Description:
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If the apc.mmap_file_mask is left at default value, which is "/tmp/apc", the APC will segfault with this backtrace:

#0  0x281bbf0b in mkstemp () from /usr/lib/libc.so.4
#1  0x288ad33c in apc_mmap (file_mask=0x288b0088 "/tmp/apc", size=31457280) at /homes/rasmus/apc/apc/apc_mmap.c:111
#2  0x288af6bd in apc_sma_init (numseg=1, segsize=31457280, mmap_file_mask=0x288b0088 "/tmp/apc")
    at /homes/rasmus/apc/apc/apc_sma.c:287
#3  0x288acf7b in apc_module_init (module_number=16) at /homes/rasmus/apc/apc/apc_main.c:342
#4  0x288a6c82 in zm_startup_apc (type=1, module_number=16) at /homes/rasmus/apc/apc/php_apc.c:188
#5  0x283cec47 in zend_startup_module_ex (module=0x80e8480)
    at /home/y/src/yahoo/packages/yahoo/php/php-5.1.4/Zend/zend_API.c:1397
#6  0x283d2c70 in zend_hash_apply (ht=0x28500940, apply_func=0x283ceb48 <zend_startup_module_ex>)
    at /home/y/src/yahoo/packages/yahoo/php/php-5.1.4/Zend/zend_hash.c:666
#7  0x283ced4f in zend_startup_modules () at /home/y/src/yahoo/packages/yahoo/php/php-5.1.4/Zend/zend_API.c:1444

I think that happens is that mktemp() expects the filename to contain X's which it overwrites with the digits, and since there are no X's, it tries to append to the string and segfaults.

Reproduce code:
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Anything.

Expected result:
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No crash.

Actual result:
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Crash.