gnu libc 13.15 File Status Flags

File status flags are used to specify attributes of the opening of a file. Unlike the file descriptor flags discussed in Descriptor Flags, the file status flags are shared by duplicated file descriptors resulting from a single opening of the file. The file status flags are specified with the flags argument to open; see Opening and Closing Files.

NOTE:

一、只有通过 open，才能够得到一个file descriptor；

File status flags fall into three categories, which are described in the following sections.

1、Access Modes, specify what type of access is allowed to the file: reading, writing, or both. They are set by open and are returned by fcntl, but cannot be changed.

2、Open-time Flags, control details of what open will do. These flags are not preserved after the open call.

3、Operating Modes, affect how operations such as read and write are done. They are set by open, and can be fetched or changed with fcntl.

The symbols in this section are defined in the header file fcntl.h.

• Access Modes:		Whether the descriptor can read or write.
• Open-time Flags:		Details of open.
• Operating Modes:		Special modes to control I/O operations.
• Getting File Status Flags:		Fetching and changing these flags.