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open(2)

open(2)                          System Calls                          open(2)



NAME
       open, openat - open a file

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

       int open(const char *path, int oflag, /* mode_t mode */);


       int openat(int fildes, const char *path, int oflag,
            /* mode_t mode */);


DESCRIPTION
       The  open()  function  establishes  the connection between a file and a
       file descriptor. It creates 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  openat()  function is identical to the open() function except that
       the path argument is interpreted relative to the starting point implied
       by  the  fildes  argument. If the fildes argument has the special value
       AT_FDCWD, a relative path argument will be  resolved  relative  to  the
       current working directory. If the path argument is absolute, the fildes
       argument is ignored.


       The open() function returns 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  does
       not  share  it with any other process in the system. The FD_CLOEXEC and
       FD_CLOFORK flags associated with the new file descriptor are cleared.


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


       The  file  status flags and file access modes of the open file descrip‐
       tion are set according to the value of oflag. The mode argument is used
       only when O_CREAT is specified (see below.)


       Values  for  oflag  are  constructed by a bitwise-inclusive-OR of flags
       from the following list, defined in <fcntl.h>. Applications must  spec‐
       ify  exactly  one of the first five values (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.


       O_EXEC      Open ordinary file for execute only.


       O_SEARCH    Open directory for search only.



       Any combination of the following may be used:

       O_APPEND

           If set, the file offset is set to the end of the file prior to each
           write.


       O_CLOEXEC

           If set, the FD_CLOEXEC flag is set for the new file descriptor.


       O_CLOFORK

           If set, the FD_CLOFORK flag is set for the new file descriptor.


       O_CREAT

           Create the file if it does not exist. This flag requires  that  the
           mode argument be specified.

           If  the  file exists, this flag has no effect except as noted under
           O_EXCL below.  Otherwise, the file is created with the user  ID  of
           the  file set to the effective user ID of the process. The group ID
           of the file is set to the effective group IDs of the process, or if
           the  S_ISGID bit is set in the directory in which the file is being
           created, the file's group ID is set to the group ID of  its  parent
           directory.   If  the  group  ID  of the new file does not match the
           effective group ID or one of  the  supplementary  groups  IDs,  the
           S_ISGID   bit   is   cleared.   The  access  permission  bits  (see
           <sys/stat.h>) of the file mode are set to the value of mode,  modi‐
           fied  as  follows (see creat(2)): a bitwise-AND is performed on the
           file-mode bits and the corresponding bits in the complement of  the
           process's  file  mode  creation  mask.  Thus,  all  bits set in the
           process's file mode creation mask (see  umask(2))  are  correspond‐
           ingly  cleared  in the file's permission mask. The "save text image
           after execution bit" of the mode is cleared (see chmod(2)).  O_SYNC
           Write  I/O operations on the file descriptor complete as defined by
           synchronized I/O file  integrity  completion   (see  fcntl.h(3HEAD)
           definition  of  O_SYNC.)  When  bits other than the file permission
           bits are set, the effect is unspecified. The mode argument does not
           affect whether the file is open for reading, writing or for both.


       O_DIRECTORY

           If  path  does not specify a directory, fail and set errno to ENOT‐
           DIR.


       O_DSYNC

           Write I/O operations on the file descriptor complete as defined  by
           synchronized I/O data integrity completion.


       O_EXCL

           If O_CREAT and O_EXCL are set, open() fails if the file exists. The
           check for the existence of the file and the creation of the file if
           it does not exist is 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() fails and sets 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_LARGEFILE

           If set, the offset maximum in the  open  file  description  is  the
           largest  value  that  can  be represented correctly in an object of
           type off64_t.


       O_NOCTTY

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


       O_NOFOLLOW

           If the path names a symbolic link, open() fails and sets  errno  to
           ELOOP.


       O_NOLINKS

           If the link count of the named file is greater than 1, open() fails
           and sets errno to EMLINK.


       O_NONBLOCK or O_NDELAY

           These flags can affect subsequent reads and writes (see read(2) and
           write(2)).  If  both  O_NDELAY  and  O_NONBLOCK are set, O_NONBLOCK
           takes precedence.

           When opening a FIFO with O_RDONLY or O_WRONLY set:

               o      If O_NONBLOCK or O_NDELAY is set, an open() for  reading
                      only  returns without delay.  An open() for writing only
                      returns an error if no process currently  has  the  file
                      open for reading.

               o      If  O_NONBLOCK  and  O_NDELAY  are  clear, an open() for
                      reading only blocks until a thread opens  the  file  for
                      writing.  An  open() for writing only blocks the calling
                      thread until a thread opens the file for reading.
           After both ends of a FIFO have been opened, there is  no  guarantee
           that  further  calls to open() O_RDONLY (O_WRONLY) will synchronize
           with later calls to open() O_WRONLY (O_RDONLY) until both  ends  of
           the  FIFO  have  been  closed by all readers and writers.  Any data
           written into a FIFO will be lost if  both  ends  of  the  FIFO  are
           closed before the data is read.

           When  opening  a  block special or character special file that sup‐
           ports non-blocking opens:

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

               o      If  O_NONBLOCK  and O_NDELAY are clear, the open() func‐
                      tion blocks the calling thread until the device is ready
                      or available before returning.
           Otherwise, the behavior of O_NONBLOCK and O_NDELAY is unspecified.


       O_RSYNC

           Read  I/O  operations  on  the file descriptor 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 complete as defined by  synchronized  I/O  data
           integrity completion.  If both O_SYNC and O_RSYNC are set in oflag,
           all I/O operations on the file descriptor complete  as  defined  by
           synchronized I/O file integrity completion.


       O_SYNC

           Write  I/O operations on the file descriptor complete as defined by
           synchronized I/O file integrity completion.


       O_TPDSAFE

           Allow opening a file for reading that is not MWAC-protected even if
           the process is part of the TPD.

           Allow  opening  STREAM  devices  or fifo when the peer is not a TPD
           process.


       O_TRUNC

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


       O_TTY_INIT

           The  O_TTY_INIT flag is ignored. Terminal devices are always opened
           in a state providing conforming behavior.


       O_XATTR

           If set in openat(), a relative path argument is  interpreted  as  a
           reference  to an extended attribute of the file associated with the
           supplied file descriptor.  This flag therefore requires  the  pres‐
           ence of a legal fildes argument. If set in open(), the implied file
           descriptor is that for  the  current  working  directory.  Extended
           attributes  must  be  referenced with a relative path; providing an
           absolute path results in a normal file reference.



       If O_CREAT is set and the file did not previously exist, upon  success‐
       ful  completion,  open()  marks  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() marks 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_NON‐
       BLOCK  or  O_NODELAY  OR-ed  with either O_RDONLY, O_WRONLY, or O_RDWR.
       Other flag values are not applicable to STREAMS  devices  and  have  no
       effect  on them.  The values O_NONBLOCK and O_NODELAY affect the opera‐
       tion of STREAMS drivers and certain functions (see read(2),  getmsg(2),
       putmsg(2),  and  write(2))  applied to file descriptors associated with
       STREAMS files.  For STREAMS drivers, the implementation  of  O_NONBLOCK
       and O_NODELAY is device-specific.


       When  open()  is  invoked to open a named stream, and the connld module
       (see connld(7M)) has been pushed on the pipe, open() blocks  until  the
       server  process  has  issued  an I_RECVFD ioctl() (see streamio(7I)) to
       receive the file descriptor.


       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.  Portable applications must call  unlockpt(3C)  before  opening
       the slave side.


       If the file is a regular file and the local file system is mounted with
       the nbmand mount option, then a mandatory share reservation is automat‐
       ically  obtained  on  the file. The share reservation is obtained as if
       fcntl(2) were called with cmd F_SHARE_NBMAND and  the  fshare_t  values
       set as follows:

       f_access    Set  to the type of read/write access for which the file is
                   opened.


       f_deny      F_NODNY


       f_id        The file descriptor value returned from open().



       If path is a symbolic link and O_CREAT and O_EXCL are set, the link  is
       not followed.


       Certain  flag  values  can  be  set  following  open()  as described in
       fcntl(2).


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

RETURN VALUES
       Upon successful completion, the open()  function  opens  the  file  and
       return  a  non-negative integer representing the lowest numbered unused
       file descriptor. Otherwise, −1 is returned, errno is  set  to  indicate
       the error, and no files are created or modified.

ERRORS
       The open() and openat() functions will fail if:

       EACCES          Search  permission is denied on a component of the path
                       prefix.

                       The file exists and the permissions specified by  oflag
                       are denied.

                       The  file does not exist and write permission is denied
                       for the parent directory of the file to be created.

                       The file is being opened for reading, the process is  a
                       TPD  process,  the  process flag PRIV_TPD_UNSAFE is not
                       set and the file can be modified by  non-TPD  processes
                       inside the immutable zone.

                       O_TRUNC is specified and write permission is denied.

                       The  {PRIV_FILE_DAC_SEARCH}  privilege allows processes
                       to search directories regardless  of  permission  bits.
                       The {PRIV_FILE_DAC_WRITE} privilege allows processes to
                       open files for writing regardless of  permission  bits.
                       See privileges(5) for special considerations when open‐
                       ing  files  owned   by   UID   0   for   writing.   The
                       {PRIV_FILE_DAC_READ} privilege allows processes to open
                       files for reading regardless of permission bits.

                       To open a file for reading or writing, the basic privi‐
                       leges  {PRIV_FILE_READ}  and {PRIV_FILE_WRITE}, respec‐
                       tively, need to be asserted in the effective set.


       EAGAIN          A mandatory share reservation  could  not  be  obtained
                       because  the  desired access conflicts with an existing
                       f_deny share reservation.


       EBADF           The file descriptor provided to openat() is invalid.


       EDQUOT          The file does not  exist,  O_CREAT  is  specified,  and
                       either  the directory where the new file entry is being
                       placed cannot be extended because the user's  quota  of
                       disk  blocks on that file system has been exhausted, or
                       the user's quota of inodes on the file system where the
                       file is being created has been exhausted.


       EEXIST          The O_CREAT and O_EXCL flags are set and the named file
                       exists.


       EILSEQ          The path argument includes non-UTF8 characters and  the
                       file  system  accepts only file names where all charac‐
                       ters are part of the UTF-8 character codeset.


       EINTR           A signal was caught during open().


       EFAULT          The path argument points to an illegal address.


       EINVAL          The system does not support synchronized I/O  for  this
                       file, or the O_XATTR flag was supplied and the underly‐
                       ing  file  system  does  not  support   extended   file
                       attributes.


       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           Too  many  symbolic links were encountered in resolving
                       path.

                       A loop exists in symbolic links encountered during res‐
                       olution of the path argument.

                       The  O_NOFOLLOW  flag is set and the final component of
                       path is a symbolic link.


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


       EMLINK          The O_NOLINKS flag is set and the named file has a link
                       count greater than 1.


       EMULTIHOP       Components of path require hopping to  multiple  remote
                       machines and the file system does not allow it.


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


       ENOEXEC         The O_EXEC access mode was specified and the file to be
                       opened is not an ordinary file.


       ENOLINK         The  path  argument points to a remote machine, and the
                       link to that machine is no longer active.


       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.


       ENOSYS          The  device specified by path does not support the open
                       operation.


       ENOTDIR         A component of the path prefix is not  a  directory,  a
                       relative  path  was  supplied  to openat(), the O_XATTR
                       flag was not supplied, and the file descriptor does not
                       refer  to  a  directory,  the  O_SEARCH access mode was
                       specified and the file to be opened is not a directory,
                       or O_DIRECTORY was specified and the path argument does
                       not specify a directory.


       ENXIO           The O_NONBLOCK flag is set, the named file is  a  FIFO,
                       the  O_WRONLY  flag is set, and no process has the file
                       open for reading; or the named file is a character spe‐
                       cial  or  block  special file and the device associated
                       with this special file  does  not  exist  or  has  been
                       retired by the fault management framework .


       EOPNOTSUPP      An  attempt was made to open a path that corresponds to
                       a AF_UNIX socket.


       EOVERFLOW       The named file is a regular file and either O_LARGEFILE
                       is  not  set  and the size of the file cannot be repre‐
                       sented correctly in an object of type off_t or O_LARGE‐
                       FILE  is  set and the size of the file cannot be repre‐
                       sented correctly in an object of type off64_t.


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



       The openat() function will fail if:

       EACCES    The permissions of the directory  underlying  fildes  do  not
                 permit directory searches.


       EBADF     The  path  argument does not specify an absolute path and the
                 fildes argument is neither AT_FDCWD nor a valid file descrip‐
                 tor open for reading or searching.



       The open() function may fail if:

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


       EINVAL          The value of the oflag argument is not valid.


       ENAMETOOLONG    Pathname resolution of  a  symbolic  link  produced  an
                       intermediate result whose length exceeds {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.


EXAMPLES
       Example 1 Open 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. 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);
         ...


       Example 2 Open 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 speci‐
       fies  the  O_EXCL  flag,  the call fails if the file already exists. In
       that case, the application 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);
         }
         ...


       Example 3 Open 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);
         }
         ...


USAGE
       The  open()  function has a transitional interface for 64-bit file off‐
       sets.  See lf64(5). Note that using open64()  is  equivalent  to  using
       open() with O_LARGEFILE set in oflag.

ATTRIBUTES
       See attributes(5) for descriptions of the following attributes:




       tab()   box;   cw(2.75i)  |cw(2.75i)  lw(2.75i)  |lw(2.75i)  ATTRIBUTE
       TYPEATTRIBUTE VALUE _ Interface StabilityCommitted _  MT-LevelAsync-
       Signal-Safe _ StandardFor open(), see standards(5).


SEE ALSO
       Intro(2),  chmod(2),  close(2),  creat(2),  dup(2),  exec(2), fcntl(2),
       getmsg(2), getrlimit(2), lseek(2),  putmsg(2),  read(2),  setpflags(2),
       stat(2),     umask(2),    write(2),    attropen(3C),    fcntl.h(3HEAD),
       stat.h(3HEAD),  unlockpt(3C),  attributes(5),  lf64(5),  privileges(5),
       standards(5), tpd(5), connld(7M), streamio(7I)

NOTES
       Hierarchical  Storage Management (HSM) file systems can sometimes cause
       long delays when opening a file, since HSM files must be recalled  from
       secondary storage.



SunOS 5.11                        9 Apr 2015                           open(2)
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