Access Modes - The GNU C Library

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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. (In the GNU system, 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.

In the GNU system (and not in 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 the GNU system, 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. In the GNU system, 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`. (In the GNU system it could also be zero, and it never includes the `O_EXEC` bit.)