Bug 79311 - mmap MAP_SHARED doesn't work in one process
mmap MAP_SHARED doesn't work in one process
Product: Red Hat Linux
Classification: Retired
Component: kernel (Show other bugs)
i686 Linux
medium Severity medium
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Assigned To: Arjan van de Ven
Brian Brock
Depends On:
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Reported: 2002-12-09 17:43 EST by Need Real Name
Modified: 2007-04-18 12:48 EDT (History)
0 users

See Also:
Fixed In Version:
Doc Type: Bug Fix
Doc Text:
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Clone Of:
Last Closed: 2003-03-25 19:11:30 EST
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RHEL 7.3 requirements from Atomic Host:
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Attachments (Terms of Use)
test program (637 bytes, text/plain)
2002-12-09 17:46 EST, Need Real Name
no flags Details

  None (edit)
Description Need Real Name 2002-12-09 17:43:21 EST
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 17:46:41 EST
Created attachment 88086 [details]
test program
Comment 2 Arjan van de Ven 2002-12-10 03:29:58 EST
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 2003-03-25 19:11:30 EST
Explanation posted seems good, resolving as notabug.
Comment 4 Need Real Name 2003-03-26 13:38:40 EST
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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