OPEN(2) — SYSTEM CALLS

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_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. 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).

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).

There is a system enforced limit on the number of open file descriptors per process, whose value is returned by the getdtablesize(2) call.

RETURN VALUE

The value −1 is returned if an error occurs, and external variable errno is set to indicate the cause of the error. Otherwise a non-negative numbered file descriptor for the new open file is returned.

ERRORS

Open fails if:

[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 file.

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

[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.

[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 was specified and the file exists.

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

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

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