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open(2) System Calls Manual open(2) NAME open, creat - Open a file for reading or writing SYNOPSIS #include <fcntl.h> #include <sys/stat.h> #include <sys/types.h> int open ( const char *path, int oflag [ , mode_t mode ] ); int creat ( const char *path, mode_t mode ); STANDARDS Interfaces documented on this reference page conform to industry stan- dards as follows: creat(): POSIX.1, XPG4, XPG4-UNIX open(): POSIX.1, XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry standards and associated tags. PARAMETERS Specifies the file to be opened or created. If the path parameter refers to a symbolic link, the open() function opens the file pointed to by the symbolic link. Specifies the type of access, special open processing, the type of update, and the initial state of the open file. The parameter value is constructed by logically ORing special open pro- cessing flags. These flags are defined in the fcntl.h header file and are described below. [DIGITAL] Specifies the read, write, and execute permissions of the file to be created (requested by the O_CREAT flag in the open() interface). If the file already exists, this parameter is ignored. This parameter is constructed by logically ORing values de- scribed in the sys/mode.h header file. DESCRIPTION The following two function calls are equivalent: creat(path, mode); open(path, O_WRONLY | O_CREAT | O_TRUNC, mode); The open() and creat() functions establish a connection between the file named by the path parameter and a file descriptor. The opened file descriptor is used by subsequent I/O functions, such as read() and write(), to access that file. The returned file descriptor is the lowest file descriptor not previ- ously open for that process. Typically, no process can have more than OPEN_MAX file descriptors open simultaneously. [DIGITAL] The per-process soft descriptor limit is checked. This num- ber is configurable; the minimum value is 64. See getrlimit(2) and setrlimit(2). The open() and creat() functions, which suspend the calling process un- til the request is completed, are redefined so that only the calling thread is suspended. The file offset, marking the current position within the file, is set to the beginning of the file. The new file descriptor is set to remain open across exec functions (see fcntl(2)). The file status flags and file access flags are designated by the oflag parameter. The oflag parameter is constructed by bitwise-inclusive OR- ing exactly one of the file access flags (O_RDONLY, O_WRONLY, or O_RDWR) with one or more of the file status flags. File Access Flags The file access flags are as follows: The file is open for reading only. The file is open for writing only. The file is open for reading and writing. Exactly one of the file access values (O_RDONLY, O_WRONLY, or O_RDWR) must be specified. If none is set, O_RDONLY is assumed. File Status Flags File status flags that specify special open processing are as follows: If the file exists, this flag has no effect except as noted under O_EXCL. If the file does not exist, a regular file is created with the following characteristics: The owner ID of the file is set to the ef- fective user ID of the process. The group ID of the file is set to the group ID of its parent directory. [DIGITAL] However, when the sys_v_mode tunable is set to 1, the group ID of the file is set to the group ID of the process. With exception, if the S_ISGID bit of the parent directory is set, the group ID of the file is set to the group ID of the par- ent directory. If the group ID of the new file does not match the effective group of the process or one of the supplementary group IDs of the process, the S_ISGID bit of the new file is cleared. The access permission bits of the file mode are set to the value of mode as follows: The corresponding bits are AND-ed with the complement of the file mode creation mask of the process. Refer to the System V Compatibility User's Guide for information on the System V habitat. The file permission and attribute bits are set to the value of the mode parameter, modified as follows: All bits set in the process file mode creation mask are cleared. The set-user ID attribute (S_ISUID bit) is cleared. The set- group ID attribute (S_ISGID bit) is cleared. The S_ISVTX at- tribute bit is cleared. The access control list of the new file is set to WILDCARD (discretionary access to the file according to traditional UNIX rules). The calling process must have write permission to the file's parent directory with respect to all access control policies to create a new file. If O_EXCL and O_CREAT are set, the open fails if the file exists. If the path parameter identifies a terminal device, this flag assures that the terminal device does not become the controlling terminal for the process. System V Compatibility [DIGITAL] In the DIGITAL UNIX operating system, O_NOCTTY is set by default and cannot be unset. In the System V habitat, how- ever, O_NOCTTY is not set by default, which means that a termi- nal device can become the controlling terminal for the process if both of the following conditions are met: The process does not already have a controlling terminal The terminal device pointed to by path is not already a controlling terminal If the file does not exist, this flag has no effect. If the file exists and is a regular file, and if the file is successfully opened O_RDWR or O_WRONLY: The length of the file is truncated to 0 (zero). The owner and group of the file are unchanged. The set-user ID attribute of the file mode is cleared. [DIGITAL] However, when the sys_v_mode tunable is set to 1, the file exists and is a regular file. Its length is truncated to 0 and the mode, owner, and group remain unchanged. The set-user ID attribute of the file is cleared. The open fails if either of the following conditions is true: The file supports enforced record locks and another process has locked a portion of the file. The file does not allow write ac- cess. A program can request some control over when updates should be made permanent for a regular file opened for write access. The calling process must have write access to the file with re- spect to all access policies. File status flags that specify special processing for subsequent reads and writes of the open file are as follows: If set, updates and writes to regular files and block devices are synchronous updates for data and file attribute information. On return from a function that performs an O_SYNC synchronous update (a write() system call when O_SYNC is set), the calling process is assured that all data and file attribute infor- mation for the file has been written to permanent storage, even if the file is also open for deferred update. If set, updates and writes to regular files and block devices are synchronous updates for data only. On return from a function that performs an O_DSYNC synchronous update (a write() system call when O_DSYNC is set), the calling process is as- sured that all data for the file has been written to permanent storage. Use of O_DSYNC does not guarantee that file-control information such as owner and modification time are updated to permanent storage. If set in combination with O_DSYNC, applies synchronized I/O data integrity completion to read operations. The calling process is assured that all pending writes of file data will have been written to permanent storage prior to a read, and that the data image has been successfully trans- ferred to the calling process. If set in combination with O_SYNC, applies synchronized I/O file integrity completion to read operations. The calling process is assured that all data and file attribute information will have been written to permanent storage prior to a read, and that the data image has been successfully transferred to the calling process. If O_RSYNC is used alone, it has no effect. If set, the file pointer is set to the end of the file prior to each write. If set, the call to open() will not block and subsequent read() or write() operations on the file will be nonblocking. General Notes on oflag Parameter Flag Values The effect of O_CREAT is immediate. When opening a FIFO with O_RDONLY: If neither O_NDELAY nor O_NONBLOCK is set, the open() function blocks until another process opens the file for writing. If the file is already open for writing (even by the call- ing process), the open() function returns without delay. If O_NDELAY or O_NONBLOCK is set, the open() function returns immediately. When opening a FIFO with O_WRONLY: If neither O_NDELAY nor O_NONBLOCK is set, the open() function blocks until another process opens the file for reading. If the file is already open for reading (even by the calling process), the open() function returns without delay. If O_NDE- LAY or O_NONBLOCK is set, the open() function returns an error if no process currently has the file open for reading. When opening a block special or character special file that supports nonblocking opens, such as a terminal device: If neither O_NDELAY nor O_NONBLOCK is set, the open() function blocks until the device is ready or available. If O_NDELAY or O_NONBLOCK is set, the open() function returns without waiting for the device to be ready or available. Subse- quent behavior of the device is device-specific. When opening a STREAMS file, oflag may be constructed from O_NDELAY or O_NONBLOCK OR-ed with either O_RDONLY, O_WRONLY or O_RDWR. Other flag values are not applicable to STREAMS devices and have no effect on them. The value of O_NDELAY or NON_BLOCK affects the operation of STREAMS drivers and certain system calls. (See read(2), getmsg(2), putmsg(2), and write(2).) For drivers, the implementation of O_NDELAY or NON_BLOCK is device-specific. Each STREAMS device driver may treat this option differently. RESTRICTIONS Since a file newly created by creat() is write_only, an fdopen() call using the r+ parameter fails when following a creat() call. A solution to this problem is to create the file using a call that adheres to the following format: open(path, O_RDWR | O_CREAT, 0666); RETURN VALUES On successful completion, the open() and creat() functions return the file descriptor, a nonnegative integer. Otherwise, a value of -1 is returned and errno is set to indicate the error. ERRORS If the open() or creat() function fails, errno may be set to one of the following values: Search permission is denied on a component of the path prefix, or the type of access specified by the oflag parameter is denied for the named file, or the file does not exist and write permis- sion is denied for the parent directory, or O_TRUNC is specified and write permission is denied. The O_TRUNC flag is set, the named file exists with enforced record locking enabled and there are record locks on the file. The named file is a block device file and the block de- vice is in use by a mounted file system. The directory in which the entry for the new link is being placed cannot be extended because the quota of disk blocks or i-nodes defined for the user on the file system containing the directory has been exhausted. The O_CREAT and O_EXCL flags are set and the named file exists. The path parameter is an in- valid address. A signal was caught by the open() function. The owner or group ID is not a value supported by this implementation. A hangup or error occurred during a STREAMS open(). The named file is a direc- tory and write access is requested. [DIGITAL] However, in a System V habitat, the named file is a directory and the oflag permission is write or read/write. Too many links were encountered in translating path. The system limit for open file descriptors per process has aleady reached OPEN_MAX. [DIGITAL] The system descriptor limit or the per-process soft descriptor limit has already been reached. Components of path require hopping to multiple remote machines. The length of the path string exceeds PATH_MAX or a pathname component is longer than NAME_MAX. [DIGITAL] The path parameter points to a remote machine and the link to that machine is no longer active. The system file table is full. The O_CREAT flag is not set and the named file does not exist; or O_CREAT is set and the path prefix does not exist; or the path parameter points to the empty string. [DIGITAL] The system was unable to allocate kernel mem- ory for more file descriptors. The directory that would contain the new file cannot be extended, the file does not exist, and O_CREAT is requested. Unable to allocate a stream. A component of the path prefix is not a directory. The named file is a character special or block special file and the device associ- ated with this special file does not exist. The named file is a multiplexed special file and the channel number is outside of the valid range or no more channels are available. The O_NONBLOCK flag is set, the named file is a FIFO, O_WRONLY is set, and no process has the file open for reading. A STREAMS module or driver open routine failed. The named file is a socket bound to the file system (a UNIX domain socket) and cannot be opened. The named file resides on a read-only file system and write access is required. [DIGITAL] For NFS file access, if the open() or creat() function fails, errno may also be set to one of the following values: Indicates a server's attempt to handle an NFS request by generating a request to another NFS server, which is not allowed. Indicates a stale NFS file handle. An opened file was deleted by the server or another client; a client cannot open a file because the server has unmounted or unex- ported the remote directory; or the directory that contains an opened file was unmounted or unexported by the server. Indicates the connec- tion timed out. For files that are mounted with the soft option, ei- ther the server is down or there is a network problem. RELATED INFORMATION Functions: chmod(2), close(2), fcntl(2), getmsg(2), lseek(2), putmsg(2), read(2), stat(2), truncate(2), umask(2), write(2), lockf(3) Standards: standards(5) delim off open(2)