262 lines
6.1 KiB
Groff
262 lines
6.1 KiB
Groff
.TH FCNTL 2
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.SH NAME
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fcntl \- miscellaneous file descriptor control functions
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.SH SYNOPSIS
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.nf
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.ft B
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#include <fcntl.h>
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int fcntl(int \fIfd\fP, int \fIcmd\fP, \fR[\fP\fIdata\fP\fR]\fP)
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.ft P
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.fi
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.SH DESCRIPTION
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.de SP
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.if t .sp 0.4
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.if n .sp
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..
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.B Fcntl()
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performs several file descriptor related functions, like duplicating a file
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descriptor, setting the "close on exec" attribute, etc. The
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.I fd
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argument is the file descriptor to operate on,
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.I cmd
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is the command code of the operation to perform, and
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.I data
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is an optional argument to give or receive parameters. The command
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codes and other symbols and types are declared in <fcntl.h>. The commands
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are:
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.SP
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.BI "fcntl(" fd ", F_DUPFD, int " fd2 ")"
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.RS
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Returns a new file descriptor that is a duplicate of file descriptor
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.IR fd .
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It shares the same file pointer and the same file status flags, but has
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separate file descriptor flags that are initially off. The value of the
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duplicate file descriptor is the first free file descriptor greater than
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or equal to
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.IR fd2 .
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.RE
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.SP
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.BI "fcntl(" fd ", F_GETFD)"
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.RS
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Returns the file descriptor flags associated with file descriptor
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.IR fd .
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The flags are the "close on exec" flag
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.B FD_CLOEXEC
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that, when set, causes the file descriptor to be closed when the process
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executes another program. The Minix-vmd specific
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.B FD_ASYNCHIO
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flag marks a file descriptor for asynchronous I/O operation.
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.RE
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.SP
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.BI "fcntl(" fd ", F_SETFD, int " flags ")"
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.RS
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Set the file descriptor flags of
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.I fd
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to
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.IR flags .
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.RE
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.SP
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.BI "fcntl(" fd ", F_GETFL)"
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.RS
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Return the file status flags and file access modes associated with the file
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associated with file descriptor
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.IR fd .
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The file status flags are
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.B O_NONBLOCK
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(non blocking I/O) and
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.B O_APPEND
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(append mode). The file access modes are
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.B O_RDONLY
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(read-only),
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.B O_WRONLY
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(write-only) and
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.B O_RDWR
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(read-write). These flags are also used in the second argument of
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.BR open (2).
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.RE
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.SP
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.BI "fcntl(" fd ", F_SETFL, int " flags ")"
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.RS
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Set the file status flags of the file referenced by
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.I fd
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to
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.IR flags .
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Only
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.B O_NONBLOCK
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and
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.B O_APPEND
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may be changed. Access mode flags are ignored.
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.RE
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.SP
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The next four commands use a parameter of type
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.B struct flock
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that is defined in <fcntl.h> as:
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.SP
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.RS
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.nf
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.ta +4n +8n +12n
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struct flock {
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short l_type; /* F_RDLCK, F_WRLCK, or F_UNLCK */
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short l_whence; /* SEEK_SET, SEEK_CUR, or SEEK_END */
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off_t l_start; /* byte offset to start of segment */
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off_t l_len; /* length of segment */
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pid_t l_pid; /* process id of the locks' owner */
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};
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.fi
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.RE
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.SP
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This structure describes a segment of a file.
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.B L_type
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is the lock operation performed on the file segment:
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.B F_RDLCK
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to set a read lock,
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.B F_WRLCK
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to set a write lock, and
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.B F_UNLCK
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to remove a lock. Several processes may have a read lock on a segment, but
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only one process can have a write lock.
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.B L_whence
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tells if the
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.B l_start
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offset must be interpreted from the start of the file
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.RB ( SEEK_SET ),
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the current file position
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.RB ( SEEK_CUR ),
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or the end of the file
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.RB ( SEEK_END ).
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This is analogous to the third parameter of
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.BR lseek (2).
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These
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.B SEEK_*
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symbols are declared in <unistd.h>.
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.B L_start
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is the starting offset of the segment of the file.
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.B L_end
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is the length of the segment. If zero then the segment extends until end of
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file.
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.B L_pid
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is the process-id of the process currently holding a lock on the segment.
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It is returned by
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.BR F_GETLK .
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.SP
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.BI "fcntl(" fd ", F_GETLK, struct flock *" lkp ")"
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.RS
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Find out if some other process has a lock on a segment of the file
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associated by file descriptor
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.I fd
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that overlaps with the segment described by the
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.B flock
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structure pointed to by
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.IR lkp .
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If the segment is not locked then
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.B l_type
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is set to
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.BR F_UNLCK .
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Otherwise an
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.B flock
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structure is returned through
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.I lkp
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that describes the lock held by the other process.
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.B L_start
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is set relative to the start of the file.
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.RE
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.SP
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.BI "fcntl(" fd ", F_SETLK, struct flock *" lkp ")"
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.RS
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Register a lock on a segment of the file associated with file descriptor
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.IR fd .
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The file segment is described by the
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.B struct flock
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pointed to by
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.IR lkp .
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This call returns an error if any part of the segment is already locked.
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.RE
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.SP
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.BI "fcntl(" fd ", F_SETLKW, struct flock *" lkp ")"
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.RS
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Register a lock on a segment of the file associated with file descriptor
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.IR fd .
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The file segment is described by the
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.B struct flock
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pointed to by
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.IR lkp .
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This call blocks waiting for the lock to be released if any part of the
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segment is already locked.
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.RE
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.SP
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.BI "fcntl(" fd ", F_FREESP, struct flock *" lkp ")"
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.RS
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This call frees a segment of disk space occupied by the
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file associated with file descriptor
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.IR fd .
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The segment is described by the
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.B struct flock
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pointed to by
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.IR lkp .
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The file is truncated in length to the byte position indicated by
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.B l_start
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if
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.B l_len
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is zero. If
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.B l_len
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is nonzero then the file keeps its size, but the freed bytes now read as
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zeros. (Other than sharing the flock structure, this call has nothing to do
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with locking.) (This call is common among UNIX(-like) systems.)
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.RE
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.SP
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.BI "fcntl(" fd ", F_SEEK, u64_t " pos ")"
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.RS
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This Minix-vmd specific call sets the file position of the file associated
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with file descriptor
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.I fd
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to the byte offset indicated by the 64-bit number
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.IR pos .
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This is analogous to the call
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.SP
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.RS
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.BI "lseek(" fd ", " pos ", SEEK_SET)"
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.RE
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.SP
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except that
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.B F_SEEK
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can be used on devices larger than 4 gigabyte.
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.RE
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.SH "SEE ALSO"
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.BR open (2),
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.BR dup (2),
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.BR lseek (2),
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.BR ftruncate (3),
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.BR int64 (3).
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.SH DIAGNOSTICS
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.B Fcntl
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returns a file descriptor, flags, or
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.B 0
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to indicate success. On error
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.B \-1
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is returned, with
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.B errno
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set to the appropriate error code. The most notable errors are:
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.TP 5
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.B EINTR
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If a blocked
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.B F_SETLKW
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operation is interrupted by a signal that is caught.
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.TP
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.B EAGAIN
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By
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.B F_SETLK
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if a segment cannot be locked.
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.TP
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.B EBADF
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A bad file descriptor in general, or an attempt to place a write lock on a
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file that is not open for writing, etc.
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.TP
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.B ENOLCK
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No locks available, the file system code has run out of internal table
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space.
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.SH AUTHOR
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Kees J. Bot <kjb@cs.vu.nl>
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.\"
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.\" $PchId: fcntl.2,v 1.2 2000/08/11 19:39:51 philip Exp $
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