fileno(3) Library Functions Manual fileno(3) fileno - stdio (libc -lc) #include int fileno(FILE *stream); glibc ( feature_test_macros(7)): fileno(): _POSIX_C_SOURCE fileno() stream . stream fclose(3). dup(2) . unlocked_stdio(3). fileno() stream. -1 errno . EBADF stream . attributes(7). +------------+------------------------------------+--------------------+ || | | +------------+------------------------------------+--------------------+ |fileno() | | MT-Safe | +------------+------------------------------------+--------------------+ POSIX.1-2008. POSIX.1-2001. open(2), fdopen(3), stdio(3), unlocked_stdio(3) 3 . . : . 6.18 8 2026 fileno(3)