NAME | SYNOPSIS | API RESTRICTIONS | FEATURES | DESCRIPTION | RETURN VALUES | ERRORS | ATTRIBUTES | SEE ALSO
#include <fcntl.h>int fcntl(int fildes, int cmd, ... /*arg */);
The function or functions documented here may not be used safely in all application contexts with all APIs provided in the ChorusOS 5.0 product.
See API(5FEA) for details.
MSDOSFS, NFS_CLIENT, UFS, POSIX_SOCKETS, POSIX_SHM
The fcntl system call provides a control mechanism for open files. The fildes filed contains a file descriptor obtained from an open(2POSIX), dup(2POSIX), accept(2POSIX), socket(2POSIX), socketpair(2POSIX), pipe(2POSIX), or shm_open(2POSIX) system call.
The commands available are:
Lowest numbered available descriptor greater than or equal to arg
Same object references as the original descriptor
New descriptor shares the same file offset if the object was a file
Same access mode (read, write or read/write)
Same file status flags (i.e., both file descriptors share the same file status flags)
The close-on-exec flag associated with the new file descriptor is set to remain open across afexec(2K) system calls.
Get the close-on-afexec flag associated with the file descriptor fildes. If the low-order bit is 0 the file will remain open across a local afexec(), otherwise the file will be closed upon execution of afexec().
Set the close-on-afexec flag associated with fildes to the low-order bit of arg (0 or 1 as above).
Get fildes status flags. This flag is not supported by message queue or shared memory objects.
Set fildes status flags to the integer value given as the third argument. Only the O_APPEND, O_NONBLOCK and O_SYNC flags can be set. This flag is not supported by message queue or shared memory objects.
The commands available for advisory record locking are as follows:
Get the first lock that blocks the lock description pointed to by the third argument, arg, taken as a pointer to a struct flock (see above). The information retrieved overwrites the information passed to fcntl() in the flock structure. If no lock is found that would prevent this lock from being created, the structure is left unchanged by this function call except for the lock type which is set to F_UNLCK.
Set or clear a file segment lock according to the lock description pointed to by the third argument, arg, taken as a pointer to a struct flock (see above). F_SETLK is used to establish shared (or read) locks (F_RDLCK) or exclusive (or write) locks, (F_WRLCK), as well as remove either type of lock (F_UNLCK). If a shared or exclusive lock cannot be set, fcntl() returns immediately with EAGAIN.
This command is the same as F_SETLK except that if a shared or exclusive lock is blocked by other locks, the process waits until the request can be satisfied. If a signal that is to be caught is received while fcntl() is waiting for a region, the fcntl() will be interrupted if the signal handler has not specified the SA_RESTART (see sigaction(2POSIX)).
When a shared lock has been set on a segment of a file, other processes can 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 was not opened with write access.
The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that the relative offset, l_start bytes, will be measured from the start of the file, current position, or end of the file, respectively. The value of l_len is the number of consecutive bytes to be locked. If l_len is negative, the result is undefined. 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 is SEEK_SET.
Locks may start and extend beyond the current end of a file, but may not start or extend before the beginning of the file. A lock is set to extend to the largest possible value of the file offset for that file if l_len is set to zero. If l_whence and l_start point to the beginning of the file, and l_len is zero, the entire file is locked. If an application wishes only to do entire file locking, the flock(2POSIX) system call is much more efficient.
There is at most one type of lock set for each byte in the file. Before a successful return from an F_SETLK or an F_SETLKW request when the calling process has previously existing locks on bytes in the region specified by the request, the previous lock type for each byte in the specified region is replaced by the new lock type. As specified above under the descriptions of shared locks and exclusive locks, an F_SETLK or an F_SETLKW request fails or blocks respectively when another process has existing locks on bytes in the specified region and the type of any of those locks conflicts with the type specified in the request.
This interface follows the completely stupid semantics of System V and IEEE Std 1003.1-1988 (``POSIX.1'') that require that all locks associated with a file for a given process are removed when any file descriptor for that file is closed by that process. This semantic means that applications must be aware of any files that a subroutine library may access. For example if an application for updating the password file locks the password file database while making the update, and then calls getpwnam(3STDC) to retrieve a record, the lock will be lost becausegetpwnam(3STDC) opens, reads, and closes the password database. The database close will release all locks that the process has associated with the database, even if the library routine never requested a lock on the database. Another minor semantic problem with this interface is that locks are not inherited by a child process created using the fork(2POSIX) function. Theflock(2POSIX) interface has much more rational last close semantics and allows locks to be inherited by child processes. flock(2POSIX) is recommended for applications that want to ensure the integrity of their locks when using library routines or wish to pass locks to their children. Note that flock(2POSIX) and fcntl(2POSIX) locks may be safely used concurrently.
All locks associated with a file for a given process are removed when the process terminates.
A potential for deadlock occurs if a process controlling a locked region is put to sleep by attempting to lock the locked region of another process. This implementation detects that sleeping until a locked region is unlocked would cause a deadlock and fails with an EDEADLK error.
Upon successful completion, the value returned depends on cmd as follows:
A new file descriptor
Value of the flag (only the low-order bit is defined)
Value other than -1
Value of file status flags. The return value will not be negative.
Value other than -1
Otherwise, a value of -1 is returned and errno is set to indicate one of the following error conditions.
fildes is not a valid open file descriptor, or is a valid message queue file descriptor.
cmd is not a valid command.
The F_GETLK, F_SETLK or F_SETLKW commands are applied to a non-regular file descriptor.
The F_GETOWN or F_SETOWN command was specified.
See attributes(5) for descriptions of the following attributes:
ATTRIBUTE TYPE | ATTRIBUTE VALUE |
---|---|
Interface Stability | Evolving |
MT-Level | Async-Signal-Safe |
accept(2POSIX), afexec(2K), close(2POSIX), dup(2POSIX), open(2POSIX), pipe(2POSIX), socket(2POSIX), shm_open(2POSIX), socketpair(2POSIX)
NAME | SYNOPSIS | API RESTRICTIONS | FEATURES | DESCRIPTION | RETURN VALUES | ERRORS | ATTRIBUTES | SEE ALSO