1.\" $NetBSD: open.2,v 1.39 2006/10/26 12:56:11 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 23, 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 EPERM 232The file's flags (see 233.Xr chflags 2 ) 234don't allow the file to be opened. 235.It Bq Er ENOTDIR 236A component of the path prefix is not a directory. 237.It Bq Er ENAMETOOLONG 238A component of a pathname exceeded 239.Dv NAME_MAX 240characters, or an entire path name exceeded 241.Dv PATH_MAX 242characters. 243.It Bq Er ENOENT 244.Dv O_CREAT 245is not set and the named file does not exist, or 246a component of the path name that must exist does not exist. 247.It Bq Er EACCES 248Search permission is denied for a component of the path prefix, 249the required permissions (for reading and/or writing) 250are denied for the given flags, or 251.Dv O_CREAT 252is specified, 253the file does not exist, 254and the directory in which it is to be created 255does not permit writing. 256.It Bq Er ELOOP 257Too many symbolic links were encountered in translating the pathname. 258.It Bq Er EISDIR 259The named file is a directory, and the arguments specify 260it is to be opened for writing. 261.It Bq Er EROFS 262The named file resides on a read-only file system, 263and the file is to be modified. 264.It Bq Er EMFILE 265The process has already reached its limit for open file descriptors. 266.It Bq Er ENFILE 267The system file table is full. 268.It Bq Er ENXIO 269The named file is a character special or block 270special file, and the device associated with this special file 271does not exist, or 272the named file is a 273.Tn FIFO , 274.Dv O_NONBLOCK 275and 276.Dv O_WRONLY 277is set and no process has the file open for reading. 278.It Bq Er EINTR 279The 280.Fn open 281operation was interrupted by a signal. 282.It Bq Er EOPNOTSUPP 283.Dv O_SHLOCK 284or 285.Dv O_EXLOCK 286is specified but the underlying filesystem does not support locking. 287.It Bq Er ENOSPC 288.Dv O_CREAT 289is specified, 290the file does not exist, 291and the directory in which the entry for the new file is being placed 292cannot be extended because there is no space left on the file 293system containing the directory. 294.It Bq Er ENOSPC 295.Dv O_CREAT 296is specified, 297the file does not exist, 298and there are no free inodes on the file system on which the 299file is being created. 300.It Bq Er EDQUOT 301.Dv O_CREAT 302is specified, 303the file does not exist, 304and the directory in which the entry for the new file 305is being placed cannot be extended because the 306user's quota of disk blocks on the file system 307containing the directory has been exhausted. 308.It Bq Er EDQUOT 309.Dv O_CREAT 310is specified, 311the file does not exist, 312and the user's quota of inodes on the file system on 313which the file is being created has been exhausted. 314.It Bq Er EIO 315An I/O error occurred while making the directory entry or 316allocating the inode for 317.Dv O_CREAT . 318.It Bq Er ETXTBSY 319The file is a pure procedure (shared text) file that is being 320executed and the 321.Fn open 322call requests write access. 323.It Bq Er EFAULT 324.Fa path 325points outside the process's allocated address space. 326.It Bq Er EEXIST 327.Dv O_CREAT 328and 329.Dv O_EXCL 330were specified and the file exists. 331.It Bq Er EOPNOTSUPP 332An attempt was made to open a socket (not currently implemented). 333.El 334.Sh SEE ALSO 335.Xr chmod 2 , 336.Xr close 2 , 337.Xr dup 2 , 338.Xr lseek 2 , 339.Xr read 2 , 340.Xr umask 2 , 341.Xr write 2 , 342.Xr getdtablesize 3 343.Sh STANDARDS 344The 345.Fn open 346function conforms to 347.St -p1003.1-90 . 348The 349.Fa flags 350values 351.Dv O_DSYNC , 352.Dv O_SYNC 353and 354.Dv O_RSYNC 355are extensions defined in 356.St -p1003.1b-93 . 357.Pp 358The 359.Dv O_SHLOCK , 360.Dv O_EXLOCK , 361and 362.Dv O_NOFOLLOW 363flags are non-standard extensions and should not be used if portability 364is of concern. 365.Sh HISTORY 366An 367.Fn open 368function call appeared in 369.At v6 . 370