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(libc.info)Access Modes

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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_RDWR'; only defined on GNU.

 - Macro: int O_WRITE
     Open the file for reading.  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.)