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fcntl(S)

fcntl -- file control

Synopsis

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

   int fcntl(int fildes, int cmd, ...  /* arg */);

Description

fcntl provides for control over open files. fildes is an open file descriptor (see intro(S)).

fcntl may take a third argument, arg, whose data type, value and use depend upon the value of cmd. cmd specifies the operation to be performed by fcntl and may be one of the following:


F_DUPFD
Return a new file descriptor with the following characteristics:

F_DUP2
Will atomically close fildes2 and replace it with a duplicate of fildes. The operation can be invoked as fcntl(fildes, F_DUP2, fildes2). (See dup2(S).)

F_GETFD
Get the close-on-exec flag associated with fildes. If the low-order bit is 0, the file will remain open across exec. Otherwise, the file will be closed upon execution of exec.

F_SETFD
Set the close-on-exec flag associated with fildes to the low-order bit of the integer value given as the third argument (0 or 1 as above).

F_GETFL
Get fildes status flags.

F_SETFL
Set fildes status flags to the integer value given as the third argument. Only certain flags can be set (see fcntl(M)).

F_FREESP
Free storage space associated with a section of the ordinary file fildes. The section is specified by a variable of data type struct flock pointed to by the third argument arg. The data type struct flock is defined in the sys/fcntl.h header file (see fcntl(M)) and contains the following members: l_whence is 0, 1, or 2 to indicate that the relative offset l_start will be measured from the start of the file, the current position, or the end of the file, respectively. l_start is the offset from the position specified in l_whence. l_len is the size of the section. F_FREESP is only supported with an l_len of 0, and frees up to the end of the file. The end of file (that is, file size) is set to the beginning of the section freed. If the beginning of the segment specified by the flock structure is beyond the current end of the file, then the file size is increased. Any data previously written into this section is no longer accessible.

F_FREESP64
Free storage space associated with a section of the ordinary file fildes. The section is specified by a variable of data type struct flock64 pointed to by the third argument arg. The data type struct flock64 is defined in the sys/fcntl.h header file (see fcntl(M)) and contains the following members: l_whence is 0, 1, or 2 to indicate that the relative offset l_start will be measured from the start of the file, the current position, or the end of the file, respectively. l_start is the offset from the position specified in l_whence. l_len is the size of the section. F_FREESP64 is only supported with an l_len of 0, and frees up to the end of the file. The end of file (that is, file size) is set to the beginning of the section freed. If the beginning of the segment specified by the flock structure is beyond the current end of the file, then the file size is increased. Any data previously written into this section is no longer accessible.

F_GETOWN
If fildes refers to a socket, get the process or process group id specified to receive 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 fildes does not refer to a socket, the results are unspecified.

F_SETOWN
If fildes refers to a socket, set the process group id specified to receive SIGURG signals when out-of-band data is available, using the value of arg taken as type int. Positive values indicate a process id, negative values other than -1 indicate a process group id. if fildes does not refer to a socket, the results are unspecified.

F_GETADV
Get the default advice that will be assumed by the system when the file is accessed. The return value of the function will be the value of the advice parameter, which will not be negative. Possible values are:

F_PLC_DEFAULT
Specifies that system default policies apply.

F_REPLICATE
Specifies that the file will be accessed in readonly fashion and replication of pages at each CPU-Group in a partitioned system is appropriate.

F_BALANCED
Specifies that the file will be accessed read/write, or readonly but such that replication at each CPU-Group in a partitioned system is not desired. It is expected that the file will be accessed from many CPU-Groups, so it is appropriate to allocate physical memory so as to spread it evenly across these CPU-Groups.

F_CPUGROUP
Specifies that the memory for the file should be preferentially allocated from just one CPU-Group.

F_FIRSTUSAGE
Specifies that the memory for the file should be preferentially allocated on the CPU-Group where it was first referenced.

F_SETADV
Set default advice to the system to take effect when the file is accessed. The third argument is an integer that passes the advice, encoded by the values given for F_GETADV, possibly modified by a bitwise or with F_MIGRATE. If F_BALANCED is set explicitly, the set of CPU-Groups used for balanced allocation is the entire set of online CPU-Groups. If F_CPUGROUP is set then the CPU-Groups to be used for allocation is the one where the caller is currently executing. If F_MIGRATE is set, the advice should apply to current allocations as well as to future ones.

The system advice given via F_SETADV may influence accesses made by other processes in the system. It remains current until the last close of the file or until it is changed by a further use of F_SETADV.

The persistence of the advice given via F_SETADV after the last close of the file is undefined. The effect on the current advice for a particular process of another process giving system advice via F_SETADV is undefined. Text files mapped by exec(S) follow the fcntl specified placement policy.


F_GETCPUGROUP
Get the CPU-Group that is to be used for allocation when F_GETCPUGROUP is set. The return value of the function will be the value of the CPU-Group identifier, which will not be negative. Should F_GETCPUGROUP not be in effect, the return value will be NOCGID.

F_SETCPUGROUP
Instruct the system that allocations for the file are to be taken from a particular CPU-Group. The third parameter contains the CPU-Group identifier. This call implicitly sets the advice to F_CPUGROUP.

F_GETGRAN
Get the granularity to be used for balanced placement. (This is akin to stripe size, but does not require a particular allocation pattern).

F_SETGRAN
Set the granularity to be used for balanced placement.

The following commands are used for record-locking. Locks may be placed on an entire file or on segments of a file.


F_SETLK
Set or clear a file segment lock according to the flock structure that arg points to (see fcntl(M)). The cmd F_SETLK is used to establish read (F_RDLCK) and write (F_WRLCK) locks, as well as remove either type of lock (F_UNLCK). If a read or write lock cannot be set, fcntl will return immediately with an error value of -1.

F_SETLK64
Set or clear a file segment lock according to the flock64 structure that arg points to (see fcntl(M)). F_SETLK64 is used to establish read (F_RDLCK) and write (F_WRLCK) locks, as well as remove either type of lock (F_UNLCK). If a read or write lock cannot be set, fcntl will return immediately with an error value of -1.

F_SETLKW
This cmd is the same as F_SETLK except that if a read or write lock is blocked by other locks, fcntl will block until the segment is free to be locked.

F_SETLKW64
This cmd is the same as F_SETLK64 except that if a read or write lock is blocked by other locks, fcntl will block until the segment is free to be locked.

F_GETLK
Get the first lock which blocks the lock description pointed to by the third argument arg, taken as a pointer to the type struct flock. The information retrieved overwrites the information passed to fcntl in the structure flock. If no lock is found that would prevent this lock from being created, the structure is left unchanged except for the lock type which is set to F_UNLCK.

If the lock request described by the flock structure that arg points to could be created, then the structure is passed back unchanged except that the lock type is set to F_UNLCK and the l_whence field will be set to SEEK_SET.

This command never creates a lock; it tests whether a particular lock could be created.


F_GETLK64
Get the first lock which blocks the lock description pointed to by the third argument arg, taken as a pointer to the type struct flock64 (see fcntl(M)). The information retrieved overwrites the information passed to fcntl in the structure flock64. If no lock is found that would prevent this lock from being created, the structure is left unchanged except for the lock type which is set to F_UNLCK.

If the lock request described by the flock64 structure that arg points to could be created, then the structure is passed back unchanged except that the lock type is set to F_UNLCK and the l_whence field will be set to SEEK_SET.

This command never creates a lock; it tests whether a particular lock could be created.


F_RSETLK
Used by the network lock daemon, lockd(NADM), to communicate with the NFS server kernel to handle locks on NFS files.

F_RSETLK64
Used by the network lock daemon, lockd(NADM), to communicate with the NFS server kernel to handle locks on NFS files.

F_RSETLKW
Used by the network lock daemon, lockd(NADM), to communicate with the NFS server kernel to handle locks on NFS files.

F_RSETLKW64
Used by the network lock daemon, lockd(NADM), to communicate with the NFS server kernel to handle locks on NFS files.

F_RGETLK
Used by the network lock daemon, lockd(NADM), to communicate with the NFS server kernel to handle locks on NFS files.

F_RGETLK64
Used by the network lock daemon, lockd(NADM), to communicate with the NFS server kernel to handle locks on NFS files.

F_RSETLK, F_RSETLK64, F_RSETLKW F_RSETLKW64 F_RGETLK and F_RGETLK64 are used by the fslock daemon and should not be used by regular applications.

A read lock prevents any other process from write locking the protected area. More than one read lock may exist for a given segment of a file at a given time. The file descriptor on which a read lock is being placed must have been opened with read access.

A write lock prevents any other process from read locking or write locking the protected area. Only one write lock and no read locks may exist for a given segment of a file at a given time. The file descriptor on which a write lock is being placed must have been opened with write access.

The flock and flock64 structures describe the type (l_type), starting offset (l_whence), relative offset (l_start), size (l_len), process ID (l_pid), and system ID (l_sysid) of the segment of the file to be affected. The process ID and system ID fields are used only with the F_GETLK cmd to return the values for a blocking lock. Locks may start and extend beyond the current end of a file, but may not be negative relative to the beginning of the file. A lock may be set to always extend to the end of file by setting l_len to 0. If such a lock also has l_whence and l_start set to 0, the whole file will be locked. Changing or unlocking a segment from the middle of a larger locked segment leaves two smaller segments 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(S) system call.

When mandatory file and record locking is active on a file (see chmod(S), creat(S), open(S), read(S) and write(S)) system calls issued on the file will be affected by the record locks in effect.

Return values

On success, fcntl returns a value that depends on cmd:

F_DUPFD
A new file descriptor.

F_DUP2
A new file descriptor.

F_GETFD
Value of flag (only the low-order bit is defined). The return value will not be negative.

F_SETFD
Value other than -1.

F_FREESP, F_FREESP64
Value of 0.

F_GETFL
Value of file status flags. The return value will not be negative.

F_SETFL
Value other than -1.

F_GETOWN
Value of the socket owner process or process group; this will not be -1.

F_SETOWN
Value other than -1.

F_GETLK, F_GETLK64
Value other than -1.

F_SETLK, F_SETLK64
Value other than -1.

F_SETLKW, F_SETLKW64
Value other than -1.

On failure, fcntl returns -1 and sets errno to identify the error.

Errors

In the following conditions, fcntl fails and sets errno to:

EACCES
cmd is F_SETLK or F_SETLK64, the type of lock (l_type) is a read lock (F_RDLCK) and the segment of a file to be locked is already write locked by another process, or the type is a write lock (F_WRLCK) and the segment of a file to be locked is already read or write locked by another process.

EACCES
cmd is F_SETFD, F_DETFL, F_SETLK, F_SETLK64, F_SETLKW, or F_SETLKW64, and either write permission on fildes is denied or fildes is already open for writing.

EACCES
cmd is F_SETLK, F_SETLK64, F_SETLKW, or F_SETLKW64, mandatory file locking bit is set for the file, and the file is currently being mapped to virtual memory via mmap or mmap64 (see mmap(S)).

EAGAIN
cmd is F_FREESP or F_FREESP64, the file exists, mandatory file/record locking is set, and there are outstanding record locks on the file.

EAGAIN
cmd is F_SETLK, F_SETLK64, F_SETLKW, or F_SETLKW64, mandatory file locking bit is set for the file, and the file is currently being mapped to virtual memory via mmap or mmap64 (see mmap(S)).

EBADF
fildes is not a valid open file descriptor.

EBADF
cmd is F_SETLK, F_SETLK64, F_SETLKW, or F_SETLKW64, the type of lock (l_type) is a read lock (F_RDLCK), and fildes is not a valid file descriptor open for reading.

EBADF
cmd is F_SETLK, F_SETLK64, F_SETLKW or F_SETLKW64, the type of lock (l_type) is a write lock (F_WRLCK), and fildes is not a valid file descriptor open for writing.

EBADF
cmd is F_FREESP or F_FREESP64, and fildes is not a valid file descriptor open for writing.

EDEADLK
cmd is F_SETLKW or F_SETLKW64, the lock is blocked by some lock from another process, and if fcntl blocked the calling process waiting for that lock to become free, a deadlock would occur.

EDEADLK
cmd is F_FREESP or F_FREESP64, mandatory record locking is enabled, O_NDELAY and O_NONBLOCK are clear and a deadlock condition was detected.

EFAULT
cmd is F_FREESP or F_FREESP64, and the value pointed to by the third argument arg resulted in an address outside the process's allocated address space.

EFAULT
cmd is F_GETLK, F_GETLK64, F_SETLK, F_SETLK64, F_SETLKW or F_SETLKW64 and the value pointed to by the third argument resulted in an address outside the program address space.

EINTR
A signal was caught during execution of the fcntl system call.

EIO
An I/O error occurred while reading from or writing to the file system.

EMFILE
cmd is F_DUPFD and the number of file descriptors currently open in the calling process is the configured value for the maximum number of open file descriptors allowed each user.

EINVAL
cmd is F_DUPFD or F_DUP2 and the third argument is either negative, or greater than or equal to the configured value for the maximum number of open file descriptors allowed each user.

EINVAL
cmd is F_SETCPUGROUP and the third parameter is not a valid CPU-Group identifier.

EINVAL
cmd is not a valid value.

EINVAL
cmd is F_GETLK, F_GETLK64, F_SETLK, F_SETLK64, F_SETLKW, or F_SETLKW64 and the third argument or the data it points to is not valid, or fildes refers to a file that does not support locking.

ENOLCK
cmd is F_SETLK, F_SETLK64, F_SETLKW, or F_SETLKW64, the type of lock is a read or write lock, and there are no more record locks available (too many file segments locked) because the system maximum has been exceeded.

ENOLINK
fildes is on a remote machine and the link to that machine is no longer active.

ENOLINK
cmd is F_FREESP or F_FREESP64, the file is on a remote machine, and the link to that machine is no longer active.

EOVERFLOW
cmd is F_GETLK or F_GETLK64 and the process ID of the process holding the requested lock is too large to be stored in the ``l_pid'' field.

EOVERFLOW
cmd is F_GETLK, F_SETLK, or F_SETLKW and the smallest, or if l_len is non-zero, the largest, offset of any byte in the requested segment cannot be represented correctly in an object of type off_t.

EOVERFLOW
One of the values to be returned cannot be represented correctly.

ENOTSUP
cmd is F_GETADV, F_SETADV, F_SETCPUGROUP, F_GETGRAN or F_SETGRAN and the system does not support placement directives on the file specified by fildes.

References

chown(S), close(S), creat(S), dup(S), exec(S), fcntl(M), fork(S), intro(S), open(S), pipe(S)

Notices

Future directions

In the future, the variable errno will be set to EAGAIN rather than EACCES when a section of a file is already locked by another process. Therefore, portable application programs should expect and test for either value.

Considerations for threads programming

Open file descriptors are a process resource and available to any sibling thread; if used concurrently, actions by one thread can interfere with those of a sibling.

File and record locks are based on process ID; consequently, all siblings share locks. It is possible for a record lock placed by one thread to be overlaid with a lock by a sibling. Other mechanisms should be used to coordinate concurrent access by multiple threads. See thread(THREAD).

New commands for fine grain shared memory placement have been added. See the descriptions of:

Considerations for large file support

fcntl supports large files. For details on programming for large file capable applications, see ``Large File Support'' in intro(S).
© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 - 01 June 2005