res = fcntl(fd, cmd, arg)
int fd, cmd, arg;
Fcntl provides for control over descriptors. The argument fd is a descriptor to be operated on by cmd as follows:
F_DUPFD Return a new descriptor as follows:
Lowest numbered available descriptor greater than or equal to arg.
Same object references as the original descriptor.
New descriptor shares the same file pointer if the object was a file.
Same access mode (read, write or read/write).
Same file status flags (i.e., both file descriptors share the same file status flags).
The close-on-exec flag associated with the new file descriptor is set to remain open across execv(2) system calls.
F_GETFD Get the close-on-exec flag associated with the file descriptor fd. If the low-order bit is 0, the file will remain open
across exec, otherwise the file will be closed upon execution of exec.
F_SETFD Set the close-on-exec flag associated with fd to the low order bit of arg (0 or 1 as above).
F_GETFL Get descriptor status flags, as described below.
F_SETFL Set descriptor status flags.
F_GETOWN Get the process ID or process group currently receiving SIGIO and SIGURG signals; process groups are returned as negative
F_SETOWN Set the process or process group to receive SIGIO and SIGURG signals; process groups are specified by supplying arg as nega-
tive, otherwise arg is interpreted as a process ID.
The flags for the F_GETFL and F_SETFL flags are as follows:
O_NONBLOCK Non-blocking I/O; if no data is available to a read call, or if a write operation would block, the call returns -1 with the
O_APPEND Force each write to append at the end of file; corresponds to the O_APPEND flag of open(2).
O_ASYNC Enable the SIGIO signal to be sent to the process group when I/O is possible, e.g., upon availability of data to be read.
Upon successful completion, the value returned depends on cmd as follows:
F_DUPFD A new file descriptor.
F_GETFD Value of flag (only the low-order bit is defined).
F_GETFL Value of flags.
F_GETOWN Value of file descriptor owner.
other Value other than -1.
Otherwise, a value of -1 is returned and errno is set to indicate the error.
Fcntl will fail if one or more of the following are true:
[EBADF] Fildes is not a valid open file descriptor.
[EMFILE] Cmd is F_DUPFD and the maximum allowed number of file descriptors are currently open.
[EINVAL] Cmd is F_DUPFD and arg is negative or greater than the maximum allowable number (see getdtablesize(2)).
[ESRCH] Cmd is F_SETOWN and the process ID given as argument is not in use.