dup / dup2 / dup3 - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Signature – int dup(int oldfd); int dup2(int oldfd, int newfd); int dup3(int oldfd, int newfd, int flags); – unistd.h.

Libc side – dup = fcntl(oldfd, F_DUPFD, 0); dup2 performs the close-and-copy atomically and treats oldfd invalid as a no-op that does not close newfd (manual, Duplicating Descriptors). dup3 is GNU (_GNU_SOURCE, symbol GLIBC_2.9 on installed glibc) and accepts only O_CLOEXEC; POSIX.1-2024 adopted it. Duplicates share one file offset and status flags but not descriptor flags. The redirection recipe (dup2 before exec) and the syscall numbers live in Descriptors and File I/O dup entry.