The open_flags argument has the same meaning as the flags argument of open() except that a database opened for write-only access opens the files for read and write access and the behavior of the O_APPEND flag is unspecified. The file_mode argument has the same meaning as the third argument of open().

Values for the open_flags argument are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>. Applications shall specify exactly one of the first three values (file access modes) below in the value of open_flags:

O_RDONLY

Open the database, and the underlying file(s) used to store the database, for reading only.

O_RDWR

Open the database, and the underlying file(s) used to store the database, for reading and writing.

O_WRONLY

Open the database for writing only. The underlying file(s) used to store the database shall be opened for reading and writing.

Any combination of the following can be used:

O_CLOEXEC

If set, the FD_CLOEXEC flag for the new file descriptor(s) used to open the underlying file(s) shall be set.

O_CREAT

If the database exists, this flag has no effect except as noted under O_EXCL below. Otherwise, the database shall be created; the user ID of the underlying file(s) shall be set to the effective user ID of the process; the group ID of the underlying file(s) shall be set to the group ID of the file's parent directory or to the effective group ID of the process. Implementations shall provide a way to initialize the group ID to the group ID of the parent directory. Implementations may, but need not, provide an implementation-defined way to initialize the group ID to the effective group ID of the calling process.

O_DSYNC

[SIO] Write I/O operations on the file(s) used to store the database shall complete as defined by synchronized I/O data integrity completion. [/SIO]

O_EXCL

If O_CREAT and O_EXCL are set, dbm_open() shall fail if the database exists according to the following criteria. If the database is stored in a file with a .db suffix, a check for the existence of the file and the creation of the file if it does not exist shall be performed as a single atomic operation. If the database is stored in files with .pag and .dir suffixes, this atomic existence check and creation operation shall be performed for each file, but it is unspecified which file is first. If the first file is successfully created and the second file is found to exist, the first file should be removed before dbm_open() returns.

If O_EXCL is set and O_CREAT is not set, the result is undefined.

O_RSYNC

[SIO] Read I/O operations on the file(s) used to store the database 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(s) 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(s) shall complete as defined by synchronized I/O file integrity completion. [/SIO]

O_SYNC

Write I/O operations on the file(s) used to store the database shall complete as defined by synchronized I/O file integrity completion.

O_TRUNC

If the database exists and is successfully opened O_RDWR or O_WRONLY, it shall be emptied, and the mode and owner of the underlying file(s) shall be unchanged. The result of using O_TRUNC without either O_RDWR or O_WRONLY is undefined.

The behaviour of other flags described for the flags argument of open() is unspecified.

The file_mode argument has the same meaning as the third argument of open() and shall apply to the underlying file(s) used to store the database.

None.

A future revision of this standard may mandate the file removal that is currently recommended (by the use of "should") in the description of O_EXCL.