Node:Getting File Status Flags, Previous:Operating Modes, Up:File Status Flags
The fcntl
function can fetch or change file status flags.
int F_GETFL | Macro |
This macro is used as the command argument to fcntl , to
read the file status flags for the open file with descriptor
filedes.
The normal return value from In case of an error,
|
int F_SETFL | Macro |
This macro is used as the command argument to fcntl , to set
the file status flags for the open file corresponding to the
filedes argument. This command requires a third int
argument to specify the new flags, so the call looks like this:
fcntl (filedes, F_SETFL, new-flags) You can't change the access mode for the file in this way; that is, whether the file descriptor was opened for reading or writing. The normal return value from |
If you want to modify the file status flags, you should get the current
flags with F_GETFL
and modify the value. Don't assume that the
flags listed here are the only ones that are implemented; your program
may be run years from now and more flags may exist then. For example,
here is a function to set or clear the flag O_NONBLOCK
without
altering any other flags:
/* Set theO_NONBLOCK
flag of desc if value is nonzero, or clear the flag if value is 0. Return 0 on success, or -1 on error witherrno
set. */ int set_nonblock_flag (int desc, int value) { int oldflags = fcntl (desc, F_GETFL, 0); /* If reading the flags failed, return error indication now. */ if (oldflags == -1) return -1; /* Set just the flag we want to set. */ if (value != 0) oldflags |= O_NONBLOCK; else oldflags &= ~O_NONBLOCK; /* Store modified flag word in the descriptor. */ return fcntl (desc, F_SETFL, oldflags); }