1.\" $NetBSD: open.2,v 1.35 2006/09/23 15:29:09 wiz Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)open.2 8.2 (Berkeley) 11/16/93 31.\" 32.Dd August 11, 2005 33.Dt OPEN 2 34.Os 35.Sh NAME 36.Nm open 37.Nd open or create a file for reading or writing 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In fcntl.h 42.Ft int 43.Fn open "const char *path" "int flags" "mode_t mode" 44.Sh DESCRIPTION 45The file name specified by 46.Fa path 47is opened 48for reading and/or writing as specified by the 49argument 50.Fa flags 51and the file descriptor returned to the calling process. 52The 53.Fa flags 54are specified by 55.Em or Ns 'ing 56the values listed below. 57Applications must specify exactly one of the first three values 58(file access methods): 59.Bl -tag -offset indent -width O_NONBLOCK 60.It Dv O_RDONLY 61Open for reading only. 62.It Dv O_WRONLY 63Open for writing only. 64.It Dv O_RDWR 65Open for reading and writing. 66.El 67.Pp 68Any combination of the following may be used: 69.Bl -tag -offset indent -width O_NONBLOCK 70.It Dv O_NONBLOCK 71Do not block on open or for data to become available. 72.It Dv O_APPEND 73Append to the file on each write. 74.It Dv O_CREAT 75Create the file if it does not exist, in which case the file is 76created with mode 77.Ar mode 78as described in 79.Xr chmod 2 80and modified by the process' umask value (see 81.Xr umask 2 ) . 82.It Dv O_TRUNC 83Truncate size to 0. 84.It Dv O_EXCL 85Error if 86.Dv O_CREAT 87and the file already exists. 88.It Dv O_SHLOCK 89Atomically obtain a shared lock. 90.It Dv O_EXLOCK 91Atomically obtain an exclusive lock. 92.It Dv O_NOFOLLOW 93If last path element is a symlink, don't follow it. This option is 94provided for compatibility with other operating systems, but its 95security value is questionable. 96.It Dv O_DSYNC 97If set, write operations will be performed according to synchronized 98I/O data integrity completion: 99each write will wait for the file data to be committed to stable 100storage. 101.It Dv O_SYNC 102If set, write operations will be performed according to synchronized 103I/O file integrity completion: 104each write will wait for both the file data and file status to be 105committed to stable storage. 106.It Dv O_RSYNC 107If set, read operations will complete at the same level of 108integrity which is in effect for write operations: 109if specified together with 110.Dv O_SYNC , 111each read will wait for the file status to be committed to stable 112storage. 113.Pp 114Combining 115.Dv O_RSYNC 116with 117.Dv O_DSYNC 118only, or specifying it without any other synchronized I/O integrity 119completion flag set, has no further effect. 120.It O_ALT_IO 121Alternate I/O semantics will be used for read and write operations 122on the file descriptor. 123Alternate semantics are defined by the underlying layers and will not 124have any alternate effect in most cases. 125.It O_NOCTTY 126If the file is a terminal device, the opened device is not 127made the controlling terminal for the session. 128This flag has no effect on 129.Nx , 130since the system defaults to the abovementioned behaviour. 131The flag is present only for standards conformance. 132.El 133.Pp 134Opening a file with 135.Dv O_APPEND 136set causes each write on the file 137to be appended to the end. 138If 139.Dv O_TRUNC 140is specified and the 141file exists, the file is truncated to zero length. 142.Pp 143If 144.Dv O_EXCL 145is set with 146.Dv O_CREAT 147and the file already 148exists, 149.Fn open 150returns an error. 151This may be used to implement a simple exclusive access locking mechanism. 152If 153.Dv O_EXCL 154is set and the last component of the pathname is 155a symbolic link, 156.Fn open 157will fail even if the symbolic 158link points to a non-existent name. 159.Pp 160If the 161.Dv O_NONBLOCK 162flag is specified, do not wait for the device or file to be ready or 163available. 164If the 165.Fn open 166call would result 167in the process being blocked for some reason (e.g., waiting for 168carrier on a dialup line), 169.Fn open 170returns immediately. 171This flag also has the effect of making all subsequent I/O on the open file non-blocking. 172.Pp 173When opening a file, a lock with 174.Xr flock 2 175semantics can be obtained by setting 176.Dv O_SHLOCK 177for a shared lock, or 178.Dv O_EXLOCK 179for an exclusive lock. 180If creating a file with 181.Dv O_CREAT , 182the request for the lock will never fail 183(provided that the underlying filesystem supports locking). 184.Pp 185If 186.Fn open 187is successful, the file pointer used to mark the current position within 188the file is set to the beginning of the file. 189.Pp 190When a new file is created it is given the group of the directory 191which contains it. 192.Pp 193The new descriptor is set to remain open across 194.Xr execve 2 195system calls; see 196.Xr close 2 197and 198.Xr fcntl 2 . 199.Pp 200The system imposes a limit on the number of file descriptors 201open simultaneously by one process. 202Calling 203.Xr getdtablesize 3 204returns the current system limit. 205.Sh RETURN VALUES 206If successful, 207.Fn open 208returns a non-negative integer, termed a file descriptor. 209Otherwise, a value of \-1 is returned and 210.Va errno 211is set to indicate the error. 212.Sh ERRORS 213The named file is opened unless: 214.Bl -tag -width Er 215.It Bq Er ENOTDIR 216A component of the path prefix is not a directory. 217.It Bq Er ENAMETOOLONG 218A component of a pathname exceeded 219.Dv NAME_MAX 220characters, or an entire path name exceeded 221.Dv PATH_MAX 222characters. 223.It Bq Er ENOENT 224.Dv O_CREAT 225is not set and the named file does not exist, or 226a component of the path name that must exist does not exist. 227.It Bq Er EACCES 228Search permission is denied for a component of the path prefix, 229the required permissions (for reading and/or writing) 230are denied for the given flags, or 231.Dv O_CREAT 232is specified, 233the file does not exist, 234and the directory in which it is to be created 235does not permit writing. 236.It Bq Er ELOOP 237Too many symbolic links were encountered in translating the pathname. 238.It Bq Er EISDIR 239The named file is a directory, and the arguments specify 240it is to be opened for writing. 241.It Bq Er EROFS 242The named file resides on a read-only file system, 243and the file is to be modified. 244.It Bq Er EMFILE 245The process has already reached its limit for open file descriptors. 246.It Bq Er ENFILE 247The system file table is full. 248.It Bq Er ENXIO 249The named file is a character special or block 250special file, and the device associated with this special file 251does not exist, or 252the named file is a 253.Tn FIFO , 254.Dv O_NONBLOCK 255and 256.Dv O_WRONLY 257is set and no process has the file open for reading. 258.It Bq Er EINTR 259The 260.Fn open 261operation was interrupted by a signal. 262.It Bq Er EOPNOTSUPP 263.Dv O_SHLOCK 264or 265.Dv O_EXLOCK 266is specified but the underlying filesystem does not support locking. 267.It Bq Er ENOSPC 268.Dv O_CREAT 269is specified, 270the file does not exist, 271and the directory in which the entry for the new file is being placed 272cannot be extended because there is no space left on the file 273system containing the directory. 274.It Bq Er ENOSPC 275.Dv O_CREAT 276is specified, 277the file does not exist, 278and there are no free inodes on the file system on which the 279file is being created. 280.It Bq Er EDQUOT 281.Dv O_CREAT 282is specified, 283the file does not exist, 284and the directory in which the entry for the new file 285is being placed cannot be extended because the 286user's quota of disk blocks on the file system 287containing the directory has been exhausted. 288.It Bq Er EDQUOT 289.Dv O_CREAT 290is specified, 291the file does not exist, 292and the user's quota of inodes on the file system on 293which the file is being created has been exhausted. 294.It Bq Er EIO 295An I/O error occurred while making the directory entry or 296allocating the inode for 297.Dv O_CREAT . 298.It Bq Er ETXTBSY 299The file is a pure procedure (shared text) file that is being 300executed and the 301.Fn open 302call requests write access. 303.It Bq Er EFAULT 304.Fa path 305points outside the process's allocated address space. 306.It Bq Er EEXIST 307.Dv O_CREAT 308and 309.Dv O_EXCL 310were specified and the file exists. 311.It Bq Er EOPNOTSUPP 312An attempt was made to open a socket (not currently implemented). 313.El 314.Sh SEE ALSO 315.Xr chmod 2 , 316.Xr close 2 , 317.Xr dup 2 , 318.Xr lseek 2 , 319.Xr read 2 , 320.Xr umask 2 , 321.Xr write 2 , 322.Xr getdtablesize 3 323.Sh STANDARDS 324The 325.Fn open 326function conforms to 327.St -p1003.1-90 . 328The 329.Fa flags 330values 331.Dv O_DSYNC , 332.Dv O_SYNC 333and 334.Dv O_RSYNC 335are extensions defined in 336.St -p1003.1b-93 . 337.Pp 338The 339.Dv O_SHLOCK , 340.Dv O_EXLOCK , 341and 342.Dv O_NOFOLLOW 343flags are non-standard extensions and should not be used if portability 344is of concern. 345.Sh HISTORY 346An 347.Fn open 348function call appeared in 349.At v6 . 350