NAME

open − open a file for reading or writing, or create a new file

SYNOPSIS

#include <sys/file.h>

open(path, flags, mode)
char ∗path;
int flags, mode;

DESCRIPTION

Open opens the file path for reading and/or writing, as specified by the flags argument and returns a descriptor for that file.  The flags argument may indicate the file is to be created if it does not already exist (by specifying the O_CREAT flag), in which case the file is created with mode mode as described in chmod(2) and modified by the process’ umask value (see umask(2)).

Path is the address of a string of ASCII characters representing a path name, terminated by a null character.  The flags specified are formed by or’ing the following values

 O_RDONLYopen for reading only
 O_WRONLYopen for writing only
 O_RDWRopen for reading and writing
 O_NDELAYdo not block on open
 O_APPENDappend on each write
 O_SYNCenable synchronous writes
 O_CREATcreate file if it does not exist
 O_TRUNCtruncate size to 0
 O_EXCLerror if create and file exists

Opening a file with O_APPEND set causes each write on the file to be appended to the end.  Opening a file with O_SYNC set causes all writes to be synchronous if the file is a regular or block special file.  If O_TRUNC is specified and the file exists, the file is truncated to zero length.  If O_EXCL is set with O_CREAT, then if the file already exists, the open returns an error.  This can be used to implement a simple exclusive access locking mechanism.  If the O_NDELAY flag is specified and the open call would result in the process being blocked for some reason (e.g. waiting for carrier on a dialup line), the open returns immediately.  The first time the process attempts to perform i/o on the open file it will block (not currently implemented, but will be added in a future release). 

Upon successful completion a non-negative integer termed a file descriptor is returned.  The file pointer used to mark the current position within the file is set to the beginning of the file. 

The new descriptor is set to remain open across execve system calls; see close(2).

No process may have more than 20 file descriptors open simultaneously. 

ERRORS

The named file is opened unless one or more of the following are true:

[EPERM] The pathname contains a character with the high-order bit set. 

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

[ENOENT] O_CREAT is not set and the named file does not exist. 

[EACCES] A component of the path prefix denies search permission. 

[EACCES] The required permissions (for reading and/or writing) are denied for the named flag. 

[EISDIR] The named file is a directory, and the arguments specify it is to be opened for writting. 

[EROFS] The named file resides on a read-only file system, and the file is to be modified. 

[EMFILE] {OPEN_MAX} file descriptors are currently open. 

[ENXIO] The named file is a character special or block special file, and the device associated with this special file does not exist. 

[ETXTBSY] The file is a pure procedure (shared text) file that is being executed and the open call requests write access. 

[EFAULT] Path points outside the process’s allocated address space. 

[ELOOP] Too many symbolic links were encountered in translating the pathname. 

[EEXIST] O_EXCL and O_CREAT were specified and the file already exists. 

[ENXIO] The O_NDELAY flag is given, and the file is a communications device on which their is no carrier present. 

[EOPNOTSUPP]
An attempt was made to open a socket (not currently implemented).

[EIO] A special file generated an I/O error. 

[EBUSY] A special file (typically a terminal device) is already open for exclusive access. 

[ENOENT] A component of the path name which must exist does not exist. 

[ENOENT] The path name is too long. 

[EACCES] O_CREAT is specified, the file does not exist, and the directory in which the entry for the file is to be created does not permit writing. 

[ENFILE] The system file table is full. 

[ENOSPC] O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because there is no space left on the file system containing the directory. 

[ENOSPC] O_CREAT is specified, the file does not exist, and there are no free inodes on teh file system in which the file is being created. 

[EIO] An I/O error occurred while reading from or writing to the file system. 

SEE ALSO

chmod(2), close(2), dup(2), lseek(2), read(2), write(2), umask(2)

4BSD