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Bug 79311 - mmap MAP_SHARED doesn't work in one process
Summary: mmap MAP_SHARED doesn't work in one process
Alias: None
Product: Red Hat Linux
Classification: Retired
Component: kernel
Version: 7.3
Hardware: i686
OS: Linux
Target Milestone: ---
Assignee: Arjan van de Ven
QA Contact: Brian Brock
Depends On:
TreeView+ depends on / blocked
Reported: 2002-12-09 22:43 UTC by Need Real Name
Modified: 2007-04-18 16:48 UTC (History)
0 users

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Clone Of:
Last Closed: 2003-03-26 00:11:30 UTC

Attachments (Terms of Use)
test program (637 bytes, text/plain)
2002-12-09 22:46 UTC, Need Real Name
no flags Details

Description Need Real Name 2002-12-09 22:43:21 UTC
From Bugzilla Helper:
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.1) Gecko/20021003

Description of problem:
It appears that if mmap is called more than once on the same file in a single
process the flag MAP_SHARED is ignored, and a new memory is allocated for the
mapping.  Test code attached, save as t.c, compile with gcc -o t t.c, and run ./t.

test system is RedHat 7.3, kernel-smp-2.4.18-18.7.x, 
glibc-kernheaders-2.4-7.16, gcc-2.96-113, glibc-2.2.5-42.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1.compile attached code
2.run attaced code
3.see pointer is different

Actual Results:  output of attached program is:
mp1 = 0x40014000
mp2 = 0x40016000

Expected Results:  mp1 = 0x40014000
mp2 = 0x40014000

Additional info:

Comment 1 Need Real Name 2002-12-09 22:46:41 UTC
Created attachment 88086 [details]
test program

Comment 2 Arjan van de Ven 2002-12-10 08:29:58 UTC
you ask for 2 mappings of the same file, you get 2 mappings; it will be the same
physical page however, just available at both addresses; MAP_SHARED doesn't mean
"return same virtual address" but "share phsyical memory between mmap users"

Comment 3 Brian Brock 2003-03-26 00:11:30 UTC
Explanation posted seems good, resolving as notabug.

Comment 4 Need Real Name 2003-03-26 18:38:40 UTC
Technically not a bug.  My problem, which was worked around with a wrapper, was
the process would run out of memory after mmaping a large file a second time. 
This seems undiserable to me,the process ran out of memory while not using any
more real memory.

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