Messages in this thread | | | Date | Fri, 22 Apr 2011 05:41:47 -0600 | From | Eric Blake <> | Subject | Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags |
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On 04/21/2011 07:22 PM, Josef Bacik wrote: >> You absolutely need to match Solaris semantics, which are documented as follows: >> >> � If whence is SEEK_HOLE, the offset of the start of the next hole >> greater than or equal to the supplied offset is returned. The def- >> inition of a hole is provided near the end of the DESCRIPTION. >> >> Note that in that definition, SEEK_HOLE does _not_ reposition the file offset >> (it returns the offset of the next hole, which might be at the end of the file >> since all files have a virtual hole at the end, but leaves the position >> unchanged). I'd have to write a test program on Solaris to see whether that >> definition is actually true, or if it is a bug in the Solaris man page. > > lseek's purpose is to reposition the file position, so I'd imagine > this is just a bug in the man page.
Confirmed that the Solaris man page is buggy and does not accurately describe the semantics that are actually implemented. My testing showed:
lseek(fd, off, SEEK_HOLE) has four cases:
off < end, in data: repositions to the next hole as if by lseek(fd, next_hole, SEEK_CUR); the next hole is guaranteed to exist, even if at end of file
off < end, in hole, and later in file has data: repositions as if by lseek(fd, off, SEEK_CUR)
off < end, in hole, and no further data in file: repositions as if by lseek(fd, 0, SEEK_END)
off == end: fails with ENXIO
lseek(fd, off, SEEK_DATA) has four cases:
off < end, in data: repositions as if by lseek(fd, off, SEEK_CUR)
off < end, in hole, and later in file has data: repositions to the next data as if by lseek(fd, next_data, SEEK_CUR)
off < end, in hole, and remainder of file is hold: fails with ENXIO
off == end: fails with ENXIO
Here's how I tested:
$ uname -sr SunOS 5.10 $ cat foo.c #include <unistd.h> #include <stdio.h> #include <errno.h> #include <fcntl.h> #include <string.h>
static int make_sparse (const char *name) { int fd = open (name, O_RDWR | O_CREAT | O_TRUNC, 0600); if (fd < 0) return fd; printf ("creating %s\n", name); if (write (fd, "a", 1) != 1) goto cleanup; if (lseek (fd, 1024*1024, SEEK_CUR) == -1) goto cleanup; if (write (fd, "b", 1) != 1) goto cleanup; if (lseek (fd, 0, SEEK_SET) != 0) goto cleanup; return fd; cleanup: close (fd); return -1; }
int main (int argc, char **argv) { off_t off; off_t end; int fd = argc > 2 ? open (argv[1], O_RDONLY) : make_sparse (argc > 1 ? argv[1] : "file");
perror(NULL); if (fd < 0) return 2;
printf ("fpathconf gives %ld, ENXIO is %d\n", fpathconf (fd, _PC_MIN_HOLE_SIZE), ENXIO);
puts ("\ntesting at start"); errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_HOLE); printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_DATA); printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_HOLE); printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
puts ("\ntesting at end");
errno = 0; off = end = lseek (fd, 0, SEEK_END); printf ("end at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, end, SEEK_HOLE); printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, end, SEEK_DATA); printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, end - 1, SEEK_HOLE); printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, end - 1, SEEK_DATA); printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
puts ("\ntesting at offset 1");
errno = 0; off = lseek (fd, 1, SEEK_HOLE); printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 1, SEEK_DATA); printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
puts ("\ntesting at offset 200000");
errno = 0; off = lseek (fd, 200000, SEEK_HOLE); printf ("HOLE gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 200000, SEEK_DATA); printf ("DATA gives offset %lld, errno %d\n", (long long) off, errno);
errno = 0; off = lseek (fd, 0, SEEK_CUR); printf ("CUR at offset %lld, errno %d\n", (long long) off, errno);
if (close (fd)) return 3; return 0; } $ ./foo creating file Error 0 fpathconf gives 512, ENXIO is 6
testing at start CUR at offset 0, errno 0 HOLE gives offset 131072, errno 0 CUR at offset 131072, errno 0 DATA gives offset 0, errno 0 CUR at offset 0, errno 0 HOLE gives offset 131072, errno 0 CUR at offset 131072, errno 0
testing at end end at offset 1048578, errno 0 HOLE gives offset -1, errno 6 CUR at offset 1048578, errno 0 DATA gives offset -1, errno 6 CUR at offset 1048578, errno 0 HOLE gives offset 1048578, errno 0 CUR at offset 1048578, errno 0 DATA gives offset 1048577, errno 0 CUR at offset 1048577, errno 0
testing at offset 1 HOLE gives offset 131072, errno 0 CUR at offset 131072, errno 0 DATA gives offset 1, errno 0 CUR at offset 1, errno 0
testing at offset 200000 HOLE gives offset 200000, errno 0 CUR at offset 200000, errno 0 DATA gives offset 1048576, errno 0 CUR at offset 1048576, errno 0 $ gtruncate -s 1M sparse $ ./foo sparse 2 Error 0 fpathconf gives 512, ENXIO is 6
testing at start CUR at offset 0, errno 0 HOLE gives offset 0, errno 0 CUR at offset 0, errno 0 DATA gives offset -1, errno 6 CUR at offset 0, errno 0 HOLE gives offset 0, errno 0 CUR at offset 0, errno 0
testing at end end at offset 1048576, errno 0 HOLE gives offset -1, errno 6 CUR at offset 1048576, errno 0 DATA gives offset -1, errno 6 CUR at offset 1048576, errno 0 HOLE gives offset 1048576, errno 0 CUR at offset 1048576, errno 0 DATA gives offset -1, errno 6 CUR at offset 1048576, errno 0
testing at offset 1 HOLE gives offset 1, errno 0 CUR at offset 1, errno 0 DATA gives offset -1, errno 6 CUR at offset 1, errno 0
testing at offset 200000 HOLE gives offset 1048576, errno 0 CUR at offset 1048576, errno 0 DATA gives offset -1, errno 6 CUR at offset 1048576, errno 0 $
-- Eric Blake eblake@redhat.com +1-801-349-2682 Libvirt virtualization library http://libvirt.org
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