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.
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.
O_RDWR ; only defined on GNU.
O_WRONLY ; 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 .
O_RDONLY , O_WRONLY , or O_RDWR . (In the GNU system it could also be zero.)