isatty - 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 isatty(int fd); – unistd.h.

Libc side – returns 1 if fd refers to a terminal, else 0 with errno set: EBADF (invalid fd) or ENOTTY (isatty(3)). That it sets errno on the zero path – rather than merely returning zero – matters for code that reports strerror(errno) unconditionally. Typical use: pick buffering or output format by isatty(STDOUT_FILENO). Terminal attribute access itself is the tcgetattr family, not this predicate.