--- name: "open" section: "3p" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux", "openbsd"] url: "https://new.carta.tech/open/3p" --- # open(3p) open file ## PROLOG This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. ## NAME open, openat — open file ## SYNOPSIS ``` #include #include ``` int open(const char \**path*, int *oflag*, ...); int openat(int *fd*, const char \**path*, int *oflag*, ...); ## DESCRIPTION The *open*() function shall establish the connection between a file and a file descriptor. It shall create an open file description that refers to a file and a file descriptor that refers to that open file description. The file descriptor is used by other I/O functions to refer to that file. The *path* argument points to a pathname naming the file. The *open*() function shall return a file descriptor for the named file, allocated as described in *Section 2.14*, *File Descriptor Allocation*. The open file description is new, and therefore the file descriptor shall not share it with any other process in the system. The FD\_CLOEXEC file descriptor flag associated with the new file descriptor shall be cleared unless the O\_CLOEXEC flag is set in *oflag*. The file offset used to mark the current position within the file shall be set to the beginning of the file. The file status flags and file access modes of the open file description shall be set according to the value of *oflag*. Values for *oflag* are constructed by a bitwise-inclusive OR of flags from the following list, defined in *\*. Applications shall specify exactly one of the first five values (file access modes) below in the value of *oflag*: - O\_EXEC Open for execute only (non-directory files). The result is unspecified if this flag is applied to a directory. - O\_RDONLY Open for reading only. - O\_RDWR Open for reading and writing. The result is undefined if this flag is applied to a FIFO. - O\_SEARCH Open directory for search only. The result is unspecified if this flag is applied to a non-directory file. - O\_WRONLY Open for writing only. Any combination of the following may be used: - O\_APPEND If set, the file offset shall be set to the end of the file prior to each write. - O\_CLOEXEC If set, the FD\_CLOEXEC flag for the new file descriptor shall be set. - O\_CREAT If the file exists, this flag has no effect except as noted under O\_EXCL below. Otherwise, if O\_DIRECTORY is not set the file shall be created as a regular file; the user ID of the file shall be set to the effective user ID of the process; the group ID of the file shall be set to the group ID of the file's parent directory or to the effective group ID of the process; and the access permission bits (see *\*) of the file mode shall be set to the value of the argument following the *oflag* argument taken as type **mode\_t** modified as follows: a bitwise AND is performed on the file-mode bits and the corresponding bits in the complement of the process' file mode creation mask. Thus, all bits in the file mode whose corresponding bit in the file mode creation mask is set are cleared. When bits other than the file permission bits are set, the effect is unspecified. The argument following the *oflag* argument does not affect whether the file is open for reading, writing, or for both. Implementations shall provide a way to initialize the file's group ID to the group ID of the parent directory. Implementations may, but need not, provide an implementation-defined way to initialize the file's group ID to the effective group ID of the calling process. - O\_DIRECTORY If *path* resolves to a non-directory file, fail and set *errno* to **\[ENOTDIR\]**. - O\_DSYNC Write I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. - O\_EXCL If O\_CREAT and O\_EXCL are set, *open*() shall fail if the file exists. The check for the existence of the file and the creation of the file if it does not exist shall be atomic with respect to other threads executing *open*() naming the same filename in the same directory with O\_EXCL and O\_CREAT set. If O\_EXCL and O\_CREAT are set, and *path* names a symbolic link, *open*() shall fail and set *errno* to **\[EEXIST\]**, regardless of the contents of the symbolic link. If O\_EXCL is set and O\_CREAT is not set, the result is undefined. - O\_NOCTTY If set and *path* identifies a terminal device, *open*() shall not cause the terminal device to become the controlling terminal for the process. If *path* does not identify a terminal device, O\_NOCTTY shall be ignored. - O\_NOFOLLOW If *path* names a symbolic link, fail and set *errno* to **\[ELOOP\]**. - O\_NONBLOCK When opening a FIFO with O\_RDONLY or O\_WRONLY set: - \* If O\_NONBLOCK is set, an *open*() for reading-only shall return without delay. An *open*() for writing-only shall return an error if no process currently has the file open for reading. - \* If O\_NONBLOCK is clear, an *open*() for reading-only shall block the calling thread until a thread opens the file for writing. An *open*() for writing-only shall block the calling thread until a thread opens the file for reading. When opening a block special or character special file that supports non-blocking opens: - \* If O\_NONBLOCK is set, the *open*() function shall return without blocking for the device to be ready or available. Subsequent behavior of the device is device-specific. - \* If O\_NONBLOCK is clear, the *open*() function shall block the calling thread until the device is ready or available before returning. Otherwise, the O\_NONBLOCK flag shall not cause an error, but it is unspecified whether the file status flags will include the O\_NONBLOCK flag. - O\_RSYNC Read I/O operations on the file descriptor shall complete at the same level of integrity as specified by the O\_DSYNC and O\_SYNC flags. If both O\_DSYNC and O\_RSYNC are set in *oflag*, all I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. If both O\_SYNC and O\_RSYNC are set in flags, all I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion. - O\_SYNC Write I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion. The O\_SYNC flag shall be supported for regular files, even if the Synchronized Input and Output option is not supported. - O\_TRUNC If the file exists and is a regular file, and the file is successfully opened O\_RDWR or O\_WRONLY, its length shall be truncated to 0, and the mode and owner shall be unchanged. It shall have no effect on FIFO special files or terminal device files. Its effect on other file types is implementation-defined. The result of using O\_TRUNC without either O\_RDWR or O\_WRONLY is undefined. - O\_TTY\_INIT If *path* identifies a terminal device other than a pseudo-terminal, the device is not already open in any process, and either O\_TTY\_INIT is set in *oflag* or O\_TTY\_INIT has the value zero, *open*() shall set any non-standard **termios** structure terminal parameters to a state that provides conforming behavior; see the Base Definitions volume of POSIX.1‐2017, *Section 11.2*, *Parameters that Can be Set*. It is unspecified whether O\_TTY\_INIT has any effect if the device is already open in any process. If *path* identifies the slave side of a pseudo-terminal that is not already open in any process, *open*() shall set any non-standard **termios** structure terminal parameters to a state that provides conforming behavior, regardless of whether O\_TTY\_INIT is set. If *path* does not identify a terminal device, O\_TTY\_INIT shall be ignored. If O\_CREAT and O\_DIRECTORY are set and the requested access mode is neither O\_WRONLY nor O\_RDWR, the result is unspecified. If O\_CREAT is set and the file did not previously exist, upon successful completion, *open*() shall mark for update the last data access, last data modification, and last file status change timestamps of the file and the last data modification and last file status change timestamps of the parent directory. If O\_TRUNC is set and the file did previously exist, upon successful completion, *open*() shall mark for update the last data modification and last file status change timestamps of the file. If both the O\_SYNC and O\_DSYNC flags are set, the effect is as if only the O\_SYNC flag was set. If *path* refers to a STREAMS file, *oflag* may be constructed from O\_NONBLOCK OR'ed with either O\_RDONLY, O\_WRONLY, or O\_RDWR. Other flag values are not applicable to STREAMS devices and shall have no effect on them. The value O\_NONBLOCK affects the operation of STREAMS drivers and certain functions applied to file descriptors associated with STREAMS files. For STREAMS drivers, the implementation of O\_NONBLOCK is device-specific. The application shall ensure that it specifies the O\_TTY\_INIT flag on the first open of a terminal device since system boot or since the device was closed by the process that last had it open. The application need not specify the O\_TTY\_INIT flag when opening pseudo-terminals. If *path* names the master side of a pseudo-terminal device, then it is unspecified whether *open*() locks the slave side so that it cannot be opened. Conforming applications shall call *unlockpt*() before opening the slave side. The largest value that can be represented correctly in an object of type **off\_t** shall be established as the offset maximum in the open file description. The *openat*() function shall be equivalent to the *open*() function except in the case where *path* specifies a relative path. In this case the file to be opened is determined relative to the directory associated with the file descriptor *fd* instead of the current working directory. If the access mode of the open file description associated with the file descriptor is not O\_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the access mode is O\_SEARCH, the function shall not perform the check. The *oflag* parameter and the optional fourth parameter correspond exactly to the parameters of *open*(). If *openat*() is passed the special value AT\_FDCWD in the *fd* parameter, the current working directory shall be used and the behavior shall be identical to a call to *open*(). ## RETURN VALUE Upon successful completion, these functions shall open the file and return a non-negative integer representing the file descriptor. Otherwise, these functions shall return -1 and set *errno* to indicate the error. If -1 is returned, no files shall be created or modified.\ ## ERRORS These functions shall fail if: - **EACCES** Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by *oflag* are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created, or O\_TRUNC is specified and write permission is denied. - **EEXIST** O\_CREAT and O\_EXCL are set, and the named file exists. - **EINTR** A signal was caught during *open*(). - **EINVAL** The implementation does not support synchronized I/O for this file. - **EIO** The *path* argument names a STREAMS file and a hangup or error occurred during the *open*(). - **EISDIR** The named file is a directory and *oflag* includes O\_WRONLY or O\_RDWR, or includes O\_CREAT without O\_DIRECTORY. - **ELOOP** A loop exists in symbolic links encountered during resolution of the *path* argument, or O\_NOFOLLOW was specified and the *path* argument names a symbolic link. - **EMFILE** All file descriptors available to the process are currently open. - **ENAMETOOLONG** The length of a component of a pathname is longer than {NAME\_MAX}. - **ENFILE** The maximum allowable number of files is currently open in the system. - **ENOENT** O\_CREAT is not set and a component of *path* does not name an existing file, or O\_CREAT is set and a component of the path prefix of *path* does not name an existing file, or *path* points to an empty string. - **ENOENT** or **ENOTDIR** O\_CREAT is set, and the *path* argument contains at least one non-\ character and ends with one or more trailing \ characters. If *path* without the trailing \ characters would name an existing file, an **\[ENOENT\]** error shall not occur. - **ENOSR** The *path* argument names a STREAMS-based file and the system is unable to allocate a STREAM. - **ENOSPC** The directory or file system that would contain the new file cannot be expanded, the file does not exist, and O\_CREAT is specified. - **ENOTDIR** A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory; or O\_CREAT and O\_EXCL are not specified, the *path* argument contains at least one non-\ character and ends with one or more trailing \ characters, and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory; or O\_DIRECTORY was specified and the *path* argument resolves to a non-directory file. - **ENXIO** O\_NONBLOCK is set, the named file is a FIFO, O\_WRONLY is set, and no process has the file open for reading. - **ENXIO** The named file is a character special or block special file, and the device associated with this special file does not exist. - **EOVERFLOW** The named file is a regular file and the size of the file cannot be represented correctly in an object of type **off\_t**. - **EROFS** The named file resides on a read-only file system and either O\_WRONLY, O\_RDWR, O\_CREAT (if the file does not exist), or O\_TRUNC is set in the *oflag* argument. The *openat*() function shall fail if: - **EACCES** The access mode of the open file description associated with *fd* is not O\_SEARCH and the permissions of the directory underlying *fd* do not permit directory searches. - **EBADF** The *path* argument does not specify an absolute path and the *fd* argument is neither AT\_FDCWD nor a valid file descriptor open for reading or searching. - **ENOTDIR** The *path* argument is not an absolute path and *fd* is a file descriptor associated with a non-directory file. These functions may fail if: - **EAGAIN** The *path* argument names the slave side of a pseudo-terminal device that is locked. - **EINVAL** The value of the *oflag* argument is not valid. - **ELOOP** More than {SYMLOOP\_MAX} symbolic links were encountered during resolution of the *path* argument. - **ENAMETOOLONG** The length of a pathname exceeds {PATH\_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH\_MAX}. - **ENOMEM** The *path* argument names a STREAMS file and the system is unable to allocate resources. - **EOPNOTSUPP** The *path* argument names a socket. - **ETXTBSY** The file is a pure procedure (shared text) file that is being executed and *oflag* is O\_WRONLY or O\_RDWR. *The following sections are informative.* ## EXAMPLES ### Opening a File for Writing by the Owner The following example opens the file **/tmp/file**, either by creating it (if it does not already exist), or by truncating its length to 0 (if it does exist). In the former case, if the call creates a new file, the access permission bits in the file mode of the file are set to permit reading and writing by the owner, and to permit reading only by group members and others. If the call to *open*() is successful, the file is opened for writing. ``` #include ... int fd; mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH; char *pathname = "/tmp/file"; ... fd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC, mode); ... ``` ### Opening a File Using an Existence Check The following example uses the *open*() function to try to create the **LOCKFILE** file and open it for writing. Since the *open*() function specifies the O\_EXCL flag, the call fails if the file already exists. In that case, the program assumes that someone else is updating the password file and exits. ``` #include #include #include ``` \#define LOCKFILE "/etc/ptmp" ... int pfd; /\* Integer for file descriptor returned by open() call. \*/ ... if ((pfd = open(LOCKFILE, O\_WRONLY \| O\_CREAT \| O\_EXCL,\ S\_IRUSR \| S\_IWUSR \| S\_IRGRP \| S\_IROTH)) == -1) {\ fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\\n");\ [exit(1)](/exit/1.md); } ... ### Opening a File for Writing The following example opens a file for writing, creating the file if it does not already exist. If the file does exist, the system truncates the file to zero bytes. ``` #include #include #include ``` \#define LOCKFILE "/etc/ptmp" ... int pfd; char pathname\[PATH\_MAX+1\]; ... if ((pfd = open(pathname, O\_WRONLY \| O\_CREAT \| O\_TRUNC,\ S\_IRUSR \| S\_IWUSR \| S\_IRGRP \| S\_IROTH)) == -1) {\ perror("Cannot open output file\\n"); [exit(1)](/exit/1.md); } ... ## APPLICATION USAGE POSIX.1‐2008 does not require that terminal parameters be automatically set to any state on first open, nor that they be reset after the last close. It is possible for a non-conforming application to leave a terminal device in a state where the next process to use that device finds it in a non-conforming state, but has no way of determining this. To ensure that the device is set to a conforming initial state, applications which perform a first open of a terminal (other than a pseudo-terminal) should do so using the O\_TTY\_INIT flag to set the parameters associated with the terminal to a conforming state. Except as specified in this volume of POSIX.1‐2017, the flags allowed in *oflag* are not mutually-exclusive and any number of them may be used simultaneously. Not all combinations of flags make sense. For example, using O\_SEARCH \| O\_CREAT will successfully open a pre-existing directory for searching, but if there is no existing file by that name, then it is unspecified whether a regular file will be created. Likewise, if a non-directory file descriptor is successfully returned, it is unspecified whether that descriptor will have execute permissions as if by O\_EXEC (note that it is unspecified whether O\_EXEC and O\_SEARCH have the same value). ## RATIONALE Some implementations permit opening FIFOs with O\_RDWR. Since FIFOs could be implemented in other ways, and since two file descriptors can be used to the same effect, this possibility is left as undefined. See *getgroups()* about the group of a newly created file. The use of *open*() to create a regular file is preferable to the use of *creat*(), because the latter is redundant and included only for historical reasons. The use of the O\_TRUNC flag on FIFOs and directories (pipes cannot be *open*()-ed) must be permissible without unexpected side-effects (for example, *creat*() on a FIFO must not remove data). Since terminal special files might have type-ahead data stored in the buffer, O\_TRUNC should not affect their content, particularly if a program that normally opens a regular file should open the current controlling terminal instead. Other file types, particularly implementation-defined ones, are left implementation-defined. POSIX.1‐2008 permits **\[EACCES\]** to be returned for conditions other than those explicitly listed. The O\_NOCTTY flag was added to allow applications to avoid unintentionally acquiring a controlling terminal as a side-effect of opening a terminal file. This volume of POSIX.1‐2017 does not specify how a controlling terminal is acquired, but it allows an implementation to provide this on *open*() if the O\_NOCTTY flag is not set and other conditions specified in the Base Definitions volume of POSIX.1‐2017, *Chapter 11*, *General Terminal Interface* are met. In historical implementations the value of O\_RDONLY is zero. Because of that, it is not possible to detect the presence of O\_RDONLY and another option. Future implementations should encode O\_RDONLY and O\_WRONLY as bit flags so that: ``` O_RDONLY | O_WRONLY == O_RDWR ``` O\_EXEC and O\_SEARCH are specified as two of the five file access modes. Since O\_EXEC does not apply to directories, and O\_SEARCH only applies to directories, their values need not be distinct. Since O\_RDONLY has historically had the value zero, implementations are not able to distinguish between O\_SEARCH and O\_SEARCH \| O\_RDONLY, and similarly for O\_EXEC. In general, the *open*() function follows the symbolic link if *path* names a symbolic link. However, the *open*() function, when called with O\_CREAT and O\_EXCL, is required to fail with **\[EEXIST\]** if *path* names an existing symbolic link, even if the symbolic link refers to a nonexistent file. This behavior is required so that privileged applications can create a new file in a known location without the possibility that a symbolic link might cause the file to be created in a different location. For example, a privileged application that must create a file with a predictable name in a user-writable directory, such as the user's home directory, could be compromised if the user creates a symbolic link with that name that refers to a nonexistent file in a system directory. If the user can influence the contents of a file, the user could compromise the system by creating a new system configuration or spool file that would then be interpreted by the system. The test for a symbolic link which refers to a nonexisting file must be atomic with the creation of a new file. In addition, the *open*() function refuses to open non-directories if the O\_DIRECTORY flag is set. This avoids race conditions whereby a user might compromise the system by substituting a hard link to a sensitive file (e.g., a device or a FIFO) while a privileged application is running, where opening a file even for read access might have undesirable side-effects. In addition, the *open*() function does not follow symbolic links if the O\_NOFOLLOW flag is set. This avoids race conditions whereby a user might compromise the system by substituting a symbolic link to a sensitive file (e.g., a device) while a privileged application is running, where opening a file even for read access might have undesirable side-effects. The POSIX.1‐1990 standard required that the group ID of a newly created file be set to the group ID of its parent directory or to the effective group ID of the creating process. FIPS 151‐2 required that implementations provide a way to have the group ID be set to the group ID of the containing directory, but did not prohibit implementations also supporting a way to set the group ID to the effective group ID of the creating process. Conforming applications should not assume which group ID will be used. If it matters, an application can use *chown*() to set the group ID after the file is created, or determine under what conditions the implementation will set the desired group ID. The purpose of the *openat*() function is to enable opening files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call to *open*(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using the *openat*() function it can be guaranteed that the opened file is located relative to the desired directory. Some implementations use the *openat*() function for other purposes as well. In some cases, if the *oflag* parameter has the O\_XATTR bit set, the returned file descriptor provides access to extended attributes. This functionality is not standardized here. ## FUTURE DIRECTIONS None. ## SEE ALSO *chmod()*, *close()*, *creat()*, *dirfd()*, *dup()*, *exec*, *fcntl()*, *fdopendir()*, *link()*, *lseek()*, *mkdtemp()*, *mknod()*, *read()*, *symlink()*, *umask()*, *unlockpt()*, *write()* The Base Definitions volume of POSIX.1‐2017, *Chapter 11*, *General Terminal Interface*, ***\***, ***\***, ***\*** ## COPYRIGHT Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting\_bugs.html .