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dup3(2) [netbsd 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

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DUP(2)								System Calls Manual							    DUP(2)

NAME
dup, dup2 - duplicate a descriptor SYNOPSIS
#include <unistd.h> int dup(int oldd) int dup2(int oldd, int newd) DESCRIPTION
Dup duplicates an existing descriptor. The argument oldd is a small non-negative integer index in the per-process descriptor table. The value must be less than OPEN_MAX, the size of the table. The new descriptor returned by the call, let's name it newd, is the lowest num- bered descriptor that is not currently in use by the process. The object referenced by the descriptor does not distinguish between references using oldd and newd in any way. Thus if newd and oldd are duplicate references 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 the second form of the call, the value of newd desired is specified. If this descriptor is already in use, the descriptor is first deallocated as if a close(2) call had been done first. Newd is not closed if it equals oldd. RETURN VALUE
The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error. ERRORS
Dup and dup2 fail if: [EBADF] Oldd or newd is not a valid active descriptor [EMFILE] Too many descriptors are active. NOTES
Dup and dup2 are now implemented using the F_DUPFD function of fcntl(2), although the old system call interfaces still exist to support old programs. SEE ALSO
open(2), close(2), fcntl(2), pipe(2). 4th Berkeley Distribution May 13, 1986 DUP(2)
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