NAME

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

USAGE

#include <sys/file.h>

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

DESCRIPTION

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

Path is the address of a string of ASCII characters representing a pathname, terminated by a null character.  The flags are formed from the logical or of 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 zero
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 and 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 dial-up line), the open returns immediately.  The first time the process’ attempts to perform I/O on the open file, it will block. 

RETURN VALUE

Upon successful completion, a non-negative integer called 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 {OPEN_MAX} file descriptors open simultaneously. 

NOTES

DOMAIN/IX does not recognize write-only files, i.e., modes with 2 in either the owner, group, or others position.  DOMAIN/IX will act as if the file is readable, even if mode is set to 2.  However, it does change the mode to include read permission for the owner.  See chmod(2). 

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 that 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. 

[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’ allocated address space. 

[ELOOP] The call encountered too many symbolic links 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 no carrier is present. 

RELATED INFORMATION

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