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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.
Open the file for read access.
Open the file for write access.
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.
Open the file for reading. Same as O_RDONLY; only defined on GNU.
Open the file for writing. Same as O_WRONLY; only defined on GNU.
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.
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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