dup create a copy of the file descriptor oldfd.
After successful return of dup, 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(2) 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(2) uses the lowest-numbered unused descriptor for the new descriptor.
dup returns the new descriptor, or -1 if an error occurred (in which case, errno is set appropriately).
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