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FCNTL(2) BSD System Calls Manual FCNTL(2)

NAME

ffccnnttll - file control

SYNOPSIS

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int ffccnnttll(int fildes, int cmd, ...);

DESCRIPTION

FFccnnttll() provides for control over descriptors. The argument fildes is a descriptor to be operated on by cmd as follows: FDUPFD Return a new descriptor as follows: ++oo Lowest numbered available descriptor greater than or equal to arg. ++oo Same object references as the original descriptor. ++oo New descriptor shares the same file offset if the object was a file. ++oo Same access mode (read, write or read/write). ++oo Same file status flags (i.e., both file descriptors share the same file status flags).

++oo The close-on-exec flag associated with the new

file descriptor is set to remain open across execv(2) system calls.

FGETFD Get the close-on-exec flag associated with the file

descriptor fildes. If the low-order bit of the

returned value is 0, the file will remain open across

eexxeecc(), otherwise the file will be closed upon execu-

tion of eexxeecc() (arg is ignored).

FSETFD Set the close-on-exec flag associated with fildes to

the low order bit of arg (0 or 1 as above). FGETFL Get descriptor status flags, as described below (arg is ignored). FSETFL Set descriptor status flags to arg.

FGETOWN Get the process ID or process group currently receiv-

ing SIGIO and SIGURG signals; process groups are returned as negative values (arg is ignored). FSETOWN Set the process or process group to receive SIGIO and

SIGURG signals; process groups are specified by sup-

plying arg as negative, otherwise arg is interpreted as a process ID.

FGETPATH Get the path of the file descriptor Fildes. The argu-

ment must be a buffer of size MMAAXXPPAATTHHLLEENN or greater. FPREALLOCATE Preallocate file storage space. FSETSIZE Truncate a file without zeroing space. The calling process must have root privileges. FRDADVISE Issue an advisory read async with no copy to user. FRDAHEAD Turn read ahead off/on. A zero value in arg disables

read ahead. A non-zero value in arg turns read ahead

on. FREADBOOTSTRAP Read bootstrap from disk. FWRITEBOOTSTRAP Write bootstrap on disk. The calling process must have root privileges.

FNOCACHE Turns data caching off/on. A non-zero value in arg

turns data caching off. A value of zero in arg turns data caching on. FLOG2PHYS Get disk device information. Currently this only includes the disk device address that corresponds to the current file offset. FFULLFSYNC Does the same thing as fsync(2) then asks the drive to flush all buffered data to the permanent storage

device (arg is ignored). This is currently imple-

mented on HFS, MS-DOS (FAT), and Universal Disk Format

(UDF) file systems. The operation may take quite a while to complete. Certain FireWire drives have also been known to ignore the request to flush their buffered data. The flags for the FGETFL and FSETFL commands are as follows:

ONONBLOCK Non-blocking I/O; if no data is available to a read

call, or if a write operation would block, the read or

write call returns -1 with the error EAGAIN.

OAPPEND Force each write to append at the end of file; corre-

sponds to the OAPPEND flag of open(2). OASYNC Enable the SIGIO signal to be sent to the process group when I/O is possible, e.g., upon availability of data to be read. Several commands are available for doing advisory file locking; they all operate on the following structure: struct flock { offt lstart; /* starting offset */ offt llen; /* len = 0 means until end of file */ pidt lpid; /* lock owner */ short ltype; /* lock type: read/write, etc. */ short lwhence; /* type of lstart */ }; The commands available for advisory record locking are as follows: FGETLK 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 ffccnnttll 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 FUNLCK. FSETLK 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). FSETLK is used to establish shared (or read) locks (FRDLCK) or exclusive (or write) locks, (FWRLCK), as well as remove either type of lock (FUNLCK). If a shared or exclusive lock cannot be set, ffccnnttll returns immediately with EACCES. FSETLKW This command is the same as FSETLK 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 ffccnnttll is waiting for a region, the

ffccnnttll will be interrupted if the signal handler has not speci-

fied the SARESTART (see sigaction(2)). 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 lwhence is SEEKSET, SEEKCUR, or SEEKEND to indicate that the relative offset, lstart bytes, will be measured from the start of the file, current position, or end of the file, respectively. The value of llen is the number of consecutive bytes to be locked. If llen is negative, the result is undefined. The lpid field is only used with FGETLK to return the process ID of the process holding a blocking lock. After a successful FGETLK request, the value of lwhence is SEEKSET. 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 llen is set to zero. If lwhence and lstart point to the beginning of

the file, and llen is zero, the entire file is locked. If an applica-

tion wishes only to do entire file locking, the flock(2) 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 FSETLK or an FSETLKW request when the call-

ing process has previously existing locks on bytes in the region speci-

fied by the request, the previous lock type for each byte in the speci-

fied region is replaced by the new lock type. As specified above under the descriptions of shared locks and exclusive locks, an FSETLK or an FSETLKW 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 applica-

tions 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 getpwname(3) to retrieve a record, the lock will be lost because

getpwname(3) opens, reads, and closes the password database. The data-

base 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(2) function. The flock(2) interface has much more rational last close semantics and allows locks to be inherited by child processes. Flock(2) 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(2) and fcntl(2) locks may be safely used con-

currently. 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. The FPREALLOCATE command operates on the following structure: typedef struct fstore { uint32t fstflags; /* IN: flags word */ int fstposmode; /* IN: indicates offset field */ offt fstoffset; /* IN: start of the region */ offt fstlength; /* IN: size of the region */ offt fstbytesalloc; /* OUT: number of bytes allocated */ } fstoret; The flags (fstflags) for the FPREALLOCATE command are as follows: FALLOCATECONTIG Allocate contiguous space. FALLOCATEALL Allocate all requested space or no space at all. The position modes (fstposmode) for the FPREALLOCATE command indicate how to use the offset field. The modes are as follows: FPEOFPOSMODE Allocate from the physical end of file. FVOLPOSMODE Allocate from the volume offset. The FRDADVISE command operates on the following structure which holds information passed from the user to the system: struct radvisory { offt raoffset; /* offset into the file */ int racount; /* size of the read */ };

The FREADBOOTSTRAP and FWRITEBOOTSTRAP commands operate on the follow-

ing structure. typedef struct fbootstraptransfer { offt fbtoffset; /* IN: offset to start read/write */ sizet fbtlength; /* IN: number of bytes to transfer */ void *fbtbuffer; /* IN: buffer to be read/written */ } fbootstraptransfert; The FLOG2PHYS command operates on the following structure. struct log2phys { uint32t l2pflags; /* unused so far */ offt l2pcontigbytes; /* unused so far */ offt l2pdevoffset; /* bytes into device */ };

RETURN VALUES

Upon successful completion, the value returned depends on cmd as follows: FDUPFD A new file descriptor.

FGETFD Value of flag (only the low-order bit is defined).

FGETFL Value of flags. FGETOWN Value of file descriptor owner.

other Value other than -1.

Otherwise, a value of -1 is returned and errno is set to indicate the

error. EERRRROORRSS The ffccnnttll() system call will fail if: [EACCES] The argument cmd is FSETLK, the type of lock (ltype) is a shared lock (FRDLCK) or exclusive lock (FWRLCK), and the segment of a file to be locked is

already exclusive-locked by another process; or the

type is an exclusive lock and some portion of the seg-

ment of a file to be locked is already shared-locked

or exclusive-locked by another process.

The argument cmd is either FSETSIZE or FWRITEBOOTSTRAP and the calling process does not have root privileges. [EBADF] Fildes is not a valid open file descriptor. The argument cmd is FSETLK or FSETLKW, the type of lock (ltype) is a shared lock (FRDLCK), and fildes is not a valid file descriptor open for reading. The argument cmd is FSETLK or FSETLKW, the type of lock (ltype) is an exclusive lock (FWRLCK), and

fildes is not a valid file descriptor open for writ-

ing. The argument cmd is FPREALLOCATE and the calling process does not have file write permission. The argument cmd is FLOG2PHYS and fildes is not a valid file descriptor open for reading. [EDEADLK] The argument cmd is FSETLKW, and a deadlock condition was detected. [EINTR] The argument cmd is FSETLKW, and the function was interrupted by a signal. [EINVAL] Cmd is FDUPFD and arg is negative or greater than the maximum allowable number (see getdtablesize(2)). The argument cmd is FGETLK, FSETLK, or FSETLKW and the data to which arg points is not valid, or fildes refers to a file that does not support locking. The argument cmd is FPREALLOCATE and the fstposmode is not a valid mode, or when FPEOFPOSMODE is set and

fstoffset is a non-zero value, or when FVOLPOSMODE

is set and fstoffset is a negative or zero value. The argument cmd is either FREADBOOTSTRAP or FWRITEBOOTSTRAP and the operation was attempted on a

non-HFS disk type.

[EMFILE] Cmd is FDUPFD and the maximum allowed number of file descriptors are currently open. [EMFILE] The argument cmd is FDUPED and the maximum number of file descriptors permitted for the process are already in use, or no file descriptors greater than or equal to arg are available.

[ENOLCK] The argument cmd is FSETLK or FSETLKW, and satisfy-

ing the lock or unlock request would result in the number of locked regions in the system exceeding a

system-imposed limit.

[EOVERFLOW] A return value would overflow its representation. For example, cmd is FGETLK, FSETLK, or FSETLKW and the

smallest (or, if llen is non-zero, the largest) off-

set of a byte in the requested segment will not fit in an object of type offt. [ESRCH] Cmd is FSETOWN and the process ID given as argument is not in use.