command is specified then the environment variable $SHELL is used.

OPTIONS

-c vtnumber: Use the given VT number and not the first available. Note you must have write access to the supplied VT for this to work.
-e: Directly execute the given command, without forking. This option is meant for use in /etc/inittab.
-s: Switch to the new VT when starting the command. The VT of the new command will be made the new current VT.
-u: Figure out the owner of the current VT, and run login as that user. Suitable to be called by init. Shouldn't be used with -c or -l.
-l: Make the command a login shell. A - is prepended to the name of the command to be executed.
-v: Be a bit more verbose.
-w: wait for command to complete. If -w and -s are used together then openvt will switch back to the controlling terminal when the command completes.
--: end of options to openvt.

NOTE

If openvt is compiled with a POSIX (GNU) getopt() and you wish to set options to the command to be run, then you must supply the end of options -- flag before the command. .SHEXAMPLES openvt can be used to start a shell on the next free VT, by using the command:

openvt bash
: To start the shell as a login shell, use:
openvt -l bash
: To get a long listing you must supply the -- separator:
openvt -- ls -l

HISTORY

Earlier, openvt was called open. It was written by Jon Tombs <jon@gtex02.us.es or jon@robots.ox.ac.uk>. The -w idea is from "sam".

Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

NOTE

HISTORY

SEE ALSO

This document was created by man2html using the manual pages.
Time: 11:39:12 GMT, February 18, 2010

2. open.2.man

Manpage of OPEN

OPEN

Section: Linux Programmer's Manual (2)
Updated: 2008-12-03
Index Return to Main Contents

NAME

open, creat - open and possibly create a file or device

SYNOPSIS

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);

int creat(const char *pathname, mode_t mode);

DESCRIPTION

Given a pathname for a file, open() returns a file descriptor, a small, non-negative integer for use in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.). The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process.

By default, the new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled; the Linux-specific O_CLOEXEC flag, described below, can be used to change this default). The file offset is set to the beginning of the file (see lseek(2)).

A call to open() creates a new open file description, an entry in the system-wide table of open files. This entry records the file offset and the file status flags (modifiable via the fcntl(2) F_SETFL operation). A file descriptor is a reference to one of these entries; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file. The new open file description is initially not shared with any other process, but sharing may arise via fork(2).

The argument flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read-only, write-only, or read/write, respectively.

In addition, zero or more file creation flags and file status flags can be bitwise-or'd in flags. The file creation flags are O_CREAT, O_EXCL, O_NOCTTY, and O_TRUNC. The file status flags are all of the remaining flags listed below. The distinction between these two groups of flags is that the file status flags can be retrieved and (in some cases) modified using fcntl(2). The full list of file creation flags and file status flags is as follows:

O_APPEND

The file is opened in append mode. Before each write(2), the file offset is positioned at the end of the file, as if with lseek(2). O_APPEND may lead to corrupted files on NFS file systems if more than one process appends data to a file at once. This is because NFS does not support appending to a file, so the client kernel has to simulate it, which can't be done without a race condition.

O_ASYNC

Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via fcntl(2)) when input or output becomes possible on this file descriptor. This feature is only available for terminals, pseudo-terminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl(2) for further details.

O_CLOEXEC (Since Linux 2.6.23)

Enable the close-on-exec flag for the new file descriptor. Specifying this flag permits a program to avoid additional fcntl(2) F_SETFD operations to set the FD_CLOEXEC flag. Additionally, use of this flag is essential in some multithreaded programs since using a separate fcntl(2) F_SETFD operation to set the FD_CLOEXEC flag does not suffice to avoid race conditions where one thread opens a file descriptor at the same time as another thread does a fork(2) plus execve(2).

O_CREAT

If the file does not exist it will be created. The owner (user ID) of the file is set to the effective user ID of the process. The group ownership (group ID) is set either to the effective group ID of the process or to the group ID of the parent directory (depending on file system type and mount options, and the mode of the parent directory, see the mount options bsdgroups and sysvgroups described in mount(8)).

mode specifies the permissions to use in case a new file is created. This argument must be supplied when O_CREAT is specified in flags; if O_CREAT is not specified, then mode is ignored. The effective permissions are modified by the process's umask in the usual way: The permissions of the created file are (mode & ~umask). Note that this mode only applies to future accesses of the newly created file; the open() call that creates a read-only file may well return a read/write file descriptor.

The following symbolic constants are provided for mode:

S_IRWXU: 00700 user (file owner) has read, write and execute permission
S_IRUSR: 00400 user has read permission
S_IWUSR: 00200 user has write permission
S_IXUSR: 00100 user has execute permission
S_IRWXG: 00070 group has read, write and execute permission
S_IRGRP: 00040 group has read permission
S_IWGRP: 00020 group has write permission
S_IXGRP: 00010 group has execute permission
S_IRWXO: 00007 others have read, write and execute permission
S_IROTH: 00004 others have read permission
S_IWOTH: 00002 others have write permission
S_IXOTH: 00001 others have execute permission

O_DIRECT (Since Linux 2.4.10)

Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching. File I/O is done directly to/from user space buffers. The I/O is synchronous, that is, at the completion of a read(2) or write(2), data is guaranteed to have been transferred. See NOTES below for further discussion.

A semantically similar (but deprecated) interface for block devices is described in raw(8).

O_DIRECTORY

If pathname is not a directory, cause the open to fail. This flag is Linux-specific, and was added in kernel version 2.1.126, to avoid denial-of-service problems if opendir(3) is called on a FIFO or tape device, but should not be used outside of the implementation of opendir(3).

O_EXCL

Ensure that this call creates the file: if this flag is specified in conjunction with O_CREAT, and pathname already exists, then open() will fail. The behavior of O_EXCL is undefined if O_CREAT is not specified.

When these two flags are specified, symbolic links are not followed: if pathname is a symbolic link, then open() fails regardless of where the symbolic link points to.

O_EXCL is only supported on NFS when using NFSv3 or later on kernel 2.6 or later. In environments where NFS O_EXCL support is not provided, programs that rely on it for performing locking tasks will contain a race condition. Portable programs that want to perform atomic file locking using a lockfile, and need to avoid reliance on NFS support for O_EXCL, can create a unique file on the same file system (e.g., incorporating hostname and PID), and use link(2) to make a link to the lockfile. If link(2) returns 0, the lock is successful. Otherwise, use stat(2) on the unique file to check if its link count has increased to 2, in which case the lock is also successful.

O_LARGEFILE

(LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to be opened. The _LARGEFILE64_SOURCE macro must be defined in order to obtain this definition. Setting the _FILE_OFFSET_BITS feature test macro to 64 (rather than using O_LARGEFILE) is the preferred method of obtaining method of accessing large files on 32-bit systems (see feature_test_macros(7)).

O_NOATIME (Since Linux 2.6.8)

Do not update the file last access time (st_atime in the inode) when the file is read(2). This flag is intended for use by indexing or backup programs, where its use can significantly reduce the amount of disk activity. This flag may not be effective on all file systems. One example is NFS, where the server maintains the access time.

O_NOCTTY

If pathname refers to a terminal device --- see tty(4) --- it will not become the process's controlling terminal even if the process does not have one.

O_NOFOLLOW

If pathname is a symbolic link, then the open fails. This is a FreeBSD extension, which was added to Linux in version 2.1.126. Symbolic links in earlier components of the pathname will still be followed.

O_NONBLOCK or O_NDELAY

When possible, the file is opened in non-blocking mode. Neither the open() nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait. For the handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NONBLOCK in conjunction with mandatory file locks and with file leases, see fcntl(2).

O_SYNC

The file is opened for synchronous I/O. Any write(2)s on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. But see NOTES below.

O_TRUNC

If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise the effect of O_TRUNC is unspecified.

Some of these optional flags can be altered using fcntl(2) after the file has been opened.

creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.

RETURN VALUE

open() and creat() return the new file descriptor, or -1 if an error occurred (in which case, errno is set appropriately).

ERRORS

EACCES: The requested access to the file is not allowed, or search permission is denied for one of the directories in the path prefix of pathname, or the file did not exist yet and write access to the parent directory is not allowed. (See also path_resolution(7).)
EEXIST: pathname already exists and O_CREAT and O_EXCL were used.
EFAULT: pathname points outside your accessible address space.
EFBIG: See EOVERFLOW.
EINTR: While blocked waiting to complete an open of a slow device (e.g., a FIFO; see fifo(7)), the call was interrupted by a signal handler; see signal(7).
EISDIR: pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or O_RDWR is set).
ELOOP: Too many symbolic links were encountered in resolving pathname, or O_NOFOLLOW was specified but pathname was a symbolic link.
EMFILE: The process already has the maximum number of files open.
ENAMETOOLONG: pathname was too long.
ENFILE: The system limit on the total number of open files has been reached.
ENODEV: pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel bug; in this situation ENXIO must be returned.)
ENOENT: O_CREAT is not set and the named file does not exist. Or, a directory component in pathname does not exist or is a dangling symbolic link.
ENOMEM: Insufficient kernel memory was available.
ENOSPC: pathname was to be created but the device containing pathname has no room for the new file.
ENOTDIR: A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was specified and pathname was not a directory.
ENXIO: O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the file open for reading. Or, the file is a device special file and no corresponding device exists.
EOVERFLOW: pathname refers to a regular file that is too large to be opened. The usual scenario here is that an application compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds (2<<31)-1 bits; see also O_LARGEFILE above. This is the error specified by POSIX.1-2001; in kernels before 2.6.24, Linux gave the error EFBIG for this case.
EPERM: The O_NOATIME flag was specified, but the effective user ID of the caller did not match the owner of the file and the caller was not privileged (CAP_FOWNER).
EROFS: pathname refers to a file on a read-only file system and write access was requested.
ETXTBSY: pathname refers to an executable image which is currently being executed and write access was requested.
EWOULDBLOCK: The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see fcntl(2)).

CONFORMING TO

SVr4, 4.3BSD, POSIX.1-2001. The O_DIRECTORY, O_NOATIME, and O_NOFOLLOW flags are Linux-specific, and one may need to define _GNU_SOURCE to obtain their definitions.

The O_CLOEXEC flag is not specified in POSIX.1-2001, but is specified in POSIX.1-2008.

O_DIRECT is not specified in POSIX; one has to define _GNU_SOURCE to get its definition.

NOTES

Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily have the intention to read or write. This is typically used to open devices in order to get a file descriptor for use with ioctl(2).

Unlike the other values that can be specified in flags, the access mode values O_RDONLY, O_WRONLY, and O_RDWR, do not specify individual bits. Rather, they define the low order two bits of flags, and are defined respectively as 0, 1, and 2. In other words, the combination O_RDONLY | O_WRONLY is a logical error, and certainly does not have the same meaning as O_RDWR. Linux reserves the special, non-standard access mode 3 (binary 11) in flags to mean: check for read and write permission on the file and return a descriptor that can't be used for reading or writing. This non-standard access mode is used by some Linux drivers to return a descriptor that is only to be used for device-specific ioctl(2) operations.

The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations. On many systems the file is actually truncated.

There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC and O_NDELAY.

POSIX provides for three different variants of synchronized I/O, corresponding to the flags O_SYNC, O_DSYNC and O_RSYNC. Currently (2.1.130) these are all synonymous under Linux.

Note that open() can open device special files, but creat() cannot create them; use mknod(2) instead.

On NFS file systems with UID mapping enabled, open() may return a file descriptor but, for example, read(2) requests are denied with EACCES. This is because the client performs open() by checking the permissions, but UID mapping is performed by the server upon read and write requests.

If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of last status change, and time of last modification; see stat(2)) are set to the current time, and so are the st_ctime and st_mtime fields of the parent directory. Otherwise, if the file is modified because of the O_TRUNC flag, its st_ctime and st_mtime fields are set to the current time.

O_DIRECT

The O_DIRECT flag may impose alignment restrictions on the length and address of userspace buffers and the file offset of I/Os. In Linux alignment restrictions vary by file system and kernel version and might be absent entirely. However there is currently no file system-independent interface for an application to discover these restrictions for a given file or file system. Some file systems provide their own interfaces for doing so, for example the XFS_IOC_DIOINFO operation in xfsctl(3).

Under Linux 2.4, transfer sizes, and the alignment of the user buffer and the file offset must all be multiples of the logical block size of the file system. Under Linux 2.6, alignment to 512-byte boundaries suffices.

The O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar to those of Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and sizes. FreeBSD 4.x introduced a flag of the same name, but without alignment restrictions.

O_DIRECT support was added under Linux in kernel version 2.4.10. Older Linux kernels simply ignore this flag. Some file systems may not implement the flag and open() will fail with EINVAL if it is used.

Applications should avoid mixing O_DIRECT and normal I/O to the same file, and especially to overlapping byte regions in the same file. Even when the file system correctly handles the coherency issues in this situation, overall I/O throughput is likely to be slower than using either mode alone. Likewise, applications should avoid mixing mmap(2) of files with direct I/O to the same files.

The behaviour of O_DIRECT with NFS will differ from local file systems. Older kernels, or kernels configured in certain ways, may not support this combination. The NFS protocol does not support passing the flag to the server, so O_DIRECT I/O will only bypass the page cache on the client; the server may still cache the I/O. The client asks the server to make the I/O synchronous to preserve the synchronous semantics of O_DIRECT. Some servers will perform poorly under these circumstances, especially if the I/O size is small. Some servers may also be configured to lie to clients about the I/O having reached stable storage; this will avoid the performance penalty at some risk to data integrity in the event of server power failure. The Linux NFS client places no alignment restrictions on O_DIRECT I/O.

In summary, O_DIRECT is a potentially powerful tool that should be used with caution. It is recommended that applications treat use of O_DIRECT as a performance option which is disabled by default.

: "The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and was probably designed by a deranged monkey on some serious mind-controlling substances." --- Linus

BUGS

Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling open(); use fcntl(2) to enable this flag.

COLOPHON

This page is part of release 3.18 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Index

O_DIRECT

BUGS

SEE ALSO

COLOPHON

This document was created by man2html using the manual pages.
Time: 11:39:12 GMT, February 18, 2010

3. open.3.man

Manpage of open

open

Section: Perl Programmers Reference Guide (3)
Updated: 2001-09-21
Index Return to Main Contents

NAME

open - perl pragma to set default PerlIO layers for input and output

SYNOPSIS

    use open IN  => ":crlf", OUT => ":bytes";
    use open OUT => ':utf8';
    use open IO  => ":encoding(iso-8859-7)";

    use open IO  => ':locale';

    use open ':utf8';
    use open ':locale';
    use open ':encoding(iso-8859-7)';

    use open ':std';

DESCRIPTION

Full-fledged support for I/O layers is now implemented provided Perl is configured to use PerlIO as its IO system (which is now the default).

The "open" pragma serves as one of the interfaces to declare default ``layers'' (also known as ``disciplines'') for all I/O. Any two-argument open(), readpipe() (aka qx//) and similar operators found within the lexical scope of this pragma will use the declared defaults. Three-argument opens are not affected by this pragma since there you (can) explicitly specify the layers and are supposed to know what you are doing.

With the "IN" subpragma you can declare the default layers of input streams, and with the "OUT" subpragma you can declare the default layers of output streams. With the "IO" subpragma you can control both input and output streams simultaneously.

If you have a legacy encoding, you can use the ":encoding(...)" tag.

if you want to set your encoding layers based on your locale environment variables, you can use the ":locale" tag. For example:

    $ENV{LANG} = 'ru_RU.KOI8-R';
    # the :locale will probe the locale environment variables like LANG
    use open OUT => ':locale';
    open(O, ">koi8");
    print O chr(0x430); # Unicode CYRILLIC SMALL LETTER A = KOI8-R 0xc1
    close O;
    open(I, "<koi8");
    printf "%#x
", ord(<I>), "
"; # this should print 0xc1
    close I;

These are equivalent

    use open ':utf8';
    use open IO => ':utf8';

as are these

    use open ':locale';
    use open IO => ':locale';

and these

    use open ':encoding(iso-8859-7)';
    use open IO => ':encoding(iso-8859-7)';

The matching of encoding names is loose: case does not matter, and many encodings have several aliases. See Encode::Supported for details and the list of supported locales.

Note that ":utf8" PerlIO layer must always be specified exactly like that, it is not subject to the loose matching of encoding names.

When open() is given an explicit list of layers they are appended to the list declared using this pragma.

The ":std" subpragma on its own has no effect, but if combined with the ":utf8" or ":encoding" subpragmas, it converts the standard filehandles (STDIN, STDOUT, STDERR) to comply with encoding selected for input/output handles. For example, if both input and out are chosen to be ":utf8", a ":std" will mean that STDIN, STDOUT, and STDERR are also in ":utf8". On the other hand, if only output is chosen to be in ":encoding(koi8r)", a ":std" will cause only the STDOUT and STDERR to be in "koi8r". The ":locale" subpragma implicitly turns on ":std".

The logic of ":locale" is as follows:

1.: If the platform supports the langinfo(CODESET) interface, the codeset returned is used as the default encoding for the open pragma.
2.: If 1. didn't work but we are under the locale pragma, the environment variables LC_ALL and LANG (in that order) are matched for encodings (the part after ".", if any), and if any found, that is used as the default encoding for the open pragma.
3.: If 1. and 2. didn't work, the environment variables LC_ALL and LANG (in that order) are matched for anything looking like UTF-8, and if any found, ":utf8" is used as the default encoding for the open pragma.

If your locale environment variables (LC_ALL, LC_CTYPE, LANG) contain the strings 'UTF-8' or 'UTF8' (case-insensitive matching), the default encoding of your STDIN, STDOUT, and STDERR, and of any subsequent file open, is UTF-8.

Directory handles may also support PerlIO layers in the future.

NONPERLIO FUNCTIONALITY

If Perl is not built to use PerlIO as its IO system then only the two pseudo-layers ":bytes" and ":crlf" are available.

The ":bytes" layer corresponds to ``binary mode'' and the ":crlf" layer corresponds to ``text mode'' on platforms that distinguish between the two modes when opening files (which is many DOS-like platforms, including Windows). These two layers are no-ops on platforms where binmode() is a no-op, but perform their functions everywhere if PerlIO is enabled.

IMPLEMENTATION DETAILS

There is a class method in "PerlIO::Layer" "find" which is implemented as XS code. It is called by "import" to validate the layers:

   PerlIO::Layer::->find("perlio")

The return value (if defined) is a Perl object, of class "PerlIO::Layer" which is created by the C code in perlio.c. As yet there is nothing useful you can do with the object at the perl level.

This document was created by man2html using the manual pages.
Time: 11:39:12 GMT, February 18, 2010

4. open.9.man

Manpage of open

open

Section: Tcl Built-In Commands (n)
Updated: 8.3
Index Return to Main Contents

NAME

open - Open a file-based or command pipeline channel

SYNOPSIS

open fileName
open fileName access
open fileName access permissions

DESCRIPTION

This command opens a file, serial port, or command pipeline and returns a channel identifier that may be used in future invocations of commands like read, puts, and close. If the first character of fileName is not | then the command opens a file: fileName gives the name of the file to open, and it must conform to the conventions described in the filename manual entry.

The access argument, if present, indicates the way in which the file (or command pipeline) is to be accessed. In the first form access may have any of the following values:

r: Open the file for reading only; the file must already exist. This is the default value if access is not specified.
r+: Open the file for both reading and writing; the file must already exist.
w: Open the file for writing only. Truncate it if it exists. If it doesn't exist, create a new file.
w+: Open the file for reading and writing. Truncate it if it exists. If it doesn't exist, create a new file.
a: Open the file for writing only. If the file doesn't exist, create a new empty file. Set the file pointer to the end of the file prior to each write.
a+: Open the file for reading and writing. If the file doesn't exist, create a new empty file. Set the initial access position to the end of the file.

In the second form, access consists of a list of any of the following flags, all of which have the standard POSIX meanings. One of the flags must be either RDONLY, WRONLY or RDWR.

RDONLY: Open the file for reading only.
WRONLY: Open the file for writing only.
RDWR: Open the file for both reading and writing.
APPEND: Set the file pointer to the end of the file prior to each write.
CREAT: Create the file if it doesn't already exist (without this flag it is an error for the file not to exist).
EXCL: If CREAT is also specified, an error is returned if the file already exists.
NOCTTY: If the file is a terminal device, this flag prevents the file from becoming the controlling terminal of the process.
NONBLOCK: Prevents the process from blocking while opening the file, and possibly in subsequent I/O operations. The exact behavior of this flag is system- and device-dependent; its use is discouraged (it is better to use the fconfigure command to put a file in nonblocking mode). For details refer to your system documentation on the open system call's O_NONBLOCK flag.
TRUNC: If the file exists it is truncated to zero length.

If a new file is created as part of opening it, permissions (an integer) is used to set the permissions for the new file in conjunction with the process's file mode creation mask. Permissions defaults to 0666.

Note that if you are going to be reading or writing binary data from the channel created by this command, you should use the fconfigure command to change the -translation option of the channel to binary before transferring any binary data. This is in contrast to the ``b'' character passed as part of the equivalent of the access parameter to some versions of the C library fopen() function.

COMMAND PIPELINES

If the first character of fileName is ``|'' then the remaining characters of fileName are treated as a list of arguments that describe a command pipeline to invoke, in the same style as the arguments for exec. In this case, the channel identifier returned by open may be used to write to the command's input pipe or read from its output pipe, depending on the value of access. If write-only access is used (e.g. access is w), then standard output for the pipeline is directed to the current standard output unless overridden by the command. If read-only access is used (e.g. access is r), standard input for the pipeline is taken from the current standard input unless overridden by the command. The id of the spawned process is accessible through the pid command, using the channel id returned by open as argument.

If the command (or one of the commands) executed in the command pipeline returns an error (according to the definition in exec), a Tcl error is generated when close is called on the channel unless the pipeline is in non-blocking mode then no exit status is returned (a silent close with -blocking 0).

It is often useful to use the fileevent command with pipelines so other processing may happen at the same time as running the command in the background.

SERIAL COMMUNICATIONS

If fileName refers to a serial port, then the specified serial port is opened and initialized in a platform-dependent manner. Acceptable values for the fileName to use to open a serial port are described in the PORTABILITY ISSUES section.

The fconfigure command can be used to query and set additional configuration options specific to serial ports (where supported):

-mode baud,parity,data,stop: This option is a set of 4 comma-separated values: the baud rate, parity, number of data bits, and number of stop bits for this serial port. The baud rate is a simple integer that specifies the connection speed. Parity is one of the following letters: n, o, e, m, s; respectively signifying the parity options of ``none'', ``odd'', ``even'', ``mark'', or ``space''. Data is the number of data bits and should be an integer from 5 to 8, while stop is the number of stop bits and should be the integer 1 or 2.
-handshake type: (Windows and Unix). This option is used to setup automatic handshake control. Note that not all handshake types maybe supported by your operating system. The type parameter is case-independent.
If type is none then any handshake is switched off. rtscts activates hardware handshake. Hardware handshake signals are described below. For software handshake xonxoff the handshake characters can be redefined with -xchar. An additional hardware handshake dtrdsr is available only under Windows. There is no default handshake configuration, the initial value depends on your operating system settings. The -handshake option cannot be queried.
-queue: (Windows and Unix). The -queue option can only be queried. It returns a list of two integers representing the current number of bytes in the input and output queue respectively.
-timeout msec: (Windows and Unix). This option is used to set the timeout for blocking read operations. It specifies the maximum interval between the reception of two bytes in milliseconds. For Unix systems the granularity is 100 milliseconds. The -timeout option does not affect write operations or nonblocking reads. This option cannot be queried.
-ttycontrol {signal boolean signal boolean ...}: (Windows and Unix). This option is used to setup the handshake output lines (see below) permanently or to send a BREAK over the serial line. The signal names are case-independent. {RTS 1 DTR 0} sets the RTS output to high and the DTR output to low. The BREAK condition (see below) is enabled and disabled with {BREAK 1} and {BREAK 0} respectively. It's not a good idea to change the RTS (or DTR) signal with active hardware handshake rtscts (or dtrdsr). The result is unpredictable. The -ttycontrol option cannot be queried.
-ttystatus: (Windows and Unix). The -ttystatus option can only be queried. It returns the current modem status and handshake input signals (see below). The result is a list of signal,value pairs with a fixed order, e.g. {CTS 1 DSR 0 RING 1 DCD 0}. The signal names are returned upper case.
-xchar {xonChar xoffChar}: (Windows and Unix). This option is used to query or change the software handshake characters. Normally the operating system default should be DC1 (0x11) and DC3 (0x13) representing the ASCII standard XON and XOFF characters.
-pollinterval msec: (Windows only). This option is used to set the maximum time between polling for fileevents. This affects the time interval between checking for events throughout the Tcl interpreter (the smallest value always wins). Use this option only if you want to poll the serial port more or less often than 10 msec (the default).
-sysbuffer inSize
-sysbuffer {inSize outSize}: (Windows only). This option is used to change the size of Windows system buffers for a serial channel. Especially at higher communication rates the default input buffer size of 4096 bytes can overrun for latent systems. The first form specifies the input buffer size, in the second form both input and output buffers are defined.
-lasterror: (Windows only). This option is query only. In case of a serial communication error, read or puts returns a general Tcl file I/O error. fconfigure -lasterror can be called to get a list of error details. See below for an explanation of the various error codes.

SERIAL PORT SIGNALS

RS-232 is the most commonly used standard electrical interface for serial communications. A negative voltage (-3V..-12V) define a mark (on=1) bit and a positive voltage (+3..+12V) define a space (off=0) bit (RS-232C). The following signals are specified for incoming and outgoing data, status lines and handshaking. Here we are using the terms workstation for your computer and modem for the external device, because some signal names (DCD, RI) come from modems. Of course your external device may use these signal lines for other purposes.

TXD(output): Transmitted Data: Outgoing serial data.
RXD(input): Received Data:Incoming serial data.
RTS(output): Request To Send: This hardware handshake line informs the modem that your workstation is ready to receive data. Your workstation may automatically reset this signal to indicate that the input buffer is full.
CTS(input): Clear To Send: The complement to RTS. Indicates that the modem is ready to receive data.
DTR(output): Data Terminal Ready: This signal tells the modem that the workstation is ready to establish a link. DTR is often enabled automatically whenever a serial port is opened.
DSR(input): Data Set Ready: The complement to DTR. Tells the workstation that the modem is ready to establish a link.
DCD(input): Data Carrier Detect: This line becomes active when a modem detects a "Carrier" signal.
RI(input): Ring Indicator: Goes active when the modem detects an incoming call.
BREAK: A BREAK condition is not a hardware signal line, but a logical zero on the TXD or RXD lines for a long period of time, usually 250 to 500 milliseconds. Normally a receive or transmit data signal stays at the mark (on=1) voltage until the next character is transferred. A BREAK is sometimes used to reset the communications line or change the operating mode of communications hardware.

ERROR CODES (Windows only)

A lot of different errors may occur during serial read operations or during event polling in background. The external device may have been switched off, the data lines may be noisy, system buffers may overrun or your mode settings may be wrong. That's why a reliable software should always catch serial read operations. In cases of an error Tcl returns a general file I/O error. Then fconfigure -lasterror may help to locate the problem. The following error codes may be returned.

RXOVER: Windows input buffer overrun. The data comes faster than your scripts reads it or your system is overloaded. Use fconfigure -sysbuffer to avoid a temporary bottleneck and/or make your script faster.
TXFULL: Windows output buffer overrun. Complement to RXOVER. This error should practically not happen, because Tcl cares about the output buffer status.
OVERRUN: UART buffer overrun (hardware) with data lost. The data comes faster than the system driver receives it. Check your advanced serial port settings to enable the FIFO (16550) buffer and/or setup a lower(1) interrupt threshold value.
RXPARITY: A parity error has been detected by your UART. Wrong parity settings with fconfigure -mode or a noisy data line (RXD) may cause this error.
FRAME: A stop-bit error has been detected by your UART. Wrong mode settings with fconfigure -mode or a noisy data line (RXD) may cause this error.
BREAK: A BREAK condition has been detected by your UART (see above).

PORTABILITY ISSUES

Windows (all versions)

Valid values for fileName to open a serial port are of the form comX:, where X is a number, generally from 1 to 4. This notation only works for serial ports from 1 to 9, if the system happens to have more than four. An attempt to open a serial port that does not exist or has a number greater than 9 will fail. An alternate form of opening serial ports is to use the filename \\.\comX, where X is any number that corresponds to a serial port; please note that this method is considerably slower on Windows 95 and Windows 98.

Windows NT

When running Tcl interactively, there may be some strange interactions between the real console, if one is present, and a command pipeline that uses standard input or output. If a command pipeline is opened for reading, some of the lines entered at the console will be sent to the command pipeline and some will be sent to the Tcl evaluator. If a command pipeline is opened for writing, keystrokes entered into the console are not visible until the pipe is closed. This behavior occurs whether the command pipeline is executing 16-bit or 32-bit applications. These problems only occur because both Tcl and the child application are competing for the console at the same time. If the command pipeline is started from a script, so that Tcl is not accessing the console, or if the command pipeline does not use standard input or output, but is redirected from or to a file, then the above problems do not occur.

Windows 95

A command pipeline that executes a 16-bit DOS application cannot be opened for both reading and writing, since 16-bit DOS applications that receive standard input from a pipe and send standard output to a pipe run synchronously. Command pipelines that do not execute 16-bit DOS applications run asynchronously and can be opened for both reading and writing.

When running Tcl interactively, there may be some strange interactions between the real console, if one is present, and a command pipeline that uses standard input or output. If a command pipeline is opened for reading from a 32-bit application, some of the keystrokes entered at the console will be sent to the command pipeline and some will be sent to the Tcl evaluator. If a command pipeline is opened for writing to a 32-bit application, no output is visible on the console until the pipe is closed. These problems only occur because both Tcl and the child application are competing for the console at the same time. If the command pipeline is started from a script, so that Tcl is not accessing the console, or if the command pipeline does not use standard input or output, but is redirected from or to a file, then the above problems do not occur.

Whether or not Tcl is running interactively, if a command pipeline is opened for reading from a 16-bit DOS application, the call to open will not return until end-of-file has been received from the command pipeline's standard output. If a command pipeline is opened for writing to a 16-bit DOS application, no data will be sent to the command pipeline's standard output until the pipe is actually closed. This problem occurs because 16-bit DOS applications are run synchronously, as described above.

Macintosh

Opening a serial port is not currently implemented under Macintosh.

Opening a command pipeline is not supported under Macintosh, since applications do not support the concept of standard input or output.

Unix

Valid values for fileName to open a serial port are generally of the form /dev/ttyX, where X is a or b, but the name of any pseudo-file that maps to a serial port may be used. Advanced configuration options are only supported for serial ports when Tcl is built to use the POSIX serial interface.

When running Tcl interactively, there may be some strange interactions between the console, if one is present, and a command pipeline that uses standard input. If a command pipeline is opened for reading, some of the lines entered at the console will be sent to the command pipeline and some will be sent to the Tcl evaluator. This problem only occurs because both Tcl and the child application are competing for the console at the same time. If the command pipeline is started from a script, so that Tcl is not accessing the console, or if the command pipeline does not use standard input, but is redirected from a file, then the above problem does not occur.

See the PORTABILITY ISSUES section of the exec command for additional information not specific to command pipelines about executing applications on the various platforms

EXAMPLE

Open a command pipeline and catch any errors:


set fl [open "| ls this_file_does_not_exist"]
set data [read $fl]
if {[catch {close $fl} err]} {
    puts "ls command failed: $err"
}

KEYWORDS

access mode, append, create, file, non-blocking, open, permissions, pipeline, process, serial

Index

NAME
SYNOPSIS
DESCRIPTION
COMMAND PIPELINES
SERIAL COMMUNICATIONS
SERIAL PORT SIGNALS
ERROR CODES (Windows only)
PORTABILITY ISSUES
EXAMPLE
SEE ALSO
KEYWORDS

This document was created by man2html using the manual pages.
Time: 11:39:12 GMT, February 18, 2010

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COMMAND PIPELINES

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PORTABILITY ISSUES

EXAMPLE

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KEYWORDS

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