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RedHat 9 (Linux i386) - man page for dup (redhat section 2)

DUP(2)							     Linux Programmer's Manual							    DUP(2)

dup, dup2 - duplicate a file descriptor
#include <unistd.h> int dup(int oldfd); int dup2(int oldfd, int newfd);
dup and dup2 create a copy of the file descriptor oldfd. After successful return of dup or dup2, the old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using lseek on one of the descriptors, the position is also changed for the other. The two descriptors do not share the close-on-exec flag, however. dup uses the lowest-numbered unused descriptor for the new descriptor. dup2 makes newfd be the copy of oldfd, closing newfd first if necessary.
dup and dup2 return the new descriptor, or -1 if an error occurred (in which case, errno is set appropriately).
EBADF oldfd isn't an open file descriptor, or newfd is out of the allowed range for file descriptors. EMFILE The process already has the maximum number of file descriptors open and tried to open a new one.
The error returned by dup2 is different to that returned by fcntl(..., F_DUPFD, ...) when newfd is out of range. On some systems dup2 also sometimes returns EINVAL like F_DUPFD.
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4 documents additional EINTR and ENOLINK error conditions. POSIX.1 adds EINTR.
fcntl(2), open(2), close(2) Linux 1.1.46 1994-08-21 DUP(2)