On Thu, May 04, 2006 at 10:14:20PM +0200, Michael Kerrisk wrote:
> > > > > > are correct; an "r+m" file is read() not mmaped.
> > > > >
> > > > > Have you written a test program that verifies (presumably via
> > > > > monitoring via 'strace') how 'm' changes the behaviour of stdio
> > > > > with respect to the use of mmap()?
> > > > Erm, yes, thats what I'm talking about (and is strace exactly what I
> > > > did). But I'm not sure if I understand you .. I verified that all 3
> > > > routines mmap() a file if "m" is given (and it is read-only), and do
> > > > not mmap() it if "m" is not given (or if it is not read-only).
> > > >
> > > > I include now the test program :)
> > >
> > > Yes, but did you verify:
> > >
> > > -- which file is being mmap()ed? (There might be many other
> > > unrelated mmap() calls going on.)
> > Yes; note that it only happens after you fread() for the first time.
> >
> > > -- whether the behaviour changes if 'm' is removed from the fopen()?
> > > (the strace output shd be different).
> > Yes:
> > > > routines mmap() a file if "m" is given (and it is read-only), and do
> > > > not mmap() it if "m" is not given (or if it is not read-only).
>
> Hmmm -- I'm not seeing this. Can you post for each of the two
> test cases:
>
> -- the fopen() call that is used.
>
> -- the command line that is used to run the program.
>
> -- the results of grep the strace output using:
>
> egrep '(mmap|open|read)'
Sure, (using strace -e mmap2,open,read instead):
fopen "r":
|...
|open("/tmp/fp.c", O_RDONLY) = 3
|mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xb7f3a000
|read(3, "#include <sys/types.h>\n#include "..., 4096) = 460
|read(3, "", 4096) = 0
fopen "r+m":
|...
|open("/tmp/fp.c", O_RDWR) = 3
|mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xb7f8e000
|read(3, "#include <sys/types.h>\n#include "..., 4096) = 462
|read(3, "", 4096) = 0
fopen "rm":
|...
|open("/tmp/fp.c", O_RDONLY) = 3
|mmap2(NULL, 461, PROT_READ, MAP_SHARED, 3, 0) = 0xb7f1c000
fdopen "rm":
|...
|open("/tmp/fp.c", O_RDONLY) = 3
|mmap2(NULL, 447, PROT_READ, MAP_SHARED, 3, 0) = 0xb7fc3000
fdopen "r+m":
|...
|open("/tmp/fp.c", O_RDONLY) = 3
|mmap2(NULL, 447, PROT_READ, MAP_SHARED, 3, 0) = 0xb7fc3000
fdopen "r":
|...
|open("/tmp/fp.c", O_RDWR) = 3
|mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xb7f06000
|read(3, "#include <sys/types.h>\n#include "..., 4096) = 444
|read(3, "", 4096) = 0
freopen "r":
|...
|open("/tmp/fp.c", O_RDONLY) = 2
|mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xb7f0f000
|read(2, "#include <sys/types.h>\n#include "..., 4096) = 466
|read(2, "", 4096) = 0
freopen "r+":
|...
|open("/tmp/fp.c", O_RDWR) = 2
|mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xb7fc1000
|read(2, "#include <sys/types.h>\n#include "..., 4096) = 468
|read(2, "", 4096) = 0
freopen "r+":
|...
|open("/tmp/fp.c", O_RDONLY) = 2
|mmap2(NULL, 467, PROT_READ, MAP_SHARED, 2, 0) = 0xb7f20000
If it matters:
Linux andromeda 2.6.15-1-686 #2 Mon Mar 6 15:27:08 UTC 2006 i686 GNU/Linux
GNU C Library stable release version 2.3.6, by Roland McGrath et al.
Copyright (C) 2005 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.0.3 (Debian 4.0.3-1).
Compiled on a Linux 2.6.13 system on 2006-04-14.
Available extensions:
GNU libio by Per Bothner
crypt add-on version 2.1 by Michael Glad and others
GNU Libidn by Simon Josefsson
linuxthreads-0.10 by Xavier Leroy
BIND-8.2.3-T5B
libthread_db work sponsored by Alpha Processor Inc
NIS(YP)/NIS+ NSS modules 0.19 by Thorsten Kukuk
Thread-local storage support included.
For bug reporting instructions, please see:
<http://www.gnu.org/software/libc/bugs.html>.
This seems to have been newly-implemented in glibc 2.3 (from the NEWS
file).
Justin
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