ttysrch is an optional file that is used by the ttyname library routine. This file contains the names of directories in /dev that contain terminal and terminal-related device files. The purpose of this file is to improve the performance of ttyname by indicating which subdirectories in /dev contain terminal-related device files and should be searched first. These subdirectory names must appear on separate lines and must begin with /dev. Those path names that do not begin with /dev will be ignored and a warning will be sent to the console. Blank lines (lines containing only white space) and lines beginning with the comment character "#" will be ignored. For each file listed (except for the special entry /dev), ttyname will recursively search through subdirectories looking for a match. If /dev appears in the ttysrch file, the /dev directory itself will be searched but there will not be a recursive search through its subdirectories.
When ttyname searches through the device files, it tries to find a file whose major/minor device number, file system identifier, and inode number match that of the file descriptor it was given as an argument. If a match is not found, it will settle for a match of just major/minor device and file system identifier, if one can be found. However, if the file descriptor is associated with a cloned device, this algorithm does not work efficiently because the inode number of the device file associated with a clonable device will never match the inode number of the file descriptor that was returned by the open of that clonable device. To help with these situations, entries can be put into the /etc/ttysrch file to improve performance when cloned devices are used as terminals on a system (for example, for remote login). However, this is only useful if the minor devices related to a cloned device are put into a subdirectory. (It is important to note that device files need not exist for cloned devices and if that is the case, ttyname will eventually fail.) An optional second field is used in the /etc/ttysrch file to indicate the matching criteria. This field is separated by white space (any combination of blanks or tabs). The letter M means major/minor device number, F means file system identifier, and I means inode number. If this field is not specified for an entry, the default is MFI which means try to match on all three. For cloned devices the field should be MF, which indicates that it is not necessary to match on the inode number.
Without the /etc/ttysrch file, ttyname will search the /dev directory by first looking in the directories /dev/term, /dev/pts, and /dev/xt. If a system has terminal devices installed in directories other than these, it may help performance if the ttysrch file is created and contains that list of directories.
A sample /etc/ttysrch file follows:
/dev/term MFI /dev/pts MFI /dev/xt MFI /dev/slan MF
This file tells ttyname that it should first search through those directories listed and that when searching through the /dev/slan directory, if a file is encountered whose major/minor devices and file system identifier match that of the file descriptor argument to ttyname, this device name should be considered a match.