llio.texi

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Here is an example showing how to use @code{dup2} to do redirection.
Typically, redirection of the standard streams (like @code{stdin}) is
done by a shell or shell-like program before calling one of the
@code{exec} functions (@pxref{Executing a File}) to execute a new
program in a child process.  When the new program is executed, it
creates and initializes the standard streams to point to the
corresponding file descriptors, before its @code{main} function is
invoked.

So, to redirect standard input to a file, the shell could do something
like:

@smallexample
pid = fork ();
if (pid == 0)
  @{
    char *filename;
    char *program;
    int file;
    @dots{}
    file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY));
    dup2 (file, STDIN_FILENO);
    TEMP_FAILURE_RETRY (close (file));
    execv (program, NULL);
  @}
@end smallexample

There is also a more detailed example showing how to implement redirection
in the context of a pipeline of processes in @ref{Launching Jobs}.


@node Descriptor Flags
@section File Descriptor Flags
@cindex file descriptor flags

@dfn{File descriptor flags} are miscellaneous attributes of a file
descriptor.  These flags are associated with particular file
descriptors, so that if you have created duplicate file descriptors
from a single opening of a file, each descriptor has its own set of flags.

Currently there is just one file descriptor flag: @code{FD_CLOEXEC},
which causes the descriptor to be closed if you use any of the
@code{exec@dots{}} functions (@pxref{Executing a File}).

The symbols in this section are defined in the header file
@file{fcntl.h}.
@pindex fcntl.h

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int F_GETFD
This macro is used as the @var{command} argument to @code{fcntl}, to
specify that it should return the file descriptor flags associated
with the @var{filedes} argument.  

The normal return value from @code{fcntl} with this command is a
nonnegative number which can be interpreted as the bitwise OR of the
individual flags (except that currently there is only one flag to use).

In case of an error, @code{fcntl} returns @code{-1}.  The following
@code{errno} error conditions are defined for this command:

@table @code
@item EBADF
The @var{filedes} argument is invalid.
@end table
@end deftypevr


@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int F_SETFD
This macro is used as the @var{command} argument to @code{fcntl}, to
specify that it should set the file descriptor flags associated with the
@var{filedes} argument.  This requires a third @code{int} argument to
specify the new flags, so the form of the call is:

@smallexample
fcntl (@var{filedes}, F_SETFD, @var{new-flags})
@end smallexample

The normal return value from @code{fcntl} with this command is an
unspecified value other than @code{-1}, which indicates an error.
The flags and error conditions are the same as for the @code{F_GETFD}
command.
@end deftypevr

The following macro is defined for use as a file descriptor flag with
the @code{fcntl} function.  The value is an integer constant usable
as a bit mask value.

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int FD_CLOEXEC
@cindex close-on-exec (file descriptor flag)
This flag specifies that the file descriptor should be closed when
an @code{exec} function is invoked; see @ref{Executing a File}.  When
a file descriptor is allocated (as with @code{open} or @code{dup}),
this bit is initially cleared on the new file descriptor, meaning that
descriptor will survive into the new program after @code{exec}.
@end deftypevr

If you want to modify the file descriptor flags, you should get the
current flags with @code{F_GETFD} and modify the value.  Don't assume
that the flags listed here are the only ones that are implemented; your
program may be run years from now and more flags may exist then.  For
example, here is a function to set or clear the flag @code{FD_CLOEXEC}
without altering any other flags:

@smallexample
/* @r{Set the @code{FD_CLOEXEC} flag of @var{desc} if @var{value} is nonzero,}
   @r{or clear the flag if @var{value} is 0.}
   @r{Return 0 on success, or -1 on error with @code{errno} set.} */ 

int
set_cloexec_flag (int desc, int value)
@{
  int oldflags = fcntl (desc, F_GETFD, 0);
  /* @r{If reading the flags failed, return error indication now.}
  if (oldflags < 0)
    return oldflags;
  /* @r{Set just the flag we want to set.} */
  if (value != 0)
    oldflags |= FD_CLOEXEC;
  else
    oldflags &= ~FD_CLOEXEC;
  /* @r{Store modified flag word in the descriptor.} */
  return fcntl (desc, F_SETFD, oldflags);
@}
@end smallexample

@node File Status Flags
@section File Status Flags
@cindex file status flags

@dfn{File status flags} are used to specify attributes of the opening of a
file.  Unlike the file descriptor flags discussed in @ref{Descriptor
Flags}, the file status flags are shared by duplicated file descriptors
resulting from a single opening of the file.  The file status flags are
specified with the @var{flags} argument to @code{open};
@pxref{Opening and Closing Files}.

File status flags fall into three categories, which are described in the
following sections.

@itemize @bullet
@item
@ref{Access Modes}, specify what type of access is allowed to the
file: reading, writing, or both.  They are set by @code{open} and are
returned by @code{fcntl}, but cannot be changed.

@item
@ref{Open-time Flags}, control details of what @code{open} will do.
These flags are not preserved after the @code{open} call.

@item
@ref{Operating Modes}, affect how operations such as @code{read} and
@code{write} are done.  They are set by @code{open}, and can be fetched or
changed with @code{fcntl}.
@end itemize

The symbols in this section are defined in the header file
@file{fcntl.h}.
@pindex fcntl.h

@menu
* Access Modes::                Whether the descriptor can read or write.
* Open-time Flags::             Details of @code{open}.
* Operating Modes::             Special modes to control I/O operations.
* Getting File Status Flags::   Fetching and changing these flags.
@end menu

@node Access Modes
@subsection File Access Modes

The file access modes allow a file descriptor to be used for reading,
writing, or both.  (In the GNU system, they can also allow none of these,
and allow execution of the file as a program.)  The access modes are chosen
when the file is opened, and never change.

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_RDONLY
Open the file for read access.
@end deftypevr

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_WRONLY
Open the file for write access.
@end deftypevr

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_RDWR
Open the file for both reading and writing.
@end deftypevr

In the GNU system (and not in other systems), @code{O_RDONLY} and
@code{O_WRONLY} are independent bits that can be bitwise-ORed together,
and it is valid for either bit to be set or clear.  This means that
@code{O_RDWR} is the same as @code{O_RDONLY|O_WRONLY}.  A file access
mode of zero is permissible; it allows no operations that do input or
output to the file, but does allow other operations such as
@code{fchmod}.  On the GNU system, since ``read-only'' or ``write-only''
is a misnomer, @file{fcntl.h} defines additional names for the file
access modes.  These names are preferred when writing GNU-specific code.
But most programs will want to be portable to other POSIX.1 systems and
should use the POSIX.1 names above instead.

@comment fcntl.h
@comment GNU
@deftypevr Macro int O_READ
Open the file for reading.  Same as @code{O_RDWR}; only defined on GNU.
@end deftypevr

@comment fcntl.h
@comment GNU
@deftypevr Macro int O_WRITE
Open the file for reading.  Same as @code{O_WRONLY}; only defined on GNU.
@end deftypevr

@comment fcntl.h
@comment GNU
@deftypevr Macro int O_EXEC
Open the file for executing.  Only defined on GNU.
@end deftypevr

To determine the file access mode with @code{fcntl}, you must extract
the access mode bits from the retrieved file status flags.  In the GNU
system, you can just test the @code{O_READ} and @code{O_WRITE} bits in
the flags word.  But in other POSIX.1 systems, reading and writing
access modes are not stored as distinct bit flags.  The portable way to
extract the file access mode bits is with @code{O_ACCMODE}.

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_ACCMODE
This macro stands for a mask that can be bitwise-ANDed with the file
status flag value to produce a value representing the file access mode.
The mode will be @code{O_RDONLY}, @code{O_WRONLY}, or @code{O_RDWR}.
(In the GNU system it could also be zero.)
@end deftypevr

@node Open-time Flags
@subsection Open-time Flags

The open-time flags specify options affecting how @code{open} will behave.
These options are not preserved once the file is open.  The exception to
this is @code{O_NONBLOCK}, which is also an I/O operating mode and so it
@emph{is} saved.  @xref{Opening and Closing Files}, for how to call
@code{open}.

There are two sorts of options specified by open-time flags.

@itemize @bullet
@item
@dfn{File name translation flags} affect how @code{open} looks up the
file name to locate the file, and whether the file can be created.
@cindex file name translation flags
@cindex flags, file name translation

@item
@dfn{Open-time action flags} specify extra operations that @code{open} will
perform on the file once it is open.
@cindex open-time action flags
@cindex flags, open-time action
@end itemize

Here are the file name translation flags.

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_CREAT
If set, the file will be created if it doesn't already exist.
@cindex create on open (file status flag)
@end deftypevr

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_EXCL
If both @code{O_CREAT} and @code{O_EXCL} are set, then @code{open} fails
if the specified file already exists.  This is guaranteed to never
clobber an existing file.
@end deftypevr

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_NONBLOCK
@cindex non-blocking open
This prevents @code{open} from blocking for a ``long time'' to open the
file.  This is only meaningful for some kinds of files, usually devices
such as serial ports.  Often opening a port to a modem blocks until the
modem reports carrier detection; if @code{O_NONBLOCK} is specified,
@code{open} will return immediately without a carrier.

Note that the @code{O_NONBLOCK} flag is overloaded as both an I/O operating
mode and a file name translation flag.  This means that specifying
@code{O_NONBLOCK} in @code{open} also sets nonblocking I/O mode;
@pxref{Operating Modes}.  To open the file without blocking but do normal
I/O that blocks, you must call @code{open} with @code{O_NONBLOCK} set and
then call @code{fcntl} to turn the bit off.
@end deftypevr

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_NOCTTY
If the named file is a terminal device, don't make it the controlling
terminal for the process.  @xref{Job Control}, for information about
what it means to be the controlling terminal.  In the GNU system and 4.4
BSD, opening a file never makes it the controlling terminal and
@code{O_NOCTTY} is zero.
@cindex controlling terminal, setting
@end deftypevr

These three file name translation flags exist only in the GNU system.

@comment fcntl.h
@comment GNU
@deftypevr Macro int O_IGNORE_CTTY
Do not recognize the named file as the controlling terminal, even if it
refers to the process's existing controlling terminal device.  Operations
on the new file descriptor will never induce job control signals.
@xref{Job Control}.
@end deftypevr

@comment fcntl.h
@comment GNU
@deftypevr Macro int O_NOLINK
If the named file is a symbolic link, open the link itself instead of
the file it refers to.  (@code{fstat} on the new file descriptor will
return the information returned by @code{lstat} on the link's name.)
@cindex symbolic link, opening
@end deftypevr

@comment fcntl.h
@comment GNU
@deftypevr Macro int O_NOTRANS
If the named file is specially translated, do not invoke the translator.
Open the bare file the translator itself sees.
@end deftypevr


The open-time action flags tell @code{open} to do additional operations
which are not really related to opening the file.  The reason to do them
as part of @code{open} instead of in separate calls is that @code{open}
can do them @i{atomically}.

@comment fcntl.h
@comment POSIX.1
@deftypevr Macro int O_TRUNC