解决gdb 在64位机器上显示sizeof(long)=4的问题

问题：gdb 在64位机器上显示sizeof(long)=4

大家都知道在64位机器上，c语言的long类型占用8个字节，但今天因验证一个功能，启动gdb，执行了print sizeof(long) 返回值为4，让人大为震惊，是服务器环境有问题吗？

赶快编写一个最简单的c程序

1 #include <stdio.h>
2 
3 int main(void){
4   printf("sizeof(int)=%d\n",sizeof(int));
5   printf("sizeof(long)=%d\n",sizeof(long));
6   printf("sizeof(long int)=%d\n",sizeof(long int));
7   printf("sizeof(long long)=%d\n",sizeof(long long));
8   return 0 ;
9 }

编译运行

1 [ansible@rocky works]$ gcc p.c
2 [ansible@rocky works]$ ./a.out 
3 sizeof(int)=4
4 sizeof(long)=8
5 sizeof(long int)=8
6 sizeof(long long)=8

看来服务器没有问题，估计是gdb的问题，在搜索引擎上搜索，得到有效资料：

1、Bug 80777 - gdb sizeof(long int)=4 but gcc sizeof(long int)=8 on x86_64-pc-cygwin
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80777
2、Bug 518712 - gdb thinks sizeof(long)==4 on 64bit
https://bugzilla.redhat.com/show_bug.cgi?id=518712

网络资料：验证过程:

[ansible@rocky ~]$ gdb
GNU gdb (GDB) Rocky Linux 10.2-13.el9
Copyright (C) 2021 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 "x86_64-redhat-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word".
(gdb) print sizeof(int)
$1 = 4
(gdb) print sizeof(long)
$2 = 4
(gdb) print sizeof(long int)
$3 = 4
(gdb) print sizeof(long long)
$4 = 8
(gdb) show arch
The target architecture is set to "auto" (currently "i386").
(gdb) set arch
Requires an argument. Valid arguments are i386, i386:x86-64, i386:x64-32, i8086, i386:intel, i386:x86-64:intel, i386:x64-32:intel, s390:64-bit, s390:31-bit, rs6000:6000, rs6000:rs1, rs6000:rsc, rs6000:rs2, powerpc:common64, powerpc:common, powerpc:603, powerpc:EC603e, powerpc:604, powerpc:403, powerpc:601, powerpc:620, powerpc:630, powerpc:a35, powerpc:rs64ii, powerpc:rs64iii, powerpc:7400, powerpc:e500, powerpc:e500mc, powerpc:e500mc64, powerpc:MPC8XX, powerpc:750, powerpc:titan, powerpc:vle, powerpc:e5500, powerpc:e6500, arm, armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5te, xscale, ep9312, iwmmxt, iwmmxt2, armv5tej, armv6, armv6kz, armv6t2, armv6k, armv7, armv6-m, armv6s-m, armv7e-m, armv8-a, armv8-r, armv8-m.base, armv8-m.main, armv8.1-m.main, arm_any, aarch64, aarch64:ilp32, aarch64:armv8-r, auto.
(gdb) set arch i386:x86-64
The target architecture is set to "i386:x86-64".
(gdb) show arch
The target architecture is set to "i386:x86-64".
(gdb) print sizeof(int)
$5 = 4
(gdb) print sizeof(long)
$6 = 8
(gdb) print sizeof(long int)
$7 = 8
(gdb) print sizeof(long long)
$8 = 8

虽然极少情况使用gdb当计算器，但万一呢？还是找一下解决方案吧

解决: 设置gdb的配置文件

vi ~/.gdbinit
set arch i386:x86-64

保存后，再执行gdb

[ansible@rocky works]$ gdb
...
For help, type "help".
Type "apropos word" to search for commands related to "word".
The target architecture is set to "i386:x86-64".
(gdb) print sizeof(int)
$3 = 4
(gdb) print sizeof(long)
$4 = 8
(gdb) print sizeof(long int)
$2 = 8
(gdb) print sizeof(long long)
$1 = 8

补充，如果gdb一个执行程序， 则gdb自动从程序处获得arch 信息

删除 .gdbinit

gdb
GNU gdb (GDB) Rocky Linux 10.2-13.el9
...
(gdb) show arch
The target architecture is set to "auto" (currently "i386").

调试a.out
[ansible@rocky works]$ gdb a.out
...
(gdb) show arch
The target architecture is set to "auto" (currently "i386:x86-64").

总结：gdb一个简单的问题，在32位机器上没有问题，但在64位机器上的缺省参数没有做到自适应，白白给人带来了困扰，或者说当64位机器成为主流后，代码的维护者没有及时修改，至少属于不积极吧。我们在开发软件过程中要引以为戒，充分考虑软件的主流运行平台的适应以及平台兼容性问题。