Problem - Kevin Tafuro

NFS older than Version 3 in particular—may not necessarily support the O_EXCL flag
to open( ). Further, because the ID of the process holding the lock is stored in the
lock file and process IDs are not shared across machines, testing for the presence of
the process holding the lock would be unreliable at best if the lock file were stored
on a network filesystem.
Three attempts are made to obtain the lock, with a pause of one second between
attempts. If the lock cannot be obtained, the return value from the function is 0. If
some kind of error occurs in attempting to obtain the lock, the return value is –1. If
the lock is successfully obtained, the return value is 1.
#include
#include
#include
#include
#include
#include
#include
#include
static int read_data(int fd, void *buf, size_t nbytes) {
size_t toread, nread = 0;
ssize_t result;
do {
if (nbytes - nread > SSIZE_MAX) toread = SSIZE_MAX;
else toread = nbytes - nread;
if ((result = read(fd, (char *)buf + nread, toread)) >= 0)
nread += result;
else if (errno != EINTR) return 0;
} while (nread < nbytes);
return 1;
}
static int write_data(int fd, const void *buf, size_t nbytes) {
size_t towrite, written = 0;
ssize_t result;
do {
if (nbytes - written > SSIZE_MAX) towrite = SSIZE_MAX;
else towrite = nbytes - written;
if ((result = write(fd, (const char *)buf + written, towrite)) >= 0)
written += result;
else if (errno != EINTR) return 0;
} while (written < nbytes);
return 1;
}
The two functions read_data( ) and write_data( ) are helper functions that ensure
that all the requested data is read or written. If the system calls for reading or writing
are interrupted by a signal, they are retried. Because such a small amount of data is
being read and written, the data should all be written atomically, but all the data may
not be read or written in a single call. These helper functions also handle this case.
Synchronizing Resource Access Across Processes on Unix | 61
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