Is the write(2) system call atomic

[Jerry Feldman]

There has been some discussion on another list regarding the write(2) system 
call on Linux. 
Here is a short code fragment to illustrate where the parent process creates 
and opens a file for writing, then forks a child. it was my impression that 
the kernel maintains a single structure for each open file. If that is true 
then we would see all 6 lines. In most of the failure cases we see that the 
first line and part of the second line of the parent are overwritten by the 
child. 
Note that I would probably use lockf(3), flock(2), or fcntl(2). 
fd = open(<filename>, O_CREAT | O_WRONLY | O_TRUNC, 0644);
if ((pid = fork()) > 0) { /* Parent code */
       sprintf(buf, "Parent first line\n");
       sprintf(str, "Parent is process %d\n", getpid());
	for (i=0; i < 2; ++i) {
           sprintf(str, "----P=%d: %d\n", getpid(), i);
           strcat(buf, str); 
 } else if (pid == 0) { /* child */
       for (i=0; i < 2; ++i) {
           sprintf(str, "****C=%d: %d\n", getpid(), i);
           strcat(buf, str);
       }
} else assert(0); /* error on fork() */
assert(write(fd, buf, strlen(buf)) == strlen(buf));
assert(close(fd) == 0);
if (pid) /* parent wait for child */
     wait(NULL);
return 0;
}

In the above case, we observed that on an SMP system, 2.6 kernel that in 
some cases the child data overwrote some of the parent data. This should 
create a file containing something like:
****C=19585: 0
****C=19585: 1
Parent first line
Parent is process 19584
----P=19584: 0
----P=19584: 1

But in the failure case, the following result may be present.
****C=18099: 0
****C=18099: 1
ocess 18098
----P=18098: 0
----P=18098: 1



-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix user group
http://www.blu.org PGP key id:C5061EA9
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