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Linux 2.6 - man page for open (linux section 3posix)

OPEN(P) 			    POSIX Programmer's Manual				  OPEN(P)

NAME
       open - open a file

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

       int open(const char *path, int oflag, ...  );

DESCRIPTION
       The  open()  function shall establish the connection between a file and a file descriptor.
       It shall create an open file description that refers to a file and a file descriptor  that
       refers  to  that open file description. The file descriptor is used by other I/O functions
       to refer to that file. The path argument points to a pathname naming the file.

       The open() function shall return a file descriptor for the named file that is  the  lowest
       file descriptor not currently open for that process. The open file description is new, and
       therefore the file descriptor shall not share it with any other process in the system. The
       FD_CLOEXEC file descriptor flag associated with the new file descriptor shall be cleared.

       The  file  offset  used	to  mark the current position within the file shall be set to the
       beginning of the file.

       The file status flags and file access modes of the open	file  description  shall  be  set
       according to the value of oflag.

       Values  for  oflag  are	constructed by a bitwise-inclusive OR of flags from the following
       list, defined in <fcntl.h>. Applications shall specify exactly one of the first three val-
       ues (file access modes) below in the value of oflag:

       O_RDONLY
	      Open for reading only.

       O_WRONLY
	      Open for writing only.

       O_RDWR Open  for reading and writing. The result is undefined if this flag is applied to a
	      FIFO.

       Any combination of the following may be used:

       O_APPEND
	      If set, the file offset shall be set to the end of the file prior to each write.

       O_CREAT
	      If the file exists, this flag has no effect except as  noted  under  O_EXCL  below.
	      Otherwise,  the  file shall be created; the user ID of the file shall be set to the
	      effective user ID of the process; the group ID of the file  shall  be  set  to  the
	      group  ID  of  the  file's  parent  directory  or  to the effective group ID of the
	      process; and the access permission bits (see <sys/stat.h>) of the file  mode  shall
	      be set to the value of the third argument taken as type mode_t modified as follows:
	      a bitwise AND is performed on the file-mode bits and the corresponding bits in  the
	      complement of the process' file mode creation mask. Thus, all bits in the file mode
	      whose corresponding bit in the file mode creation mask is  set  are  cleared.  When
	      bits  other  than  the file permission bits are set, the effect is unspecified. The
	      third argument does not affect whether the file is open for  reading,  writing,  or
	      for  both. Implementations shall provide a way to initialize the file's group ID to
	      the group ID of the parent directory.  Implementations may, but need  not,  provide
	      an  implementation-defined  way  to initialize the file's group ID to the effective
	      group ID of the calling process.

       O_DSYNC
	      Write I/O operations on the file descriptor shall complete as defined  by  synchro-
	      nized I/O data integrity completion.

       O_EXCL If  O_CREAT and O_EXCL are set, open() shall fail if the file exists. The check for
	      the existence of the file and the creation of the file if it does not  exist  shall
	      be  atomic  with respect to other threads executing open() naming the same filename
	      in the same directory with O_EXCL and O_CREAT set. If O_EXCL and O_CREAT	are  set,
	      and  path  names	a  symbolic  link,  open()  shall fail and set errno to [EEXIST],
	      regardless of the contents of the symbolic link. If O_EXCL is set  and  O_CREAT  is
	      not set, the result is undefined.

       O_NOCTTY
	      If  set  and path identifies a terminal device, open() shall not cause the terminal
	      device to become the controlling terminal for the process.

       O_NONBLOCK
	      When opening a FIFO with O_RDONLY or O_WRONLY set:

	       * If O_NONBLOCK is set, an open() for reading-only shall return without delay.  An
		 open()  for  writing-only  shall return an error if no process currently has the
		 file open for reading.

	       * If O_NONBLOCK is clear, an open()  for  reading-only  shall  block  the  calling
		 thread  until	a  thread  opens the file for writing. An open() for writing-only
		 shall block the calling thread until a thread opens the file for reading.

       When opening a block special or character special file that supports non-blocking opens:

	       * If O_NONBLOCK is set, the open() function shall return without blocking for  the
		 device  to  be ready or available.  Subsequent behavior of the device is device-
		 specific.

	       * If O_NONBLOCK is clear, the open() function shall block the calling thread until
		 the device is ready or available before returning.

       Otherwise, the behavior of O_NONBLOCK is unspecified.

       O_RSYNC
	      Read  I/O  operations  on  the  file descriptor shall complete at the same level of
	      integrity as specified by the O_DSYNC and O_SYNC flags. If both O_DSYNC and O_RSYNC
	      are  set	in  oflag,  all  I/O  operations on the file descriptor shall complete as
	      defined by synchronized I/O data integrity completion. If both O_SYNC  and  O_RSYNC
	      are  set	in  flags,  all  I/O  operations on the file descriptor shall complete as
	      defined by synchronized I/O file integrity completion.

       O_SYNC Write I/O operations on the file descriptor shall complete as defined  by  synchro-
	      nized I/O file integrity completion.

       O_TRUNC
	      If  the  file  exists  and  is  a regular file, and the file is successfully opened
	      O_RDWR or O_WRONLY, its length shall be truncated to 0,  and  the  mode  and  owner
	      shall  be  unchanged.  It  shall	have  no effect on FIFO special files or terminal
	      device files. Its effect on other file types is implementation-defined. The  result
	      of using O_TRUNC with O_RDONLY is undefined.

       If  O_CREAT  is	set  and  the  file did not previously exist, upon successful completion,
       open() shall mark for update the st_atime, st_ctime, and st_mtime fields of the	file  and
       the st_ctime and st_mtime fields of the parent directory.

       If  O_TRUNC  is	set and the file did previously exist, upon successful completion, open()
       shall mark for update the st_ctime and st_mtime fields of the file.

       If both the O_SYNC and O_DSYNC flags are set, the effect is as if only the O_SYNC flag was
       set.

       If  path  refers  to  a	STREAMS file, oflag may be constructed from O_NONBLOCK OR'ed with
       either O_RDONLY, O_WRONLY, or O_RDWR. Other flag values	are  not  applicable  to  STREAMS
       devices	and  shall  have no effect on them. The value O_NONBLOCK affects the operation of
       STREAMS drivers and certain functions applied to file descriptors associated with  STREAMS
       files. For STREAMS drivers, the implementation of O_NONBLOCK is device-specific.

       If  path names the master side of a pseudo-terminal device, then it is unspecified whether
       open() locks the slave side so that it cannot be  opened.  Conforming  applications  shall
       call unlockpt() before opening the slave side.

       The  largest  value  that can be represented correctly in an object of type off_t shall be
       established as the offset maximum in the open file description.

RETURN VALUE
       Upon successful completion, the function shall open the file  and  return  a  non-negative
       integer	representing  the  lowest numbered unused file descriptor. Otherwise, -1 shall be
       returned and errno set to indicate the error. No files shall be created or modified if the
       function returns -1.

ERRORS
       The open() function shall fail if:

       EACCES Search  permission  is denied on a component of the path prefix, or the file exists
	      and the permissions specified by oflag are denied, or the file does not  exist  and
	      write  permission  is denied for the parent directory of the file to be created, or
	      O_TRUNC is specified and write permission is denied.

       EEXIST O_CREAT and O_EXCL are set, and the named file exists.

       EINTR  A signal was caught during open().

       EINVAL The implementation does not support synchronized I/O for this file.

       EIO    The path argument names a STREAMS file and a hangup or error  occurred  during  the
	      open().

       EISDIR The named file is a directory and oflag includes O_WRONLY or O_RDWR.

       ELOOP  A loop exists in symbolic links encountered during resolution of the path argument.

       EMFILE {OPEN_MAX} file descriptors are currently open in the calling process.

       ENAMETOOLONG
	      The  length  of  the  path  argument  exceeds {PATH_MAX} or a pathname component is
	      longer than {NAME_MAX}.

       ENFILE The maximum allowable number of files is currently open in the system.

       ENOENT O_CREAT is not set and the named file does not exist; or O_CREAT is set and  either
	      the path prefix does not exist or the path argument points to an empty string.

       ENOSR  The path argument names a STREAMS-based file and the system is unable to allocate a
	      STREAM.

       ENOSPC The directory or file system that would contain the new file  cannot  be	expanded,
	      the file does not exist, and O_CREAT is specified.

       ENOTDIR
	      A component of the path prefix is not a directory.

       ENXIO  O_NONBLOCK  is  set,  the named file is a FIFO, O_WRONLY is set, and no process has
	      the file open for reading.

       ENXIO  The named file is a character special or block special file, and the device associ-
	      ated with this special file does not exist.

       EOVERFLOW
	      The  named  file	is  a regular file and the size of the file cannot be represented
	      correctly in an object of type off_t.

       EROFS  The named file resides on a read-only file  system  and  either  O_WRONLY,  O_RDWR,
	      O_CREAT (if the file does not exist), or O_TRUNC is set in the oflag argument.

       The open() function may fail if:

       EAGAIN The path argument names the slave side of a pseudo-terminal device that is locked.

       EINVAL The value of the oflag argument is not valid.

       ELOOP  More  than  {SYMLOOP_MAX}  symbolic links were encountered during resolution of the
	      path argument.

       ENAMETOOLONG
	      As a result of encountering a symbolic link in resolution of the path argument, the
	      length of the substituted pathname string exceeded {PATH_MAX}.

       ENOMEM The  path  argument  names  a  STREAMS  file  and  the system is unable to allocate
	      resources.

       ETXTBSY
	      The file is a pure procedure (shared text) file that is being executed and oflag is
	      O_WRONLY or O_RDWR.

       The following sections are informative.

EXAMPLES
   Opening a File for Writing by the Owner
       The  following  example	opens  the  file /tmp/file, either by creating it (if it does not
       already exist), or by truncating its length to 0 (if it does exist). In the  former  case,
       if  the	call  creates a new file, the access permission bits in the file mode of the file
       are set to permit reading and writing by the owner, and to permit reading  only	by  group
       members and others.

       If the call to open() is successful, the file is opened for writing.

	      #include <fcntl.h>
	      ...
	      int fd;
	      mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
	      char *filename = "/tmp/file";
	      ...
	      fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, mode);
	      ...

   Opening a File Using an Existence Check
       The following example uses the open() function to try to create the LOCKFILE file and open
       it for writing. Since the open() function specifies the O_EXCL flag, the call fails if the
       file  already  exists. In that case, the program assumes that someone else is updating the
       password file and exits.

	      #include <fcntl.h>
	      #include <stdio.h>
	      #include <stdlib.h>

	      #define LOCKFILE "/etc/ptmp"
	      ...
	      int pfd; /* Integer for file descriptor returned by open() call. */
	      ...
	      if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
		  S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
	      {
		  fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
		  exit(1);
	      }
	      ...

   Opening a File for Writing
       The following example opens a file for writing, creating the file if it does  not  already
       exist. If the file does exist, the system truncates the file to zero bytes.

	      #include <fcntl.h>
	      #include <stdio.h>
	      #include <stdlib.h>

	      #define LOCKFILE "/etc/ptmp"
	      ...
	      int pfd;
	      char filename[PATH_MAX+1];
	      ...
	      if ((pfd = open(filename, O_WRONLY | O_CREAT | O_TRUNC,
		  S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
	      {
		  perror("Cannot open output file\n"); exit(1);
	      }
	      ...

APPLICATION USAGE
       None.

RATIONALE
       Except as specified in this volume of IEEE Std 1003.1-2001, the flags allowed in oflag are
       not mutually-exclusive and any number of them may be used simultaneously.

       Some implementations permit opening FIFOs with O_RDWR. Since FIFOs could be implemented in
       other  ways, and since two file descriptors can be used to the same effect, this possibil-
       ity is left as undefined.

       See getgroups() about the group of a newly created file.

       The use of open() to create a regular file is preferable to the use  of	creat(),  because
       the latter is redundant and included only for historical reasons.

       The  use  of the O_TRUNC flag on FIFOs and directories (pipes cannot be open()-ed) must be
       permissible without unexpected side effects (for example,  creat()  on  a  FIFO	must  not
       remove  data).  Since terminal special files might have type-ahead data stored in the buf-
       fer, O_TRUNC should not affect their content, particularly  if  a  program  that  normally
       opens  a  regular  file	should	open the current controlling terminal instead. Other file
       types, particularly implementation-defined ones, are left implementation-defined.

       IEEE Std 1003.1-2001 permits [EACCES] to be  returned  for  conditions  other  than  those
       explicitly listed.

       The  O_NOCTTY  flag  was  added to allow applications to avoid unintentionally acquiring a
       controlling terminal as a  side	effect	of  opening  a	terminal  file.  This  volume  of
       IEEE Std 1003.1-2001  does  not	specify  how  a  controlling terminal is acquired, but it
       allows an implementation to provide this on open() if the O_NOCTTY flag	is  not  set  and
       other conditions specified in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter
       11, General Terminal Interface are met. The O_NOCTTY flag is an	effective  no-op  if  the
       file being opened is not a terminal device.

       In  historical  implementations	the value of O_RDONLY is zero. Because of that, it is not
       possible to detect the presence of O_RDONLY and	another  option.  Future  implementations
       should encode O_RDONLY and O_WRONLY as bit flags so that:

	      O_RDONLY | O_WRONLY == O_RDWR

       In  general,  the open() function follows the symbolic link if path names a symbolic link.
       However, the open() function, when called with O_CREAT and O_EXCL,  is  required  to  fail
       with [EEXIST] if path names an existing symbolic link, even if the symbolic link refers to
       a nonexistent file.  This behavior is required so that privileged applications can  create
       a  new  file  in a known location without the possibility that a symbolic link might cause
       the file to be created in a different location.

       For example, a privileged application that must create a file with a predictable name in a
       user-writable  directory,  such	as the user's home directory, could be compromised if the
       user creates a symbolic link with that name that refers to a nonexistent file in a  system
       directory. If the user can influence the contents of a file, the user could compromise the
       system by creating a new system configuration or spool file that would then be interpreted
       by  the	system.  The  test for a symbolic link which refers to a nonexisting file must be
       atomic with the creation of a new file.

       The POSIX.1-1990 standard required that the group ID of a newly created file be set to the
       group  ID  of  its  parent directory or to the effective group ID of the creating process.
       FIPS 151-2 required that implementations provide a way to have the group ID be set to  the
       group ID of the containing directory, but did not prohibit implementations also supporting
       a way to set the group ID to the effective group ID of the  creating  process.  Conforming
       applications  should not assume which group ID will be used. If it matters, an application
       can use chown() to set the group ID after the file is created,  or  determine  under  what
       conditions the implementation will set the desired group ID.

FUTURE DIRECTIONS
       None.

SEE ALSO
       chmod()	, close() , creat() , dup() , fcntl() , lseek() , read() , umask() , unlockpt() ,
       write() , the Base Definitions volume of  IEEE Std 1003.1-2001,	<fcntl.h>,  <sys/stat.h>,
       <sys/types.h>

COPYRIGHT
       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					  OPEN(P)


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