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For most
file system In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
s, a program initializes access to a
file File or filing may refer to: Mechanical tools and processes * File (tool) A file is a tool used to remove fine amounts of material from a workpiece. It is common in woodworking, metalworking, and other similar trade and hobby tasks. Most are ...
in a
file system In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
using the open
system call In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and so ...
. This allocates resources associated to the file (the
file descriptor In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a unique identifier (Handle (computing), handle) for a file (computing), file or other input/output System resource, resource, such as a pipe (Unix) ...
), and returns a
handle A handle is a part of, or attachment to, an object that allows it to be ed and d by . The design of each type of handle involves substantial issues, even where these are dealt with intuitively or by following tradition. Handles for are an ...

handle
that the
process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process A business process, business method ...
will use to refer to that file. In some cases the open is performed by the first access. The same file may be opened simultaneously by several processes, and even by the same process, resulting in several file descriptors for the same file; depending on the file organization and filesystem. Operations on the descriptors such as moving the file pointer or closing it are independentthey do not affect other descriptors for the same file. Operations on the file, such as a
write Writing is a medium of human communication Communication (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area ...
, can be seen by operations on the other descriptors: a later read can read the newly written data. During the open, the filesystem may allocate memory for
buffers Buffer may refer to: Science * Buffer gas, an inert or nonflammable gas * Buffer solution, a solution used to prevent changes in pH * Buffering agent, the weak acid or base in a buffer solution * Lysis buffer, in cell biology * Metal ion buffer * M ...
, or it may wait until the first operation. The absolute file path is resolved. This may include connecting to a remote host and notifying an operator that a removable medium is required. It may include the initialization of a communication device. At this point an error may be returned if the host or medium is not available. The first access to at least the
directory Directory may refer to: * Directory (computing) In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and developmen ...
within the filesystem is performed. An error will usually be returned if the higher level components of the path (
directories Directory may refer to: * Directory (computing) In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and developmen ...
) cannot be located or accessed. An error will be returned if the file is expected to exist and it does not or if the file should not already exist and it does. If the file is expected to exist and it does, the file access, as restricted by permission flags within the file meta data or
access control list In computer security Computer security, cybersecurity, or information technology security (IT security) is the protection of computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic o ...
, is validated against the requested type of operations. This usually requires an additional filesystem access although in some filesystems meta-flags may be part of the directory structure. If the file is being created, the filesystem may allocate the default initial amount of storage or a specified amount depending on the file system capabilities. If this fails an error will be returned. Updating the directory with the new entry may be performed or it may be delayed until the close is performed. Various other errors which may occur during the open include directory update failures, un-permitted multiple connections, media failures, communication link failures and device failures. The return value must always be examined and an error specific action taken. In many cases programming language-specific run-time library opens may perform additional actions including initializing a run-time library structure related to the file. As soon as a file is no longer needed, the program should close it. This will cause run-time library and filesystem buffers to be updated to the physical media and permit other processes to access the data if exclusive use had been required. Some run-time libraries may close a file if the program calls the run-time exit. Some filesystems may perform the necessary operations if the program terminates. Neither of these is likely to take place in the event of a kernel or power failure. This can cause damaged filesystem structures requiring the running of privileged and lengthy filesystem utilities during which the entire filesystem may be inaccessible.


open call arguments

#The pathname to the file, #The kind of access requested on the file (read, write, append etc.), #The initial file permission is requested using the third argument called mode. This argument is relevant only when a new file is being created. After using the file, the process should close the file using close call, which takes the file descriptor of the file to be closed. Some filesystems include a disposition to permit releasing the file. Some computer languages include run-time libraries which include additional functionality for particular filesystems. The open (or some auxiliary routine) may include specifications for key size, record size, connection speed. Some open routines include specification of the program code to be executed in the event of an error.


Perl language form

open FILEHANDLE,MODE EXPR for example: open(my $fh, ">", "output.txt");
Perl Perl is a family of two high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific Objective (goal), goals of a systematic operation; and are applied in a wide range of contexts, such as, for ...
also uses the tie function of the Tie::File module to associate an array with a file. The tie::AnyDBM_File function associates a hash with a file.


C library POSIX definition

The
open
call is standardized by the
POSIX The Portable Operating System Interface (POSIX) is a family of standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requirement * St ...
specification for
C language C (, as in the C, letter ''c'') is a General-purpose language, general-purpose, procedural programming, procedural computer programming language supporting structured programming, lexical variable scope, and Recursion (computer science), recurs ...
: int open(const char *path, int oflag, .../*,mode_t mode */); int openat(int fd, const char *path, int oflag, ...); int creat(const char *path, mode_t mode); FILE *fopen(const char *restrict filename, const char *restrict mode); The value returned is a file descriptor which is a reference to a process specific structure which contains, among other things, a position pointer that indicates which place in the file will be acted upon by the next operation. Open may return
−1 In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...
indicating a failure with errno detailing the error. The file system also updates a global table of all open files which is used for determining if a file is currently in use by any process.


path

The name of the file to open. It includes the file path defining where, in which file system, the file is found (or should be created). openat expects a relative path.


oflag

This argument formed by OR'ing together optional parameters and (from < fcntl.h>) one of: :O_RDONLY, O_RDWR and O_WRONLY Option parameters include: :O_APPEND data written will be appended to the end of the file. The file operations will always adjust the position pointer to the end of the file. :O_CREAT Create the file if it does not exist; otherwise the open fails setting
errno errno.h is a header file Many programming languages and other computer files have a directive (programming), directive, often called include (sometimes copy or import), that causes the contents of a second file to be inserted into the original fi ...
to ENOENT. :O_EXCL Used with O_CREAT if the file already exists, then fail, setting errno to EEXIST. :O_TRUNC If the file already exists then discard its previous contents, reducing it to an empty file. Not applicable for a device or named pipe. Additional flags and errors are defined i
open
call. creat() is implemented as: int creat(const char *path, mode_t mode)

uses string flags such as r, w, a and + and returns a file pointer used wit

an


mode

Optional and relevant only when creating a new file, defines the
file permissionsMost file system In computing, a file system or filesystem (often abbreviated to fs) controls how data is Computer data storage, stored and retrieved. Without a file system, data placed in a storage medium would be one large body of data with no wa ...
. These include read, write or execute the file by the owner, group or all users. The mode is masked by the calling process's
umask In computing, is a command COMMAND.COM is the default command-line interpreter A command-line interface (CLI) processes commands to a computer program in the form of lines of text. The program which handles the interface is called a co ...

umask
: bits set in the umask are cleared in the mode.


See also

*
File descriptor In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a unique identifier (Handle (computing), handle) for a file (computing), file or other input/output System resource, resource, such as a pipe (Unix) ...
– how it works and other functions related to open


Notes


References

* Advanced Programming in the UNIX Environment by W. Richard Stevens *UNIX concept & application by Sumitabh Das {{Computer files C POSIX library System calls Articles with example C code