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BSD 2.11 - man page for fcntl (bsd section 2)

FCNTL(2)							System Calls Manual							  FCNTL(2)

NAME
fcntl - file control
SYNOPSIS
#include <fcntl.h> res = fcntl(fd, cmd, arg) int res; int fd, cmd, arg;
DESCRIPTION
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 values. 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 error EWOULDBLOCK. 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.
RETURN VALUE
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.
ERRORS
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.
SEE ALSO
close(2), execve(2), getdtablesize(2), open(2), sigvec(2)
BUGS
The asynchronous I/O facilities of O_NONBLOCK and O_ASYNC are currently available only for tty and socket operations. 4.2 Berkeley Distribution Nov 30, 1994 FCNTL(2)
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