求助：fgets 讀文件死鎖！

mailsafe

問題：難道fgets函數(shù)也死鎖，系統(tǒng)上線后出現(xiàn)過幾次這類問題。 暫未解決，只能kill掉讀取進程。

環(huán)境：HP-UX 安騰 64。
數(shù)據(jù)文件：放在共享存儲上，不在本機硬盤上。
描述：兩個并發(fā)進程進入文件拆分函數(shù)，讀取同一待拆分文件，然后就沒有下文，僵在里面。
錯誤跟蹤日志如下：

--進程A，GDB堆棧信息
xxxx@/home/db/xxxx> gdb xxxxFileLoad 20166
HP gdb 6.2 for HP Itanium (32 or 64 bit) and target HP-UX 11iv2 and 11iv3.
Copyright 1986 - 2011 Free Software Foundation, Inc.
Hewlett-Packard Wildebeest 6.2 (based on GDB) is covered by the
GNU General Public License. Type "show copying" to see the conditions to
change it and/or distribute copies. Type "show warranty" for warranty/support.
..
Attaching to program: /home/db/xxxx/xxxxbatch/bin/xxxxFileLoad, process 20166

warning: The shared libraries were not privately mapped; setting a
breakpoint in a shared library will not work until you rerun the program;
stepping over longjmp calls will not work as expected.
Please set the kernel variable "shlib_debug_enable" to 1 to enable the shared library debugging

warning: Load module /home/db/oracle/product/10.2.0/lib/libnnz10.so has been stripped.  
Debugging information is not available.

0xc00000000054a490:0 in _read_sys+0x30 () from /usr/lib/hpux64/libc.so.1
(gdb) bt
#0  0xc00000000054a490:0 in _read_sys+0x30 () from /usr/lib/hpux64/libc.so.1
#1  0xc00000000055ed80:0 in read+0xe0 () from /usr/lib/hpux64/libc.so.1
#2  0xc000000000527a20:0 in _filbuf+0x190 () from /usr/lib/hpux64/libc.so.1
#3  0xc00000000053a0d0:0 in __fgets_unlocked+0x1f0 () from /usr/lib/hpux64/libc.so.1
#4  0xc00000000053a3e0:0 in fgets+0x140 () from /usr/lib/hpux64/libc.so.1
#5  0x4000000000030320:0 in g_GetFSegment () at p_file.c:301  文件拆分函數(shù)：這一行就是 fgets()，其讀取緩存大小為一次讀取 400K。
#6  0x4000000000006130:0 in InsxxxxFile () at xxxxFileLoad.pc:1060
#7  0x4000000000005380:0 in main () at xxxxFileLoad.pc:107
(gdb) quit
The program is running.  Quit anyway (and detach it)? (y or n) y
Detaching from program: /home/db/xxxx/xxxxbatch/bin/xxxxFileLoad, process 20166


--進程B，GDB堆棧信息
xxxx@/home/db/xxxx> gdb xxxxFileLoad 20167
HP gdb 6.2 for HP Itanium (32 or 64 bit) and target HP-UX 11iv2 and 11iv3.
Copyright 1986 - 2011 Free Software Foundation, Inc.
Hewlett-Packard Wildebeest 6.2 (based on GDB) is covered by the
GNU General Public License. Type "show copying" to see the conditions to
change it and/or distribute copies. Type "show warranty" for warranty/support.
..
Attaching to program: /home/db/xxxx/xxxxbatch/bin/xxxxFileLoad, process 20167

warning: The shared libraries were not privately mapped; setting a
breakpoint in a shared library will not work until you rerun the program;
stepping over longjmp calls will not work as expected.
Please set the kernel variable "shlib_debug_enable" to 1 to enable the shared library debugging

warning: Load module /home/db/oracle/product/10.2.0/lib/libnnz10.so has been stripped.  
Debugging information is not available.

0xc00000000054a490:0 in _read_sys+0x30 () from /usr/lib/hpux64/libc.so.1
(gdb) bt
#0  0xc00000000054a490:0 in _read_sys+0x30 () from /usr/lib/hpux64/libc.so.1
#1  0xc00000000055ed80:0 in read+0xe0 () from /usr/lib/hpux64/libc.so.1
#2  0xc000000000527a20:0 in _filbuf+0x190 () from /usr/lib/hpux64/libc.so.1
#3  0xc00000000053a0d0:0 in __fgets_unlocked+0x1f0 () from /usr/lib/hpux64/libc.so.1
#4  0xc00000000053a3e0:0 in fgets+0x140 () from /usr/lib/hpux64/libc.so.1
#5  0x4000000000030320:0 in g_GetFSegment () at p_file.c:301 文件拆分函數(shù)：這一行就是 fgets()，其讀取緩存大小為一次讀取 400K。
#6  0x4000000000006130:0 in InsxxxxFile () at xxxxFileLoad.pc:1060
#7  0x4000000000005380:0 in main () at xxxxFileLoad.pc:107
(gdb) quit
The program is running.  Quit anyway (and detach it)? (y or n) y
Detaching from program: /home/db/xxxx/xxxxbatch/bin/xxxxFileLoad, process 20167

請教一下可能是什么原因?qū)е麓祟悊栴}隨機產(chǎn)生。

csumck

man手冊里說明了可以用*_unlocked()來做非阻塞操作。

liangzhishao

因為fgets的讀操作（read系統(tǒng)調(diào)用）是在網(wǎng)絡(luò)設(shè)備上讀取文件導(dǎo)致永遠阻塞（就是死鎖）。

APUE 14.2中描述

1. 低速系統(tǒng)調(diào)用是可能會使進程永遠阻塞的一類系統(tǒng)調(diào)用，它們包括下列調(diào)用：
   
2.   * 如果某些文件類型（例如管道、終端設(shè)備和[color=Red]網(wǎng)絡(luò)設(shè)備[/color]）的數(shù)據(jù)并不存在，則讀操作可能會使調(diào)用者永遠阻塞。
   
3.   * 。。。。

復(fù)制代碼

你的系統(tǒng)使用的共享存儲，它不是本地文件系統(tǒng)，所以它屬于低速設(shè)備。

另外，APUE 中還說

1. 我們也曾說過，雖然讀、寫磁盤文件會使調(diào)用者在短暫時間內(nèi)阻塞，但并不能將和磁盤I/O有關(guān)的系統(tǒng)調(diào)用視為“低速”。

復(fù)制代碼

這一點我不能給出進一步解釋，為何磁盤文件不屬于“低速”，它是否會永遠阻塞，我沒有找到明確的答案。
望得到指點。。。

mailsafe

意思是：網(wǎng)絡(luò)設(shè)備可能在某一時點失效，從而使讀取發(fā)生死鎖，可以采用 fgets_unlocked 來規(guī)避？