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RedHat 9 (Linux i386) - man page for open (redhat section 2)

OPEN(2) 				   System calls 				  OPEN(2)

NAME
       open, creat - open and possibly create a file or device

SYNOPSIS
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode);
       int creat(const char *pathname, mode_t mode);

DESCRIPTION
       The open() system call is used to convert a pathname into a file descriptor (a small, non-
       negative integer for use in subsequent I/O as with read, write, etc.).  When the  call  is
       successful,  the file descriptor returned will be the lowest file descriptor not currently
       open for the process.  This call creates a new  open  file,  not  shared  with  any  other
       process.   (But	shared	open  files may arise via the fork(2) system call.)  The new file
       descriptor is set to remain open across exec functions (see fcntl(2)).  The file offset is
       set to the beginning of the file.

       The  parameter flags is one of O_RDONLY, O_WRONLY or O_RDWR which request opening the file
       read-only, write-only or read/write, respectively, bitwise-or'd with zero or more  of  the
       following:

       O_CREAT
	      If  the file does not exist it will be created.  The owner (user ID) of the file is
	      set to the effective user ID of the process. The group ownership (group ID) is  set
	      either  to  the  effective group ID of the process or to the group ID of the parent
	      directory (depending on filesystem type and mount options, and the mode of the par-
	      ent  directory,  see,  e.g., the mount options bsdgroups and sysvgroups of the ext2
	      filesystem, as described in mount(8)).

       O_EXCL When used with O_CREAT, if the file already exists it is an error and the open will
	      fail.  In  this context, a symbolic link exists, regardless of where its points to.
	      O_EXCL is broken on NFS file systems, programs which  rely  on  it  for  performing
	      locking  tasks  will  contain a race condition.  The solution for performing atomic
	      file locking using a lockfile is to create a unique file	on  the  same  fs  (e.g.,
	      incorporating  hostname  and  pid),  use link(2) to make a link to the lockfile. If
	      link() returns 0, the lock is successful.  Otherwise, use  stat(2)  on  the  unique
	      file  to check if its link count has increased to 2, in which case the lock is also
	      successful.

       O_NOCTTY
	      If pathname refers to a terminal device -- see tty(4) -- it  will  not  become  the
	      process's controlling terminal even if the process does not have one.

       O_TRUNC
	      If  the  file already exists and is a regular file and the open mode allows writing
	      (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0.  If the file  is	a
	      FIFO  or terminal device file, the O_TRUNC flag is ignored. Otherwise the effect of
	      O_TRUNC is unspecified.  (On many Linux versions it will be ignored; on other  ver-
	      sions it will return an error.)

       O_APPEND
	      The  file  is  opened  in append mode. Before each write, the file pointer is posi-
	      tioned at the end of the file, as if with lseek.	O_APPEND may  lead  to	corrupted
	      files  on NFS file systems if more than one process appends data to a file at once.
	      This is because NFS does not support appending to a file, so the client kernel  has
	      to simulate it, which can't be done without a race condition.

       O_NONBLOCK or O_NDELAY
	      When  possible,  the  file is opened in non-blocking mode. Neither the open nor any
	      subsequent operations on the file descriptor which is returned will cause the call-
	      ing  process  to	wait.  For the handling of FIFOs (named pipes), see also fifo(4).
	      This mode need not have any effect on files other than FIFOs.

       O_SYNC The file is opened for synchronous I/O. Any writes on the resulting file descriptor
	      will  block  the	calling process until the data has been physically written to the
	      underlying hardware.  See RESTRICTIONS below, though.

       O_NOFOLLOW
	      If pathname is a symbolic link, then the open fails.  This is a FreeBSD  extension,
	      which  was added to Linux in version 2.1.126.  Symbolic links in earlier components
	      of the pathname will still be followed.  The headers from glibc 2.0.100  and  later
	      include a definition of this flag; kernels before 2.1.126 will ignore it if used.

       O_DIRECTORY
	      If  pathname  is	not a directory, cause the open to fail.  This flag is Linux-spe-
	      cific, and was added in kernel version 2.1.126, to avoid denial-of-service problems
	      if opendir(3) is called on a FIFO or tape device, but should not be used outside of
	      the implementation of opendir.

       O_DIRECT
	      Try to minimize cache effects of the I/O to and from this file.	In  general  this
	      will  degrade  performance,  but	it  is useful in special situations, such as when
	      applications do their own caching.  File I/O is done directly  to/from  user  space
	      buffers.	 The  I/O  is  synchronous,  i.e.,  at	the  completion of the read(2) or
	      write(2) system call, data is guaranteed to have been transferred.  Transfer sizes,
	      and the alignment of user buffer and file offset must all be multiples of the logi-
	      cal block size of the file system.
	      This flag is supported on a number of Unix-like systems; support	was  added  under
	      Linux in kernel version 2.4.10.
	      A semantically similar interface for block devices is described in raw(8).

       O_ASYNC
	      Generate	a  signal  (SIGIO  by default, but this can be changed via fcntl(2)) when
	      input or output becomes possible on this file descriptor.   This	feature  is  only
	      available  for  terminals,  pseudo-terminals, and sockets. See fcntl(2) for further
	      details.

       O_LARGEFILE
	      On 32-bit systems that support the Large Files System, allow files whose sizes can-
	      not be represented in 31 bits to be opened.

       Some of these optional flags can be altered using fcntl after the file has been opened.

       The  argument  mode  specifies the permissions to use in case a new file is created. It is
       modified by the process's umask in the usual way: the permissions of the created file  are
       (mode & ~umask).  Note that this mode only applies to future accesses of the newly created
       file; the open call that creates a read-only  file  may	well  return  a  read/write  file
       descriptor.

       The following symbolic constants are provided for mode:

       S_IRWXU
	      00700 user (file owner) has read, write and execute permission

       S_IRUSR (S_IREAD)
	      00400 user has read permission

       S_IWUSR (S_IWRITE)
	      00200 user has write permission

       S_IXUSR (S_IEXEC)
	      00100 user has execute permission

       S_IRWXG
	      00070 group has read, write and execute permission

       S_IRGRP
	      00040 group has read permission

       S_IWGRP
	      00020 group has write permission

       S_IXGRP
	      00010 group has execute permission

       S_IRWXO
	      00007 others have read, write and execute permission

       S_IROTH
	      00004 others have read permission

       S_IWOTH
	      00002 others have write permisson

       S_IXOTH
	      00001 others have execute permission

       mode must be specified when O_CREAT is in the flags, and is ignored otherwise.

       creat is equivalent to open with flags equal to O_CREAT|O_WRONLY|O_TRUNC.

RETURN VALUE
       open  and creat return the new file descriptor, or -1 if an error occurred (in which case,
       errno is set appropriately).  Note that open can open device special files, but creat can-
       not create them - use mknod(2) instead.

       On  NFS	file systems with UID mapping enabled, open may return a file descriptor but e.g.
       read(2) requests are denied with EACCES.  This is because  the  client  performs  open  by
       checking  the  permissions, but UID mapping is performed by the server upon read and write
       requests.

       If the file is newly created, its atime, ctime, mtime fields are set to the current  time,
       and  so are the ctime and mtime fields of the parent directory.	Otherwise, if the file is
       modified because of the O_TRUNC flag, its ctime and mtime fields are set  to  the  current
       time.

ERRORS
       EEXIST pathname already exists and O_CREAT and O_EXCL were used.

       EISDIR pathname	refers to a directory and the access requested involved writing (that is,
	      O_WRONLY or O_RDWR is set).

       EACCES The requested access to the file is not allowed, or one of the directories in path-
	      name  did  not allow search (execute) permission, or the file did not exist yet and
	      write access to the parent directory is not allowed.

       ENAMETOOLONG
	      pathname was too long.

       ENOENT O_CREAT is not set and the named file does not exist.  Or, a directory component in
	      pathname does not exist or is a dangling symbolic link.

       ENOTDIR
	      A  component  used  as  a  directory  in	pathname is not, in fact, a directory, or
	      O_DIRECTORY was specified and pathname was not a directory.

       ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the  file
	      open  for  reading.   Or,  the  file  is a device special file and no corresponding
	      device exists.

       ENODEV pathname refers to a device special file and no corresponding device exists.  (This
	      is a Linux kernel bug - in this situation ENXIO must be returned.)

       EROFS  pathname refers to a file on a read-only filesystem and write access was requested.

       ETXTBSY
	      pathname	refers to an executable image which is currently being executed and write
	      access was requested.

       EFAULT pathname points outside your accessible address space.

       ELOOP  Too many symbolic links were encountered in resolving pathname, or  O_NOFOLLOW  was
	      specified but pathname was a symbolic link.

       ENOSPC pathname	was  to be created but the device containing pathname has no room for the
	      new file.

       ENOMEM Insufficient kernel memory was available.

       EMFILE The process already has the maximum number of files open.

       ENFILE The limit on the total number of files open on the system has been reached.

CONFORMING TO
       SVr4, SVID, POSIX, X/OPEN, BSD 4.3 The O_NOFOLLOW and  O_DIRECTORY  flags  are  Linux-spe-
       cific.  One may have to define the _GNU_SOURCE macro to get their definitions.

RESTRICTIONS
       There  are  many  infelicities  in  the	protocol underlying NFS, affecting amongst others
       O_SYNC and O_NDELAY.

       POSIX provides for three different variants of  synchronised  I/O,  corresponding  to  the
       flags  O_SYNC,  O_DSYNC	and  O_RSYNC.  Currently (2.1.130) these are all synonymous under
       Linux.

SEE ALSO
       read(2), write(2), fcntl(2), close(2), link(2),	mknod(2),  mount(2),  stat(2),	umask(2),
       unlink(2), socket(2), fopen(3), fifo(4)

Linux					    1999-06-03					  OPEN(2)


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