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Old 10-07-2011, 02:19 PM   #1
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Registered: Apr 2006
Location: San Jose, California
Distribution: SuSE, STX, Debian
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Question Inconsistent error 75 using fchmod on shared memory file descriptors (x64)

My C application initialization loops 14 times allocating 12MB blocks of shared memory, assigns each to the video group, and grants RW permissions to video members. This works for the first about 7-10 blocks (a little different each run), after which fchmod starts returning error 75 - "Value too large for defined data type"! The same steps work when run in a bash script. I tried inserting a 2 milli-sec delay and a fsync in before the call to fchmod to emulate running from a script, but the problem still occurs. The values passed to fchmod are a file descriptor (varied from 5 to 19 in this test) and the octal permissions (0664). This leaves me with three questions:
1> Which value does the error message refer to? The fd, the permissions, or some internal value I can't see?
2> Why does the fchmod call work properly for the first several calls then suddenly start failing after about ten calls?
3> What can I do in my code to get this working?

> I am using Fedora 15 with a kernel with 12 GB RAM.
> With the x32 kernel 12 MB shm buffers are difficult to get to work.
> I've tried increasing the maximum per process limit on file descriptors to 32K but it did not help with this problem.
Old 10-07-2011, 07:13 PM   #2
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Registered: Apr 2006
Location: San Jose, California
Distribution: SuSE, STX, Debian
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Smile I cannot understand why this workaround works, but it does!

The last step in each loop after allocating a memory block and setting the group and permissions was to also to set some flags in a structure at the head of the memory block (to indicate that the block was ready for use). Moving the initialization of these flags until after ALL the file allocations and group and permissions were complete eliminated the errors! This makes a good workaround but I would still like to understand why it was failing before and not now!

Grant Q


chmod, shm, x64

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