MOUNT_FDESC(8) BSD System Manager's Manual MOUNT_FDESC(8) NNAAMMEE mmoouunntt__ffddeesscc - mount the file-descriptor file system SSYYNNOOPPSSIISS mmoouunntt__ffddeesscc [--FF _f_s_o_p_t_i_o_n_s] _f_d_e_s_c _m_o_u_n_t___p_o_i_n_t DDEESSCCRRIIPPTTIIOONN The mmoouunntt__ffddeesscc command attaches an instance of the per-process file de- scriptor namespace to the global filesystem namespace. The conventional mount point is _/_d_e_v and the filesystem should be union mounted in order to augment, rather than replace, the existing entries in _/_d_e_v. This com- mand is normally executed by mount(8) at boot time. The contents of the mount point are _f_d, _s_t_d_e_r_r, _s_t_d_i_n, _s_t_d_o_u_t and _t_t_y. _f_d is a directory whose contents appear as a list of numbered files which correspond to the open files of the process reading the directory. The files _/_d_e_v_/_f_d_/_0 through _/_d_e_v_/_f_d_/_# refer to file descriptors which can be accessed through the file system. If the file descriptor is open and the mode the file is being opened with is a subset of the mode of the exist- ing descriptor, the call: fd = open("/dev/fd/0", mode); and the call: fd = fcntl(0, F_DUPFD, 0); are equivalent. The files _/_d_e_v_/_s_t_d_i_n, _/_d_e_v_/_s_t_d_o_u_t and _/_d_e_v_/_s_t_d_e_r_r appear as symlinks to the relevant entry in the _/_d_e_v_/_f_d sub-directory. Opening them is equiva- lent to the following calls: fd = fcntl(STDIN_FILENO, F_DUPFD, 0); fd = fcntl(STDOUT_FILENO, F_DUPFD, 0); fd = fcntl(STDERR_FILENO, F_DUPFD, 0); Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored. The _/_d_e_v_/_t_t_y entry is an indirect reference to the current process's con- trolling terminal. It appears as a named pipe (FIFO) but behaves in ex- actly the same way as the real controlling terminal device. FFIILLEESS /dev/fd/# /dev/stdin /dev/stdout /dev/stderr /dev/tty SSEEEE AALLSSOO mount(2), unmount(2), tty(4), fstab(5) CCAAVVEEAATTSS No . and _._. entries appear when listing the contents of the _/_d_e_v_/_f_d di- rectory. This makes sense in the context of this filesystem, but is in- consistent with usual filesystem conventions. However, it is still pos- sible to refer to both . and _._. in a pathname. This filesystem may not be NFS-exported. HHIISSTTOORRYY The mmoouunntt__ffddeesscc utility first appeared in 4.4BSD. 4.4BSD June 9, 1993 2