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13.14.1 File Access Modes

The file access modes allow a file descriptor to be used for reading, writing, or both. (On GNU/Hurd systems, they can also allow none of these, and allow execution of the file as a program.) The access modes are chosen when the file is opened, and never change.

Macro: int O_RDONLY

Open the file for read access.

Macro: int O_WRONLY

Open the file for write access.

Macro: int O_RDWR

Open the file for both reading and writing.

On GNU/Hurd systems (and not on other systems), O_RDONLY and O_WRONLY are independent bits that can be bitwise-ORed together, and it is valid for either bit to be set or clear. This means that O_RDWR is the same as O_RDONLY|O_WRONLY. A file access mode of zero is permissible; it allows no operations that do input or output to the file, but does allow other operations such as fchmod. On GNU/Hurd systems, since “read-only” or “write-only” is a misnomer, fcntl.h defines additional names for the file access modes. These names are preferred when writing GNU-specific code. But most programs will want to be portable to other POSIX.1 systems and should use the POSIX.1 names above instead.

Macro: int O_READ

Open the file for reading. Same as O_RDONLY; only defined on GNU.

Macro: int O_WRITE

Open the file for writing. Same as O_WRONLY; only defined on GNU.

Macro: int O_EXEC

Open the file for executing. Only defined on GNU.

To determine the file access mode with fcntl, you must extract the access mode bits from the retrieved file status flags. On GNU/Hurd systems, you can just test the O_READ and O_WRITE bits in the flags word. But in other POSIX.1 systems, reading and writing access modes are not stored as distinct bit flags. The portable way to extract the file access mode bits is with O_ACCMODE.

Macro: int O_ACCMODE

This macro stands for a mask that can be bitwise-ANDed with the file status flag value to produce a value representing the file access mode. The mode will be O_RDONLY, O_WRONLY, or O_RDWR. (On GNU/Hurd systems it could also be zero, and it never includes the O_EXEC bit.)

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