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fcntl

SYNOPSIS

int fcntl(int fd, int cmd);

int fcntl(int fd, int cmd, long arg);

PARAMETERS

fd: [in] the file descriptor affected by the call.

cmd: [in] the operation to apply on the file descriptor.

arg: [in] an optionnal argument to the operation.

DESCRIPTION

This call directly applies an operation on a file descriptor. The possible operations are:

F_DUPFD

Duplicates fd the the new file descriptor specified by arg. If arg specifies a file descriptor that was already opened, then the file descriptor is first closed. It has the same effect has dup2. See section dup and dup2

F_GETFD

Returns the close-on-exec flag of fd.

F_SETFD

Sets the close-on-exec flag of fd.

F_GETFL

Returns the file descriptor flags (as specified by open).

F_SETFL

Sets the file descriptor flags to arg. The only flags that can be modified are O_APPEND and O_NONBLOCK. See section creat and open

F_GETLK

Determine if the lock described by arg can be set on the file. If so, the l_type member is set to F_UNLCK. Otherwise, arg is modified to describe the lock preventing the set operation.

F_SETLK

Set the lock described by arg on the file or release an already existing lock.

F_SETLKW

Same as F_SETLK but block if the lock can not be set.

F_GETOWN

Returns the process id or the process group of a socket. A process group is returned as a negative value.

F_SETOWN

Sets the process id or the process group that owns a socket. The owner of the socket receives the SIGIO and SIGURG signals. A process group is specified by a negative value.

struct flock {
    short l_type;   /* read, write or unlock */
    short l_whence; /* how to interpret l_start */
    off_t l_start;  /* where to begin the locking area */
    off_t l_len;    /* the lenght of the area to lock */
    pid_t l_pid;    /* the pid of the task holding the lock:
                       returned by F_GETLK */
};

The l_whence member has the same meaning as for lseek. See section lseek l_type can take one of the following values:

F_RDLCK

for a shared read lock on.

F_WRLCK

for an exclusive write lock.

F_UNLCK

to unlock the region.

The system merges adjacent locking regions of the same type and owned by the same task. When a subregion inside a region is unlocked, the region is split in two parts.

RETURN VALUE

On success, it depends on the cmd parameter:

F_DUPFD

the new file descriptor.

F_GETFD

the value of the close-on-exec flag.

F_GETFL

the value of the file descriptor flags.

F_GETOWN

the owner of the file descriptor.

On error, the call returns -1 and errno is set to one of the following values:

• EINVAL: for F_DUPFD, arg is invalid or the maximum number of opened file descriptor is reached. For F_GETLK, specified a lock with F_UNLCK. Or unlock operation failed.
• EBADF: the file descriptor is invalid, or the task requested a lock for reading or writing while it does not have the corresponding read or write access right on the file.
• EAGAIN: impossible to set the lock.
• ENOLCK: impossible to allocate memory for the new lock.
• EFAULT, ERESTARTSYS.

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