GNU/Linux man pages
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GNU/Linux |
RedHat 5.2(Apollo) |
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open(2) |
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open, creat − open and possibly create a file or device
#include
<sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int
open(const char *pathname, int
flags);
int open(const char *pathname, int
flags, mode_t mode);
int creat(const char *pathname, mode_t
mode);
open attempts to open a file and return a file descriptor (a small, non-negative integer for use in read, write, etc.)
flags is one of O_RDONLY, O_WRONLY or O_RDWR which request opening the file read-only, write-only or read/write, respectively.
flags may also be bitwise-or’d with one or more of the following:
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O_CREAT |
If the file does not exist it will be created. | ||
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O_EXCL |
When used with O_CREAT, if the file already exists it is an error and the open will fail. O_EXCL is broken on NFS file systems, programs which rely on it for performing locking tasks will contain a race condition. The solution for performing atomic file locking using a lockfile is to create a unique file on the same fs (e.g., incorporating hostname and pid), use link(2) to make a link to the lockfile and use stat(2) on the unique file to check if its link count has increased to 2. Do not use the return value of the link() call. |
O_NOCTTY
If pathname refers to a terminal device — see tty(4) — it will not become the process’s controlling terminal even if the process does not have one.
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O_TRUNC |
If the file already exists it will be truncated. |
O_APPEND
The file is opened in append mode. Initially, and before each write, the file pointer is positioned at the end of the file, as if with lseek. O_APPEND may lead to corrupted files on NFS file systems if more than one process appends data to a file at once. This is because NFS does not support appending to a file, so the client kernel has to simulate it, which can’t be done without a race condition.
O_NONBLOCK or O_NDELAY
The file is opened in non-blocking mode. Neither the open nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait.
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O_SYNC |
The file is opened for synchronous I/O. Any writes on the resulting file descriptor will block the calling process until the data has been physically written to the underlying hardware. See RESTRICTIONS below, though. |
Some of these optional flags can be altered using fcntl after the file has been opened.
mode specifies the permissions to use if a new file is created. It is modified by the process’s umask in the usual way: the permissions of the created file are (mode & ~umask).
The following
symbolic constants are provided for mode:
S_IRWXU
00700 user (file owner) has read, write and execute permission
S_IRUSR (S_IREAD)
00400 user has read permission
S_IWUSR (S_IWRITE)
00200 user has write permission
S_IXUSR (S_IEXEC)
00100 user has execute permission
S_IRWXG
00070 group has read, write and execute permission
S_IRGRP
00040 group has read permission
S_IWGRP
00020 group has write permission
S_IXGRP
00010 group has execute permission
S_IRWXO
00007 others have read, write and execute permission
S_IROTH
00004 others have read permission
S_IWOTH
00002 others have write permisson
S_IXOTH
00001 others have execute permission
mode should always be specified when O_CREAT is in the flags, and is ignored otherwise.
creat is equivalent to open with flags equal to O_CREAT|O_WRONLY|O_TRUNC.
open and creat return the new file descriptor, or −1 if an error occurred (in which case, errno is set appropriately). Note that open can open device special files, but creat cannot create them - use mknod(2) instead.
On NFS file systems with UID mapping enabled, open may return a file descriptor but e.g. read(2) requests are denied with EACCES. This is because the client performs open by checking the permissions, but UID mapping is performed by the server upon read and write requests.
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EEXIST |
pathname already exists and O_CREAT and O_EXCL were used. | ||
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EISDIR |
pathname refers to a directory and the access requested involved writing. | ||
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ETXTBSY |
pathname refers to an executable image which is currently being executed and write access was requested. | ||
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EFAULT |
pathname points outside your accessible address space. | ||
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EACCES |
The requested access to the file is not allowed, or one of the directories in pathname did not allow search (execute) permission. |
ENAMETOOLONG
pathname was too long.
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ENOENT |
A directory component in pathname does not exist or is a dangling symbolic link. | ||
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ENOTDIR |
A component used as a directory in pathname is not, in fact, a directory. | ||
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EMFILE |
The process already has the maximum number of files open. | ||
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ENFILE |
The limit on the total number of files open on the system has been reached. | ||
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ENOMEM |
Insufficient kernel memory was available. | ||
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EROFS |
pathname refers to a file on a read-only filesystem and write access was requested. | ||
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ELOOP |
Too many symbolic links were encountered in resolving pathname. | ||
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ENOSPC |
pathname was to be created but the device containing pathname has no room for the new file. |
SVr4, SVID, POSIX, X/OPEN, BSD 4.3
There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC and O_NDELAY.
read(2), write(2), fcntl(2), close(2), unlink(2), mknod(2), stat(2), umask(2), mount(2), socket(2), socket(2), fopen(3), link(2).
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open(2) | |