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fcntl(2) [bsd man page]

FCNTL(2)							System Calls Manual							  FCNTL(2)

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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DUP(2)							      BSD System Calls Manual							    DUP(2)

dup, dup2, dup3 -- duplicate an existing file descriptor LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> int dup(int oldd); int dup2(int oldd, int newd); int dup3(int oldd, int newd, int flags); DESCRIPTION
dup() duplicates an existing object descriptor and returns its value to the calling process (newd = dup(oldd)). The argument oldd is a small non-negative integer index in the per-process descriptor table. The value must be less than the size of the table, which is returned by getdtablesize(3). The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process. The object referenced by the descriptor does not distinguish between oldd and newd in any way. Thus if newd and oldd are duplicate refer- ences to an open file, read(2), write(2) and lseek(2) calls all move a single pointer into the file, and append mode, non-blocking I/O and asynchronous I/O options are shared between the references. If a separate pointer into the file is desired, a different object reference to the file must be obtained by issuing an additional open(2) call. The close-on-exec flag on the new file descriptor is unset. In dup2(), the value of the new descriptor newd is specified. If this descriptor is already in use, the descriptor is first deallocated as if a close(2) call had been done first. If newd and oldd are the same, the call has no effect. dup3() behaves exactly like dup2() only it allows extra flags to be set on the returned file descriptor. The following flags are valid: O_CLOEXEC Set the ``close-on-exec'' property. O_NONBLOCK Sets non-blocking I/O. O_NOSIGPIPE Return EPIPE instead of raising SIGPIPE. RETURN VALUES
The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error. ERRORS
All three functions may fail if: [EBADF] oldd is not a valid active descriptor or newd is not in the range of valid file descriptors. The dup() function may also fail if: [EMFILE] Too many descriptors are active. The dup3() function will also fail if: [EINVAL] flags is other than O_NONBLOCK or O_CLOEXEC. SEE ALSO
accept(2), close(2), fcntl(2), open(2), pipe(2), socket(2), socketpair(2), getdtablesize(3) STANDARDS
The dup() and dup2() functions conform to ISO/IEC 9945-1:1990 (``POSIX.1''). HISTORY
The dup3() function is inspired from Linux and appeared in NetBSD 6.0. BSD
January 23, 2012 BSD

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