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lam_rfwrite

NAME

lam_rfposix - LAM POSIX-like remote file service

DESCRIPTION

The remote file daemon, filed, provides POSIX file access for any client process in the LAM multicomputer. Some services are LAM specific (see lam_rfrmfd(2))). Most of services mirror local POSIX services and these are listed below.

 
 open(2)        open file
 close(2)       close file descriptor
 read(2)        read input
 write(2)       write output
 lseek(2)       set or increment position in file
 access(2)      check permissions and/or existance of a file
 mkdir(2)       create a directory
 chdir(2)       change current working directory
 fstat(2)       get status on a file descriptor
 stat(2)        get status on a file name
 dup(2)         duplicate a file descriptor
 dup2(2)        duplicate a file descriptor to a specific handle
 system(3)      issue a command to sh(1)
 rmdir(2)       remove a directory
 unlink(2)      remove a file
 getwd(3)       get name of current working directory
 ftruncate(2)   set open file to specified length
 truncate(2)    set named file to specified length
 

The names of the LAM versions of these functions are prepended with 'lam_rf', for example lam_rfopen(), lam_rfclose(), etc. The parameters are the same as in the POSIX calls. Special flag bits passed to lam_rfopen(2) will invoke special LAM functionality. These flags, defined in <freq.h>, are listed below.

 
 LAM_O_LOCK     lock a file permanently active
 LAM_O_REUSE    reuse existing open descriptor with same file name
 LAM_O_1WAY     do not reply to write requests
 LAM_O_DINT4    data are four-byte integers
 LAM_O_DFLT4    data are four-byte reals (single precision)
 LAM_O_DFLT8    data are eight-byte reals (double precision)
 

By default, files are accessed on the origin node. The node ID of a file can be specified by prepending the file name with the node ID and a `:'. The example below opens the file "foo" on node 15.

       lam_rfopen("15:foo", O_RDWR, 0);

An active LAM file descriptor has an associated open POSIX file descriptor. Since the daemon can maintain only a limited number of open POSIX files, it periodically has to close them and reopen them for a different LAM file descriptor. The global number of LAM file descriptors is unlimited, as are the clients of the remote file service. Thus, performance problems (like thrashing) may be avoided by locking certain files in an active state. Locked LAM file descriptors will maintain their open POSIX descriptors until closed by the client. Do not overuse this facility.

Most of the POSIX-like services behave exactly as in POSIX (in fact, the genuine article is eventually called). The exception is lam_rfwrite() when the data size is greater than the LAM network message size (as defined in <net.h>). Such requests have to be packetized. The daemon treats each packet as an individual write request in order to remain simple and stateless. Therefore, long writes on asynchronous descriptors are not atomic.

BUGS

The daemon does not allow read access to any filename starting with "/dev/". This is done to prevent the possibility of the daemon blocking on a slow device, such as a tty. This limitation will be removed in the future.

SEE ALSO

lam_rfstate(2), lam_rfrmfd(2)