Linux and UNIX Man Pages

Test Your Knowledge in Computers #126
Difficulty: Easy
Linux distro is an OS created from a collection of software built upon the Linux kernel.
True or False?
Linux & Unix Commands - Search Man Pages

dup(2) [freebsd man page]

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

NAME
dup, dup2 -- 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); DESCRIPTION
The dup() system call 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 new descriptor returned by the call is the lowest num- bered 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) system 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 and oldd != newd, the descriptor is first deallocated as if the close(2) system call had been used. If oldd is not a valid descriptor, then newd is not closed. If oldd == newd and oldd is a valid descriptor, then dup2() is successful, and does nothing. RETURN VALUES
These calls return the new file descriptor if successful; otherwise the value -1 is returned and the external variable errno is set to indi- cate the cause of the error. ERRORS
The dup() system call fails if: [EBADF] The oldd argument is not a valid active descriptor [EMFILE] Too many descriptors are active. The dup2() system call fails if: [EBADF] The oldd argument is not a valid active descriptor or the newd argument is negative or exceeds the maximum allowable descriptor number SEE ALSO
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2), dup3(3) STANDARDS
The dup() and dup2() system calls are expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1''). HISTORY
The dup() and dup2() functions appeared in Version 7 AT&T UNIX. BSD
June 1, 2013 BSD

Check Out this Related Man Page

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

NAME
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

Featured Tech Videos