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fcntl(2)
NAME
fcntl, dup, dup2 - Control open file descriptors
SYNOPSIS
#include <fcntl.h>
#include <sys/types.h>
#include <unistd.h>
int fcntl(
int filedes,
int request [,
int argument |,
struct flock *argument |,
advfs_opT argument] );
int dup(
int filedes );
int dup2(
int old,
int new );
STANDARDS
Interfaces documented on this reference page conform to industry standards
as follows:
dup(), dup2(): XSH4.0, XSH4.2, XSH5.0
fcntl(): XSH4.0, XSH4.2,XSH5.0, XNS4.0, XNS5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
filedes
Specifies an open file descriptor obtained from a successful open(),
fcntl(), or pipe() function.
request
Specifies the operation to be performed.
argument
Specifies a variable that depends on the value of the request
parameter. The standard allows for a list of variables, but does not
specify them. These can vary with each vendor's implementation of this
function.
old Specifies an open file descriptor that is returned by the dup2()
function.
new Specifies an open file descriptor that is returned by the dup2()
function.
DESCRIPTION
The fcntl() function performs controlling operations on the open file
specified by the filedes parameter.
When the fcntl(), dup() and dup2() functions need to block, only the
calling thread is suspended rather than all threads in the calling process.
The following are values for the request parameter:
F_DUPFD
Returns a new file descriptor as follows:
· Lowest numbered available file descriptor greater than or equal to
the argument parameter, taken as type int.
· Same object references as the original file.
· Same file pointer as the original file. (That is, both file
descriptors share one file pointer if the object is a file).
· Same access mode (read, write, or read-write).
· Same file status options. (That is, both file descriptors share
the same file status options).
· The close-on-exec option (FD_CLOEXEC bit) associated with the new
file descriptor is cleared so that the file will remain open
across exec functions.
F_GETFD
Gets the value of the close-on-exec option associated with the file
descriptor filedes. File descriptor options are associated with a
single file descriptor and do not affect other file descriptors that
refer to the same file. The argument parameter is ignored.
F_SETFD
Sets the close-on-exec option associated with the filedes parameter to
the value of the argument parameter, taken as type int. If the argument
parameter is 0 (zero), the file remains open across the exec functions.
If the argument parameter is FD_CLOEXEC, the file is closed on
successful execution of the next exec function.
F_GETTIMES
[Tru64 UNIX] Retrieves the current times for the file identified by
filedes. For the F_GETTIMES and F_SETTIMES requests, the function's
third argument is a pointer to the struct attr_timbuf structure
(defined in sys/fcntl1.h), which contains the file's atime (access
time), mtime (modification time), and ctime (file attribute change
time) values. These requests are useful for operations, such as backup
and archiving, when it is necessary to save a file's current time
values and then, after copying or moving the file, set back atime or
mtime without changing ctime.
F_SETTIMES
[Tru64 UNIX] Sets the current times for the file identified by
filedes. This request requires superuser privilege and returns the
[EPERM] error on an attempt to set the file's times without this
privilege.
The following example illustrates how to use the F_GETTIMES and
F_SETTIMES requests:
# include <stdio.h>
# include <sys/fcntl.h>
# include <sys/fcntl1.h>
main()
{
char buffer[1024];
int fd,bytesread;
struct attr_timbuf tstamp;
/* Create a file*/
fd=open("/usr/tmp/foo",O_CREAT|O_RDONLY);
if(fd > 0) {
/* Display the atime and ctime of the file */
printf("atime before reading the file:\n");
system("ls -lu /usr/tmp/foo");
printf("ctime before reading the file:\n");
system("ls -lc /usr/tmp/foo");
if(fcntl(fd,F_GETTIMES,&tstamp) < 0) {
perror("fcntl:F_GETTIMES");
exit(1);
}
}
else {
perror("open");
exit(1);
}
printf("Sleeping for one minute because ls commands can \
show time change only in terms of hours and minutes...\n");
sleep(60);
/* Access the file */
bytesread=read(fd,buffer,1024);
if(bytesread >= 0) {
/* Again display the atime and ctime of the file */
printf("\n\natime after reading the file:\n");
system( "ls -lu /usr/tmp/foo");
printf("ctime after reading the file:\n");
system( "ls -lc /usr/tmp/foo");
/* Now use F_SETTIMES to reinstate the original atime */
if(fcntl(fd,F_SETTIMES,&tstamp) < 0) {
perror("fcntl:F_SETTIMES");
exit(1);
}
else {
printf("\n\nAfter using F_SETTIMES, atime is reset \
without affecting ctime\n");
printf("\tatime:\n");
system( "ls -lu /usr/tmp/foo");
printf("\tctime:\n");
system( "ls -lc /usr/tmp/foo");
}
}
else
perror("read");
system("rm -rf /usr/tmp/foo");
}
F_GETFL
Gets the file status options and file access modes for the file
referred to by the filedes parameter. The file access modes can be
extracted by using the mask O_ACCMODE on the return value. File status
options and file access modes are associated with the file description
and do not affect other file descriptors that refer to the same file
with different open file descriptions. The argument parameter is
ignored.
F_SETFL
Sets the file status options to the argument parameter, taken as type
int, for the file to which the filedes parameter refers. The file
access mode is not changed.
F_GETOWN
[XNS5.0] If filedes refers to a socket, gets the process or process
group ID currently receiving SIGURG signals when out-of-band data is
available. Positive values indicate a process ID; negative values,
other than -1, indicate a process group ID. If filedes does not refer
to a socket, the results are unknown.
F_SETOWN
[XNS5.0] If filedes refers to a socket, sets the process or process
group ID to receive SIGURG signals when out-of-band data is available,
using the value of the argument parameter, taken as type int. Positive
values indicate a process ID; negative values, other than -1, indicate
a process group ID. If filedes does not refer to a socket, the results
are unknown.
F_RSETLK
[Tru64 UNIX] Reserved for use by the network lock daemon
(rpc.lockd(8)) on NFS servers to acquire locks requested by F_SETLK on
NFS clients.
F_RSETLKW
[Tru64 UNIX] Reserved for use by the network lock daemon
(rpc.lockd(8)) on NFS servers to acquire locks requested by F_SETLK on
NFS clients.
F_RGETLK
[Tru64 UNIX] Reserved for use by the network lock daemon
(rpc.lockd(8)) on NFS servers to acquire locks requested by F_SETLK on
NFS clients.
F_CNVT
[Tru64 UNIX] Is used by the network lock daemon (rpc.lockd(8)) to
communicate with the NFS server kernel to handle locks on the NFS
files.
F_ADVFS_OP
[Tru64 UNIX] Performs AdvFS-specific operations on AdvFS files. The
value expects that the argument parameter is a pointer to a advfs_opT
structure as described in the <fcntl.h> header file.
The following values for the request parameter are available for record
locking:
F_GETLK
Gets the first lock that blocks the lock description pointed to by the
argument parameter, taken as a pointer to type struct flock. The
information retrieved overwrites the information passed to the fcntl()
function in the flock structure. If no lock is found that would prevent
this lock from being created, then the structure is left unchanged
except for the lock type, which is set to F_UNLCK.
F_SETLK
Sets or clears a file segment lock according to the lock description
pointed to by argument, taken as a pointer to type struct flock.
F_SETLK is used to establish shared locks (F_RDLCK), or exclusive locks
(F_WRLCK), as well as remove either type of lock (F_UNLCK). If a shared
(read) or exclusive (write) lock cannot be set, the fcntl() function
returns immediately with a value of -1.
An unlock (F_UNLCK) request in which l_len is nonzero and the offset of
the last byte of the requested segment is the maximum value for an
object of type off_t, when the process has an existing lock in which
l_len is 0 and which includes the last byte of the requested segment,
is treated as a request to unlock from the start of the requested
segment with an l_len equal to 0. Otherwise, an unlock (F_UNLCK)
request attempts to unlock only the requested file.
F_SETLKW
Same as F_SETLK except that if a shared or exclusive lock is blocked by
other locks, the process will wait until it is unblocked. If a signal
is received while fcntl() is waiting for a region, the function is
interrupted, -1 is returned, and errno is set to [EINTR].
AdvFS-only request Parameters
[Tru64 UNIX] The following values for the request parameter are available
for AdvFS only, and relate to performing direct I/O. The arguments used
with these request parameters are in the <fcntl.h> file. FCACHE is defined
as zero to indicate that the file's cache policy is the file system's
default cache policy. FDIRECTIO is defined as one to indicate that the
file's cache policy is direct I/O.
F_GETCACHEPOLICY
Gets the cache policy for the file, which is either direct I/O or
caching.
[Tru64 UNIX] The following value for the request parameter is valid only
when the filedes parameter describes an AdvFS or UFS file.
F_GETMAP
[Tru64 UNIX] The F_GETMAP request gets the sparseness map of the file
referred to by the fildes parameter. The argument parameter, taken as a
pointer to type struct extentmap, is filled in with data that describes
the extent map of the file.
Each map entry is declared as:
struct extentmapentry {
unsigned long offset;
unsigned long size;
};
struct extentmap {
unsigned long arraysize;
unsigned long numextents;
unsigned long offset;
struct extentmapentry *extent;
};
The map returned by this function can be different from the actual
number of extents (or their definition) when the file is being written.
It is recommended that you use this function only on files that are not
being written.
[Tru64 UNIX] The O_NDELAY and O_NONBLOCK requests affect only operations
against file descriptors derived from the same open() function. In BSD,
these apply to all file descriptors that refer to the object.
When a shared lock is set on a segment of a file, other processes are able
to set shared locks on that segment or a portion of it. A shared lock
prevents any other process from setting an exclusive lock on any portion of
the protected area. A request for a shared lock fails if the file
descriptor was not opened with read access.
An exclusive lock prevents any other process from setting a shared lock or
an exclusive lock on any portion of the protected area. A request for an
exclusive lock fails if the file descriptor was not opened with write
access.
The flock structure describes the type (l_type), starting offset
(l_whence), relative offset (l_start), size (l_len) and process ID (l_pid)
of the segment of the file to be affected.
The value of l_whence is set to SEEK_SET, SEEK_CUR or SEEK_END, to indicate
that the relative offset l_start bytes is measured from the start of the
file, from the current position, or from the end of the file, respectively.
The value of l_len is the number of consecutive bytes to be locked. The
l_len value may be negative (where the definition of off_t permits negative
values of l_len). The l_pid field is only used with F_GETLK to return the
process ID of the process holding a blocking lock. After a successful
F_GETLK request, the value of l_whence becomes SEEK_SET.
If l_len is positive, the area affected starts at l_start and ends at
l_start + l_len - 1. If l_len is negative, the area affected starts at
l_start + l_len and ends at l_start - 1. Locks may start and extend beyond
the current end of a file, but may not be negative relative to the
beginning of the file. If l_len is set to 0 (zero), a lock may be set to
always extend to the largest possible value of the file offset for that
file. If such a lock also has l_start set to 0 (zero) and l_whence is set
to SEEK_SET, the whole file is locked.
Changing or unlocking a portion from the middle of a larger locked segment
leaves a smaller segment at either end. Locking a segment that is already
locked by the calling process causes the old lock type to be removed and
the new lock type to take effect.
All locks associated with a file for a given process are removed when a
file descriptor for that file is closed by that process or the process
holding that file descriptor terminates. Locks are not inherited by a child
process in a fork() function.
[Tru64 UNIX] The fcntl() record locks are implemented in the kernel for
local locks and throughout the network by the network lock daemon
(rpc.lockd(8)) for remote locks on NFS files. If the file server crashes
and has to be rebooted, the lock daemon attempts to recover all locks that
were associated with that server. If a lock cannot be reclaimed, the
process that held the lock is issued a SIGLOST signal.
[Tru64 UNIX] In order to maintain consistency in the network case, data
must not be cached on client machines. For this reason, file buffering for
an NFS file is turned off when the first lock is attempted on the file.
Buffering remains off as long as the file is open. Programs that do I/O
buffering in the user address space, however, may have inconsistent
results. The standard I/O package, for instance, is a common source of
unexpected buffering.
[Tru64 UNIX] If a regular file has enforced record locking enabled,
record locks on the file will affect calls to other calls, including
creat(), open(), read(), write(), truncate(), and ftruncate().
A potential for deadlock occurs if a process controlling a locked region is
put to sleep by attempting to lock another process' locked region. If the
system detects that sleeping until a locked region is unlocked would cause
a deadlock, the fcntl() function fails with an [EDEADLK] error.
[Tru64 UNIX] The F_ADVFS_OP request is used to perform operations on
AdvFS files which do not have an analog on other file systems. The
argument parameter is expected to be a pointer to an advfs_opT structure.
The operation field of the advfs_opT structure specifies the general kind
of operation. The action field of the advfs_opT structure refines the
operation field in order to specify more exactly the operation to be
performed. If the action specified is ADVFS_GET_INFO, the info_buf and
info_buf_size fields also must be used. The info_buf field is a pointer to
the buffer that will contain the requested information. The info_buf_size
field specifies the size of the buffer. See the <fcntl.h> header file for a
description of the operations, actions, and values returned by
ADVFS_GET_INFO.
To use the F_ADVFS_OP request on AdvFS files that are mounted across NFS,
the NFS property list daemon, proplistd, must be running on the NFS client
and the fileset must have been mounted on the client using the proplist
option.
The following code fragment shows how to activate atomic write data logging
on an AdvFS file:
.
.
.
advfs_opT myop;
int fd;
.
.
.
myop.operation = ADVFS_AW_DATA_LOGGING;
myop.action = ADVFS_ACTIVATE;
ret = fcntl(fd, F_ADVFS_OP, &myop);
.
.
.
The following code fragment shows how to query the current I/O mode for an
AdvFS file:
.
.
.
advfs_opT myop;
int fd;
int io_mode;
.
.
.
myop.operation = advfs_aw_data_logging;
myop.action = ADVFS_GET_INFO;
myop.info_buf = &io_mode;
myop.info_buf_size = sizeof(int):
ret = fcntl(fd, F_ADVFS_OP, &myop):
if (ret) {
perror("fcntl failed");
}
if (io_mode == ADVFS_ASYNC_IO)
printf("I/O mode is asynchronous.\n");
else if (io_mode == ADVFS_DATA_LOGGING_IO)
printf("I/O mode is atomic write data logging.\n");
else if (io_mode == ADVFS_SYNC_IO)
printf("I/O mode is forced synchronous writes.\n");
.
.
.
See chfile(8) for information on the file's I/O mode. Note that the
previous example is attempting to determine the I/O mode setting. The
setting could also have been specified as ADVFS_SYNC_WRITE.
NOTES
The dup(filedes) function is equivalent to fcntl(filedes, F_DUPFD, 0).
The dup2(oldfiledes, newfiledes) function has similar functionality to:
close(newfiledes) and fcntl(oldfiledes, F_DUPFD, newfiledes).
RETURN VALUES
Upon successful completion, the value returned depends on the value of the
request parameter as follows:
F_DUPFD
Returns a new file descriptor.
F_GETFD
Returns FD_CLOEXEC or 0 (zero).
F_SETFD
Returns a value other than -1.
F_GETFL
Returns the value of file status options and access modes. (The return
value will not be negative.)
F_SETFL
Returns a value other than -1.
F_GETLK
Returns a value other than -1.
F_SETLK
Returns a value other than -1.
F_GETOWN
[XNS5.0] Returns the value of the socket owner process or process
group; this will not be -1.
F_SETOWN
[XNS5.0] Returns a value other than -1.
F_SETLKW
Returns a value other than -1.
F_GETCACHEPOLICY
[Tru64 UNIX] Returns a value other than -1.
F_GETTIMES
[Tru64 UNIX] Returns a value other than -1.
F_SETTIMES
[Tru64 UNIX] Returns a value other than -1.
F_GETMAP
[Tru64 UNIX] Returns a value other than -1.
If the fcntl() function fails, a value of -1 is returned and errno is set
to indicate the error.
ERRORS
The fcntl() function sets errno to the specified values for the following
conditions:
[EACCES or EAGAIN]
The request parameter is F_SETLK; the type of lock (l_type) is a shared
(F_RDLCK) or exclusive (F_WRLCK) lock, and the segment of a file to be
locked is already exclusive-locked by another process.
The type is an exclusive lock and some portion of the segment of a file
to be locked is already shared-locked or exclusive-locked by another
process.
[EBADF]
The filedes or old parameter is not a valid open file descriptor and
the argument parameter file descriptor is negative or greater than or
equal to the per-process limit.
[Tru64 UNIX] The request parameter is F_GETMAP and the filedes
parameter does not point to an open file descriptor of an AdvFS or UFS
file.
The request parameter is F_SETLK or F_SETLKW, the type of lock (l_type)
is a shared lock (F_RDLCK), and filedes is not a valid file descriptor
open for reading.
The type of lock (l_type) is an exclusive lock (F_WRLCK), and filedes
is not a valid file descriptor open for writing.
[EDEADLK]
The request parameter is F_SETLKW, the lock is blocked by some lock
from another process and putting the calling process to sleep, and
waiting for that lock to become free would cause a deadlock.
[EFAULT]
The argument parameter is an invalid address.
[EINVAL]
The request parameter is F_DUPFD and the argument parameter is negative
or greater than or equal to OPEN_MAX.
[Tru64 UNIX] Either the OPEN_MAX value or the per-process soft
descriptor limit is checked.
An illegal value was provided for the request parameter.
The request parameter is F_GETLK, F_SETLK, or F_SETLKW and the data
pointed to by argument is invalid, or filedes refers to a file that
does not support locking.
[Tru64 UNIX] The F_ADVFS_OP request was performed and the fd referred
to a socket; or the action was ADVFS_GET_INFO and the info_buf_size was
zero; or the operation to be performed was undefined; or the action to
be taken was undefined.
[EMFILE]
The request parameter is F_DUPFD and too many or OPEN_MAX file
descriptors are currently open in the calling process, or no file
descriptors greater than or equal to argument are available.
[Tru64 UNIX] Either the OPEN_MAX value or the per-process soft
descriptor limit is checked.
[EOVERFLOW]
One of the values to be returned cannot be represented correctly.
The request argument is F_BETLK, F_SETLK, or F_SETLKW and the smallest
or, if l_len is nonzero, the largest offset of any byte in the
requested segment cannot be represented correctly in an object of type
off_t.
[ESRCH]
[Tru64 UNIX] The value of the request parameter is F_SETOWN and the
process ID given as argument is not in use.
[EINTR]
The request parameter is F_SETLKW and the fcntl() function was
interrupted by a signal which was caught.
[EIO]
[Tru64 UNIX] The request parameter is F_GETMAP and an I/O error
occurred on the disk where the file is located.
[ENOLCK]
The request parameter is F_SETLK or F_SETLKW and satisfying the lock or
unlock request would exceed the configurable system limit of
NLOCK_RECORD.
[Tru64 UNIX] The file is an NFS file, and either the client or server
system is not running rpc.lockd, which is the NFS lock manager.
[ENOMEM]
[Tru64 UNIX] The system was unable to allocate kernel memory for the
requested file descriptor.
[ENOTTY]
[Tru64 UNIX] The request parameter is F_SETOWN and the calling
process does not have a controlling terminal, the file is not the
controlling terminal, or the controlling terminal is no longer
associated with the calling process' session.
[EPERM]
[Tru64 UNIX] The request parameter is F_SETOWN and the argument
specified by the pgrp_id is valid, but matches a process ID or process
group ID of a process in another session.
[Tru64 UNIX] The request parameter is F_SETTIMES and the user does not
have superuser privilege.
The dup() and dup2() functions set errno to the specified values for the
following conditions:
[EACCES]
The request parameter is F_SETLK; the type of lock (l_type) is a read
(F_RDLCK) lock, and the segment of a file to be locked is already
write-locked by another process; or the type is a write (F_WRLCK) lock
and the segment of a file to be locked is already read- or write-locked
by another process.
[EBADF]
The filedes or old parameter is not a valid open file descriptor or the
new parameter file descriptor is negative or greater than OPEN_MAX.
[Tru64 UNIX] Either the OPEN_MAX value or the per-process soft
descriptor limit is checked.
[EINTR]
The dup2() function was interrupted by a signal which was caught.
[EMFILE]
The number of file descriptors exceeds OPEN_MAX or the per-process
limit, or there is no file descriptor above the value of the new
parameter.
[ENETUNREACH]
[Tru64 UNIX] The file descriptor specified by filedes is on a remote
machine and the link to that machine is no longer active.
[ENOMEM]
[Tru64 UNIX] The system was unable to allocate kernel memory for the
requested file descriptor.
Because in the future the variable errno is set to EAGAIN rather than
EACCES when a section of a file is already locked by another process,
portable application programs should expect and test for either value.
SEE ALSO
Functions: close(2), creat(2), dup(2), exec(2), flock(2), fork(2),
getdtablesize(2), open(2), pipe(2), read(2), truncate(2), write(2),
lockf(3)
Commands: rpc.lockd(8), rpc.statd(8)
Standards: standards(5)
Network Programmer's Guide
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Alphabetical
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