1.\" $NetBSD: open.2,v 1.37 2006/10/13 00:19:21 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 October 5, 2006 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. 94This option is provided for compatibility with other operating 95systems, but its security 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 Dv 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 Dv 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.It Dv O_DIRECT 133If set on a regular file, data I/O operations will not buffer the data 134being transferred in the kernel's cache, but rather transfer the data 135directly between user memory and the underlying device driver if possible. 136This flag is advisory; the request may be performed in the normal 137buffered fashion if certain conditions are not met, e.g. if the request 138is not sufficiently aligned or if the file is mapped. 139.Pp 140To meet the alignment requirements for direct I/O, the file offset, 141the length of the I/O and the address of the buffer in memory must all 142be multiples of 143.Dv DEV_BSIZE 144(512 bytes). 145If the I/O request is made 146using an interface that supports scatter/gather via struct iovec, each 147element of the request must meet the above alignment constraints. 148.El 149.Pp 150Opening a file with 151.Dv O_APPEND 152set causes each write on the file 153to be appended to the end. 154If 155.Dv O_TRUNC 156is specified and the 157file exists, the file is truncated to zero length. 158.Pp 159If 160.Dv O_EXCL 161is set with 162.Dv O_CREAT 163and the file already 164exists, 165.Fn open 166returns an error. 167This may be used to implement a simple exclusive access locking mechanism. 168If 169.Dv O_EXCL 170is set and the last component of the pathname is 171a symbolic link, 172.Fn open 173will fail even if the symbolic 174link points to a non-existent name. 175.Pp 176If the 177.Dv O_NONBLOCK 178flag is specified, do not wait for the device or file to be ready or 179available. 180If the 181.Fn open 182call would result 183in the process being blocked for some reason (e.g., waiting for 184carrier on a dialup line), 185.Fn open 186returns immediately. 187This flag also has the effect of making all subsequent I/O on the open file non-blocking. 188.Pp 189When opening a file, a lock with 190.Xr flock 2 191semantics can be obtained by setting 192.Dv O_SHLOCK 193for a shared lock, or 194.Dv O_EXLOCK 195for an exclusive lock. 196If creating a file with 197.Dv O_CREAT , 198the request for the lock will never fail 199(provided that the underlying filesystem supports locking). 200.Pp 201If 202.Fn open 203is successful, the file pointer used to mark the current position within 204the file is set to the beginning of the file. 205.Pp 206When a new file is created it is given the group of the directory 207which contains it. 208.Pp 209The new descriptor is set to remain open across 210.Xr execve 2 211system calls; see 212.Xr close 2 213and 214.Xr fcntl 2 . 215.Pp 216The system imposes a limit on the number of file descriptors 217open simultaneously by one process. 218Calling 219.Xr getdtablesize 3 220returns the current system limit. 221.Sh RETURN VALUES 222If successful, 223.Fn open 224returns a non-negative integer, termed a file descriptor. 225Otherwise, a value of \-1 is returned and 226.Va errno 227is set to indicate the error. 228.Sh ERRORS 229The named file is opened unless: 230.Bl -tag -width Er 231.It Bq Er ENOTDIR 232A component of the path prefix is not a directory. 233.It Bq Er ENAMETOOLONG 234A component of a pathname exceeded 235.Dv NAME_MAX 236characters, or an entire path name exceeded 237.Dv PATH_MAX 238characters. 239.It Bq Er ENOENT 240.Dv O_CREAT 241is not set and the named file does not exist, or 242a component of the path name that must exist does not exist. 243.It Bq Er EACCES 244Search permission is denied for a component of the path prefix, 245the required permissions (for reading and/or writing) 246are denied for the given flags, or 247.Dv O_CREAT 248is specified, 249the file does not exist, 250and the directory in which it is to be created 251does not permit writing. 252.It Bq Er ELOOP 253Too many symbolic links were encountered in translating the pathname. 254.It Bq Er EISDIR 255The named file is a directory, and the arguments specify 256it is to be opened for writing. 257.It Bq Er EROFS 258The named file resides on a read-only file system, 259and the file is to be modified. 260.It Bq Er EMFILE 261The process has already reached its limit for open file descriptors. 262.It Bq Er ENFILE 263The system file table is full. 264.It Bq Er ENXIO 265The named file is a character special or block 266special file, and the device associated with this special file 267does not exist, or 268the named file is a 269.Tn FIFO , 270.Dv O_NONBLOCK 271and 272.Dv O_WRONLY 273is set and no process has the file open for reading. 274.It Bq Er EINTR 275The 276.Fn open 277operation was interrupted by a signal. 278.It Bq Er EOPNOTSUPP 279.Dv O_SHLOCK 280or 281.Dv O_EXLOCK 282is specified but the underlying filesystem does not support locking. 283.It Bq Er ENOSPC 284.Dv O_CREAT 285is specified, 286the file does not exist, 287and the directory in which the entry for the new file is being placed 288cannot be extended because there is no space left on the file 289system containing the directory. 290.It Bq Er ENOSPC 291.Dv O_CREAT 292is specified, 293the file does not exist, 294and there are no free inodes on the file system on which the 295file is being created. 296.It Bq Er EDQUOT 297.Dv O_CREAT 298is specified, 299the file does not exist, 300and the directory in which the entry for the new file 301is being placed cannot be extended because the 302user's quota of disk blocks on the file system 303containing the directory has been exhausted. 304.It Bq Er EDQUOT 305.Dv O_CREAT 306is specified, 307the file does not exist, 308and the user's quota of inodes on the file system on 309which the file is being created has been exhausted. 310.It Bq Er EIO 311An I/O error occurred while making the directory entry or 312allocating the inode for 313.Dv O_CREAT . 314.It Bq Er ETXTBSY 315The file is a pure procedure (shared text) file that is being 316executed and the 317.Fn open 318call requests write access. 319.It Bq Er EFAULT 320.Fa path 321points outside the process's allocated address space. 322.It Bq Er EEXIST 323.Dv O_CREAT 324and 325.Dv O_EXCL 326were specified and the file exists. 327.It Bq Er EOPNOTSUPP 328An attempt was made to open a socket (not currently implemented). 329.El 330.Sh SEE ALSO 331.Xr chmod 2 , 332.Xr close 2 , 333.Xr dup 2 , 334.Xr lseek 2 , 335.Xr read 2 , 336.Xr umask 2 , 337.Xr write 2 , 338.Xr getdtablesize 3 339.Sh STANDARDS 340The 341.Fn open 342function conforms to 343.St -p1003.1-90 . 344The 345.Fa flags 346values 347.Dv O_DSYNC , 348.Dv O_SYNC 349and 350.Dv O_RSYNC 351are extensions defined in 352.St -p1003.1b-93 . 353.Pp 354The 355.Dv O_SHLOCK , 356.Dv O_EXLOCK , 357and 358.Dv O_NOFOLLOW 359flags are non-standard extensions and should not be used if portability 360is of concern. 361.Sh HISTORY 362An 363.Fn open 364function call appeared in 365.At v6 . 366