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fd(4) Kernel Interfaces Manual fd(4) NAME fd - file descriptor files DESCRIPTION The /dev/fd file system is a pseudo-file system layered beneath the Virtual File System (VFS). The file descriptor files (fd*) are those files that are accessible through file descriptors. The file descrip- tors use the naming convention /dev/fd/0, /dev/fd/1, /dev/fd/2 and so on up to any number. To make the /dev/fd file system known to the operating system, you must create the directory with the correct privileges, then you must mount the file system. The following steps describe how to create the direc- tory, mount the file system both manually and automatically, and how to dismount the file system: Create the directory using the mkdir and chmod commands: mkdir /dev/fd; chmod 777 /dev/fd Mount the file system manually using the mount command: mount -t fdfs /dev/fd /dev/fd Mount the file system automatically by editing either the /etc/fstab file or the /sbin/bcheckrc file. Add the following entry to the /etc/fstab file: /dev/fd /dev/fd fdfs rw 0 0 This entry mounts the pseudodevice /dev/fd on the /dev/fd directory with read/write privileges. The file system type is fdfs and the zeros (0) in the remaining fields specify that the file system is not to be backed up nor can file system checks be performed by the fsck command as this is a virtual file system. Add the following entry to the /sbin/bcheckrc file: # # mount fdfs # echo 'Mounting /dev/fd filesystem' /sbin/mount -a -v -t fdfs Again, the /dev/fd file system should not be mounted in this manner if an entire system is to be backed up starting from the root directory. Dismount the file system using the umount command: umount /dev/fd For correct truncate() behavior on fd files, you must load your program using the -lsys5 flag. RESTRICTIONS The /dev/fd file descriptors should not be exported. EXAMPLES The following example show how the open and dup functions have the same effect if file descriptor n is opened: fd = open("/dev/fd/n", mode); fd = dup(n); In the above example, the open function is equal to the creat function and mode is ignored. Using the dup function, subsequent reads or writes on the fd file descriptor files fail unless the original file descriptor enables the operation. ERRORS The following error condition exists: The file descriptor is not valid. RELATED INFORMATION Commands: chmod(1), mkdir(1), mount(8). Functions: creat(2), dup(2), open(2). delim off fd(4)