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Manual page for fcntl(2)

fcntl - file control

SYNOPSIS

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

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

MT-LEVEL

Async_Signal_Safe

DESCRIPTION

fcntl() provides for control over open files. fildes is an open file descriptor (see intro.2

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:
: Lowest numbered available file descriptor greater than or equal to the integer value given as the third argument.
: Same open file (or pipe) as the original file.
: Same file pointer as the original file (that is, both file descriptors share one file pointer).
: Same access mode (read, write, or read/write) as the original file.
: Shares any locks associated with the original file descriptor.
: Same file status flags (that is, both file descriptors share the same file status flags) as the original file.
: The close-on-exec flag (see F_GETFD) associated with the new file descriptor is set to remain open across exec.2 functions.
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.5
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 <fcntl.h> header (see fcntl.5 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.
: An l_len of 0 frees up to the end of the file; in this case, the end of file (that is, file size) is set to the beginning of the section freed. Any data previously written into this section is no longer accessible.
: Note that all filesystems might not support all possible variations of F_FREESP arguments. In particular, many filesystems only allow space to be freed at the end of a file.

The following values for cmd 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.5 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_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_GETLK: 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.
: If a lock is found that would prevent this lock from being created, then the structure is overwritten with a description of the first lock that is preventing such a lock from being created. The structure also contains the process ID and the system ID of the process holding the lock.
: This command never creates a lock; it tests whether a particular lock could be created.

A read lock prevents any 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 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 record to be locked or unlocked is described by the flock structure defined in
<sys/fcntl.h> (included in <fcntl.h>) as follows:

typedef struct flock {
	short	l_type;
	short	l_whence;
	off_t	l_start;
	off_t	l_len;          /* len == 0 means until end of file */
	long	l_sysid;
	pid_t	l_pid;
	long	pad[4];         /* reserve area */
} flock_t;

The flock structure describes 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.2 function.

When mandatory file and record locking is active on a file (see chmod.2 creat.2 open.2 read.2 and write.2 functions issued on the file will be affected by the record locks in effect. When mandatory file and record locking is active on a file, it cannot be memory mapped.

RETURN VALUES

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

F_DUPFD: 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: Value of 0.
F_GETFL: Value of file status flags. The return value will not be negative.
F_SETFL: Value other than -1.
F_GETLK: Value other than -1.
F_SETLK: Value other than -1.
F_SETLKW: Value other than -1.

On failure, fcntl() returns -1 and sets errno to indicate the error.

ERRORS

fcntl() will fail if one or more of the following are true:

EAGAIN: cmd is F_SETLK, 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.
Note that in the past this function was returned as EACCES.
EAGAIN: cmd is F_FREESP, the file exists, mandatory file/record locking is set, and there are outstanding record locks on the file.
EAGAIN: cmd is F_SETLK or F_SETLKW, mandatory file/record locking is set, and the file is currently being mapped to virtual memory using mmap.2
EBADF: fildes is not a valid open file descriptor.
EBADF: cmd is F_SETLK or F_SETLKW, 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 or F_SETLKW, 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, and fildes is not a valid file descriptor open for writing.
EDEADLK: cmd is F_SETLKW, 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, mandatory record locking is enabled, O_NDELAY and O_NONBLOCK are clear and a deadlock condition was detected.
EFAULT: cmd is F_FREESP and the third argument arg points to an illegal address.
EFAULT: cmd is F_GETLK, F_SETLK or F_SETLKW and the third argument points to an illegal address.
EINTR: A signal was caught during execution of the fcntl() function.
EINVAL: cmd is F_DUPFD 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 not a valid value.
EINVAL: cmd is F_GETLK, F_SETLK, or F_SETLKW and the third argument or the data it points to is not valid, or fildes refers to a file that does not support locking.
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.
ENOLCK: cmd is F_SETLK or F_SETLKW, 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, the file is on a remote machine, and the link to that machine is no longer active.
EOVERFLOW: cmd is F_GETLK and the process ID of the process holding the requested lock is too large to be stored in the l_pid field.

WARNINGS

Mandatory record locks are dangerous. If a runaway or otherwise out-of-control process should hold a mandatory lock on a file critical to the system and fail to release that lock, the entire system could hang or crash. For this reason, mandatory record locks may be removed in a future SunOS release. Use advisory record locking whenever possible.

NOTES

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

Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee exclusive access. Files can be accessed without advisory locks, but inconsistencies may result.

read.2 and write.2 system calls on files are affected by mandatory file and record locks (see chmod.2

Last modified 21/April/97