Apparently POSIX states that
Either a file descriptor or a stream is called a "handle" on the open file description to which it refers; an open file description may have several handles. […] All activity by the application affecting the file offset on the first handle shall be suspended until it again becomes the active file handle. […] The handles need not be in the same process for these rules to apply. -- POSIX.1-2008
and
If two threads each call [the write() function], each call shall either see all of the specified effects of the other call, or none of them. -- POSIX.1-2008
My understanding of this is that when the first process issues a
write(handle, data1, size1) and the second process issues
write(handle, data2, size2), the writes can occur in any order but
the data1 and data2 must be both pristine and contiguous.
But running the following code gives me unexpected results.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/wait.h>
die(char *s)
{
perror(s);
abort();
}
main()
{
unsigned char buffer[3];
char *filename = "/tmp/atomic-write.log";
int fd, i, j;
pid_t pid;
unlink(filename);
/* XXX Adding O_APPEND to the flags cures it. Why? */
fd = open(filename, O_CREAT|O_WRONLY/*|O_APPEND*/, 0644);
if (fd < 0)
die("open failed");
for (i = 0; i < 10; i++) {
pid = fork();
if (pid < 0)
die("fork failed");
else if (! pid) {
j = 3 + i % (sizeof(buffer) - 2);
memset(buffer, i % 26 + 'A', sizeof(buffer));
buffer[0] = '-';
buffer[j - 1] = '\n';
for (i = 0; i < 1000; i++)
if (write(fd, buffer, j) != j)
die("write failed");
exit(0);
}
}
while (wait(NULL) != -1)
/* NOOP */;
exit(0);
}
I tried running this on Linux and Mac OS X 10.7.4 and using grep -a
'^[^-]\|^..*-' /tmp/atomic-write.log shows that some writes are not
contiguous or overlap (Linux) or plain corrupted (Mac OS X).
Adding the flag O_APPEND in the open(2) call fixes this
problem. Nice, but I do not understand why. POSIX says
O_APPEND If set, the file offset shall be set to the end of the file prior to each write.
but this is not the problem here. My sample program never does
lseek(2) but share the same file description and thus same file
offset.
I have already read similar questions on Stackoverflow but they still do not fully answer my question.
Atomic write on file from two process does not specifically address the case where the processes share the same file description (as opposed to the same file).
How does one programmatically determine if “write” system call is atomic on a particular file? says that
The
writecall as defined in POSIX has no atomicity guarantee at all.
But as cited above it does have some. And what’s more,
O_APPEND seems to trigger this atomicity guarantee although it seems
to me that this guarantee should be present even without O_APPEND.
Can you explain further this behaviour ?
unsigned char buffer[3]but then usej = 3 + i(becausesizeof(buffer) - 2 == 3 - 2 == 1and the%therefore being meaningless), and then dobuf[j-1] = ...andwrite(..., buffer, j)- you corrupt parts of the stack and then write that out. The result of that is not well-specified, and the only reason your app doesn't crash is because it never returns frommain()but callsexit()instead. – Luganskj = 3:x % 1 == 0whateverxis. I did this to experiment with different sizes ofbuffer. – Golanka