#0 0x000000303b2f508e in __lll_lock_wait_private () from /lib64/libc.so.6
#1 0x000000303b2e22a8 in _L_lock_798 () from /lib64/libc.so.6
#2 0x000000303b2e1d72 in __vsyslog_chk () from /lib64/libc.so.6
#3 0x000000000041232c in DEBUG ()
#4 0x0000000000408b20 in sync_config_on_signal ()
#5 <strong><signal handler called></strong>
#6 0x000000303b2e6812 in send () from /lib64/libc.so.6
#7 0x000000303b2e1e80 in __vsyslog_chk () from /lib64/libc.so.6
#8 0x000000000041232c in DEBUG ()
#12 0x00000000004073c3 in main ()
原来是锁住了。Google “syslog signal handler” ，看到：
- http://www.pgpool.net/mantisbt/view.php?id=9：Don’t call syslog(3) family in signal handler. It causes process deadlock.
- https://cwe.mitre.org/data/definitions/479.html：在这篇名为Signal handler use of a non-reentrant function的文章里，特别拿出syslog举例来说，摘录如下：
Non-reentrant functions are functions that cannot safely be called, interrupted, and then recalled before the first call has finished without resulting in memory corruption. This can lead to an unexpected system state，an unpredictable result with a variety of potential consequences depending on context, including denial of service and code execution.
Many functions are not reentrant, but some of them can result in the corruption of memory if they are used in a signal handler. The function call syslog() is an example of this. In order to perform its functionality, it allocates a small amount of memory as “scratch space.” If syslog() is suspended by a signal call and the signal handler calls syslog(), the memory used by both of these functions enters an undefined, and possibly, exploitable state. Implementations of malloc() and free() manage metadata in global structures in order to track which memory is allocated versus which memory is available, but they are non-reentrant. Simultaneous calls to these functions can cause corruption of the metadata.
再翻开TLPI或者APUE.3e中关于signal handler，reentrant and async-signal-safe functions的解释，于这一点上便更明白了。
- APUE.3e 10.6 Reentrant Functions
- TLPI 21.1.2 Reentrant and Async-Signal-Safe Functions