用gdb来调试fwrite异常产生的core

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fwrite的用法，想必大家肯定很熟悉了。但是，很多人在调用fopen后，不定返回的fp进行非空判断, 恕我直言，确实有点小流氓。实际上，在linux开发中，受权限影响，很多时候调用fopen会失败，返回NULL. 在本文中，我们来看看，万一产生了core, 该如下定位。

实际上，对于gdb调试core，我们已经比较熟悉了，现在来强化一下，如下：

[taoge@localhost test]$ lstest.c[taoge@localhost test]$ cat test.c -n     1  #include <stdio.h>     2     3  int main()     4  {     5          FILE *fp = NULL;     6          int a = 1;     7          int b = 2;     8          int c = a + b;     9    10          fwrite("abc", sizeof("abc"), 1, fp);    11    12          return 0;    13  }[taoge@localhost test]$ gcc -g test.c [taoge@localhost test]$ lsa.out  test.c[taoge@localhost test]$ ./a.out Segmentation fault (core dumped)[taoge@localhost test]$ lsa.out  core.3355  test.c[taoge@localhost test]$ [taoge@localhost test]$ [taoge@localhost test]$ [taoge@localhost test]$ gdb a.out core.3355 GNU gdb (GDB) Red Hat Enterprise Linux (7.1-29.el6)Copyright (C) 2010 Free Software Foundation, Inc.License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>This is free software: you are free to change and redistribute it.There is NO WARRANTY, to the extent permitted by law.  Type "show copying"and "show warranty" for details.This GDB was configured as "i686-redhat-linux-gnu".For bug reporting instructions, please see:<http://www.gnu.org/software/gdb/bugs/>...Reading symbols from /home/taoge/test/a.out...done.[New Thread 3355]Missing separate debuginfo for Try: yum --disablerepo='*' --enablerepo='*-debuginfo' install /usr/lib/debug/.build-id/74/d23352fd770753e375bd0caecf375bd77bded5Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.Loaded symbols for /lib/libc.so.6Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.Loaded symbols for /lib/ld-linux.so.2Core was generated by `./a.out'.Program terminated with signal 11, Segmentation fault.#0  0x008de17c in fwrite () from /lib/libc.so.6Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.7.el6.i686(gdb) (gdb) (gdb) (gdb) bt#0  0x008de17c in fwrite () from /lib/libc.so.6#1  0x08048418 in main () at test.c:10(gdb)

定位到了代码的第10行， fwrite产生了core, 在追根溯源，发现fp为NULL, 于是一切得到了合理解释。

最后再次说明一下，在实际开发中，一定要对fopen的返回值进行校验，我至少碰到过一次了。项目复杂后，这种问题搞得人满头大汗。如果是概率性问题，那就头要大了。偶尔，我在博客中，为了图简便，省略了一些检验，呵呵。

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