fcntl - 操作檔案描述符

內容簡介

#include <unistd.h> 
#include <fcntl.h> 

int fcntl(int fd, int cmd); int fcntl(int fd, int cmd, long arg); int fcntl(int fd, int cmd, struct flock *lock);

描述

fcntl() 執行下述就開檔案描述符fd的操作之一。該操作是由 cmd 確定。

複製一個檔案描述符

檔案描述符標誌

The following commands manipulate the flags associated with a file descriptor. Currently, only one such flag is defined: FD_CLOEXEC, the close-on-exec flag. If theFD_CLOEXEC bit is 0, the file descriptor will remain open across an execve(2), otherwise it will be closed.

檔案狀態標誌

Each open file description has certain associated status flags, initialized by open(2) and possibly modified by fcntl(2). Duplicated file descriptors (made with dup(),fcntl(F_DUPFD), fork(), etc.) refer to the same open file description, and thus share the same file status flags.

The file status flags and their semantics are described in open(2).

咨詢鎖

F_GETLK, F_SETLK and F_SETLKW are used to acquire, release, and test for the existence of record locks (also known as file-segment or file-region locks). The third argument lock is a pointer to a structure that has at least the following fields (in unspecified order).

struct flock {
    ...
    short l_type;    /* Type of lock: F_RDLCK,
                        F_WRLCK, F_UNLCK */
    short l_whence;  /* How to interpret l_start:
                        SEEK_SET, SEEK_CUR, SEEK_END */
    off_t l_start;   /* Starting offset for lock */
    off_t l_len;     /* Number of bytes to lock */
    pid_t l_pid;     /* PID of process blocking our lock
                        (F_GETLK only) */
    ...
};

The l_whence, l_start, and l_len fields of this structure specify the range of bytes we wish to lock. l_start is the starting offset for the lock, and is interpreted relative to either: the start of the file (if l_whence is SEEK_SET); the current file offset (if l_whenceis SEEK_CUR); or the end of the file (if l_whence is SEEK_END). In the final two cases,l_start can be a negative number provided the offset does not lie before the start of the file. l_len is a non-negative integer (but see the NOTES below) specifying the number of bytes to be locked. Bytes past the end of the file may be locked, but not bytes before the start of the file. Specifying 0 for l_len has the special meaning: lock all bytes starting at the location specified by l_whence and l_start through to the end of file, no matter how large the file grows.

The l_type field can be used to place a read (F_RDLCK) or a write (F_WRLCK) lock on a file. Any number of processes may hold a read lock (shared lock) on a file region, but only one process may hold a write lock (exclusive lock). An exclusive lock excludes all other locks, both shared and exclusive. A single process can hold only one type of lock on a file region; if a new lock is applied to an already-locked region, then the existing lock is converted to the new lock type. (Such conversions may involve splitting, shrinking, or coalescing with an existing lock if the byte range specified by the new lock does not precisely coincide with the range of the existing lock.)

In order to place a read lock, fd must be open for reading. In order to place a write lock,fd must be open for writing. To place both types of lock, open a file read-write.

As well as being removed by an explicit F_UNLCK, record locks are automatically released when the process terminates or if it closes any file descriptor referring to a file on which locks are held. This is bad: it means that a process can lose the locks on a file like /etc/passwd or /etc/mtab when for some reason a library function decides to open, read and close it.

Record locks are not inherited by a child created via fork(2), but are preserved across an execve(2).

Because of the buffering performed by the stdio(3) library, the use of record locking with routines in that package should be avoided; use read(2) and write(2) instead.

強制鎖

Advisory locks are not enforced and are useful only between cooperating processes.

Mandatory locks are enforced for all processes. If a process tries to perform an incompatible access (e.g., read(2) or write(2)) on a file region that has an incompatible mandatory lock, then the result depends upon whether the O_NONBLOCK flag is enabled for its open file description. If the O_NONBLOCK flag is not enabled, then system call is blocked until the lock is removed or converted to a mode that is compatible with the access. If the O_NONBLOCK flag is enabled, then the system call fails with the error EAGAIN or EWOULDBLOCK.

To make use of mandatory locks, mandatory locking must be enabled both on the file system that contains the file to be locked, and on the file itself. Mandatory locking is enabled on a file system using the "-o mand" option to mount(8), or theMS_MANDLOCK flag for mount(2). Mandatory locking is enabled on a file by disabling group execute permission on the file and enabling the set-group-ID permission bit (seechmod(1) and chmod(2)).

管理信號

F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O availability signals:

Using these mechanisms, a program can implement fully asynchronous I/O without using select(2) or poll(2) most of the time.

The use of O_ASYNC, F_GETOWN, F_SETOWN is specific to BSD and Linux. F_GETSIGand F_SETSIG are Linux-specific. POSIX has asynchronous I/O and the aio_sigeventstructure to achieve similar things; these are also available in Linux as part of the GNU C Library (Glibc).

租約

When a process (the "lease breaker") performs an open() or truncate() that conflicts with a lease established via F_SETLEASE, the system call is blocked by the kernel and the kernel notifies the lease holder by sending it a signal (SIGIO by default). The lease holder should respond to receipt of this signal by doing whatever cleanup is required in preparation for the file to be accessed by another process (e.g., flushing cached buffers) and then either remove or downgrade its lease. A lease is removed by performing anF_SETLEASE command specifying arg as F_UNLCK. If we currently hold a write lease on the file, and the lease breaker is opening the file for reading, then it is sufficient to downgrade the lease to a read lease. This is done by performing an F_SETLEASEcommand specifying arg as F_RDLCK.

If the lease holder fails to downgrade or remove the lease within the number of seconds specified in /proc/sys/fs/lease-break-time then the kernel forcibly removes or downgrades the lease holder’s lease.

Once the lease has been voluntarily or forcibly removed or downgraded, and assuming the lease breaker has not unblocked its system call, the kernel permits the lease breaker’s system call to proceed.

If the lease breaker’s blocked open() or truncate() is interrupted by a signal handler, then the system call fails with the error EINTR, but the other steps still occur as described above. If the lease breaker is killed by a signal while blocked in open() ortruncate(), then the other steps still occur as described above. If the lease breaker specifies the O_NONBLOCK flag when calling open(), then the call immediately fails with the error EWOULDBLOCK, but the other steps still occur as described above.

The default signal used to notify the lease holder is SIGIO, but this can be changed using the F_SETSIG command to fcntl(). If a F_SETSIG command is performed (even one specifying SIGIO), and the signal handler is established using SA_SIGINFO, then the handler will receive a siginfo_t structure as its second argument, and the si_fd field of this argument will hold the descriptor of the leased file that has been accessed by another process. (This is useful if the caller holds leases against multiple files).

檔案和目錄更改通知

返回值

對於一個成功的呼叫，返回值取決於操作：

On error, -1 is returned, and errno is set appropriately.

錯誤

注意

The errors returned by dup2() are different from those returned by F_DUPFD.

Since kernel 2.0, there is no interaction between the types of lock placed by flock(2) and fcntl(2).

POSIX.1-2001 allows l_len to be negative. (And if it is, the interval described by the lock covers bytes l_start+l_len up to and including l_start-1.) This is supported by Linux since Linux 2.4.21 and 2.5.49.

Several systems have more fields in struct flock such as e.g. l_sysid. Clearly, l_pid alone is not going to be very useful if the process holding the lock may live on a different machine.

BUGS

A limitation of the Linux system call conventions on some architectures (notably x86) means that if a (negative) process group ID to be returned by F_GETOWN falls in the range -1 to -4095, then the return value is wrongly interpreted by glibc as an error in the system call; that is, the return value of fcntl() will be -1, and errno will contain the (positive) process group ID.

In Linux 2.4 and earlier, there is bug that can occur when an unprivileged process usesF_SETOWN to specify the owner of a socket file descriptor as a process (group) other than the caller. In this case, fcntl() can return -1 with errno set to EPERM, even when the owner process (group) is one that the caller has permission to send signals to. Despite this error return, the file descriptor owner is set, and signals will be sent to the owner.

遵循於

SVr4, 4.3BSD, POSIX.1-2001. Only the operations F_DUPFD, F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK, F_SETLKW, F_GETOWN, and F_SETOWN are specified in POSIX.1-2001.

F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux specific. (Define the _GNU_SOURCE macro to obtain these definitions.)

另請參閱

• dup2 (2)

• flock (2)

• open (2)

• socket (2)