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open system call wikipedia jump to content main menu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 open call arguments toggle open call arguments subsection 1 1 c library posix definition 1 1 1 path 1 1 2 oflag 1 1 3 mode 2 see also 3 notes 4 references toggle the table of contents open system call 4 languages català čeština euskara norsk bokmål edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia system calls in some operating systems this article includes a list of references related reading or external links but its sources remain unclear because it lacks inline citations please help improve this article by introducing more precise citations september 2025 learn how and when to remove this message in unix like operating systems a program initializes access to a file in a file system using the open system call this allocates resources associated to the file the file descriptor and returns a handle that the process 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 independent they do not affect other descriptors for the same file operations on the file such as a write 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 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 within the filesystem is performed an error will usually be returned if the higher level components of the path directories 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 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 edit 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 c library posix definition edit the open call is standardized by the posix specification for the c language int open const char path int oflag mode_t mode int openat int fd const char path int oflag mode_t mode 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 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 edit 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 edit 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 it has no effect except as noted under o_excl below 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 in open call creat is implemented as int creat const char path mode_t mode return open path o_wronly o_creat o_trunc mode fopen uses string flags such as r w a and and returns a file pointer used with fgets fputs and fclose mode edit optional and relevant only when creating a new file defines the file permissions 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 bits set in the umask are cleared in the mode see also edit file descriptor how it works and other functions related to open notes edit references edit advanced programming in the unix environment by w richard stevens isbn 81 7808 096 6 unix concept application by sumitabh das v t e computer files types binary file text file data file file format list of file formats list of file signatures magic number open file formats proprietary file formats metafile sidecar file sparse file swap file system file temporary file zero byte file properties filename 8 3 filename long filename filename mangling filename extension lists of filename extensions file attribute extended file attributes file size hidden file and hidden directory organisation directory folder ntfs links temporary folder directory structure file system filesystem hierarchy standard grid file system semantic file system path operations open close read write linking file descriptor hard link shortcut alias shadow symbolic link management backup file comparison file copying data compression file manager comparison of file managers file system fragmentation file system permissions file transfer file sharing file synchronization file verification retrieved from https en wikipedia org w index php title open_ system_call oldid 1322715763 categories c posix library system calls hidden categories articles with short description short description matches wikidata articles lacking in text citations from september 2025 all articles lacking in text citations articles with example c code this page was last edited on 17 november 2025 at 19 27 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by using this site you agree to the terms of use and privacy policy wikipedia is a registered trademark of the wikimedia foundation inc a non profit organization privacy policy about wikipedia disclaimers contact wikipedia legal safety contacts code of conduct developers statistics cookie statement mobile view search search toggle the table of contents open system call 4 languages add topic