服务器评测
第一：DEBUG实例
[root@localhost debug]# vim null.c
int a(int *p);
int main(void)
{
int* p = 0;
return a(p);
}
int a(int *p)
{
int y = *p;
return y;
}
~
[root@localhost debug]# ulimit -c unlimited
[root@localhost debug]# gcc -Wall –g null.c
[root@localhost debug]# ./a.out
Segmentation fault (core dumped)
[root@localhost debug]# ls
null.c a.out core.21982
[root@localhost debug]# yum -y install gdb
[root@localhost debug]# gdb a.out core.21982
……………..
……………..
……………..
(gdb) print p
No symbol table is loaded. Use the “file” command.
(gdb) print p
$1 = (int *) 0x0
(gdb) backtrace
#0 0x08048389 in a (p=0x0) at null.c:10
#1 0x08048377 in main () at null.c:6
(gdb) quit (gdb)
第二：优化实例(这里主要采用的是-O0—3和no-loops方式)
[root@localhost opm]# cat test.c
#include <stdio.h>
double powern(double d, unsigned n)
{
double x = 1.0;
unsigned j;
for(j=1;j<n;j++)
x *=d;
return x;
}
int main(void)
{
double sum = 0.0;
unsigned i;
for(i=1;i<=100000000;i++ )
{
sum += powern(i,i%5);
}
printf(“sum = %g\n”,sum);
return 0;
}
[root@localhost opm]# gcc -Wall -O0 test.c -o o0 //优化等级为0—不优化
[root@localhost opm]# time ./o0
sum = 5e+30
real 0m2.815s
user 0m2.799s
sys 0m0.013s
[root@localhost opm]# gcc -Wall -O1 test.c -o o1 //优化等级为1
[root@localhost opm]# time ./o1
sum = 5e+30
real 0m1.849s
user 0m1.843s
sys 0m0.006s
[root@localhost opm]# gcc -Wall -O2 test.c -o o2 //优化等级为2
[root@localhost opm]# time ./o2
sum = 5e+30
real 0m1.923s
user 0m1.910s
sys 0m0.011s
[root@localhost opm]# gcc -Wall -O3 test.c -o o3 //优化等级为1
[root@localhost opm]# time ./o3
sum = 5e+30
real 0m1.460s
user 0m1.453s
sys 0m0.006s
[root@localhost opm]# gcc -Wall -O3 -funroll-loops test.c -o o4 //加入no—loop优化
[root@localhost opm]# time ./o4
sum = 5e+30
real 0m1.322s
user 0m1.308s
sys 0m0.014s
第三：优化帮助发现DEBUG
[root@localhost O]# vim uninit.c
int sign(int x)
{
int s;
if (x>0)
s = 1;
else if (x<0)
s = -1;
return s;
}
[root@localhost O]# gcc -Wall -c uninit.c
[root@localhost O]# gcc -Wall -O1 -c uninit.c
uninit.c: In function ‘sign’:
uninit.c:3: warning: ‘s’ may be used uninitialized in this function
[root@localhost O]# gcc -Wall -O2 -c uninit.c
uninit.c: In function ‘sign’:
uninit.c:3: warning: ‘s’ may be used uninitialized in this function
[root@localhost O]# gcc -Wall -O3 -c uninit.c
uninit.c: In function ‘sign’:
uninit.c:3: warning: ‘s’ may be used uninitialized in this function
编译过程描述实例
[root@localhost hello]# vim hello.c
#include <stdio.h>
int main(void)
{
printf(“Hello World!!!\n”);
return 0;
}
第四：最后列举下编译的过程实例
1、 预处理器阶段（cpp hello.c > hello.i）
[root@localhost hello]# cpp hello.c > hello.i
2、 编译器阶段（gcc -Wall -S hello.i）
[root@localhost hello]# gcc -Wall -S hello.i
3、 汇编器阶段（as hello.s -o hello.o）
[root@localhost hello]# as hello.s -o hello.o
4、 连接器阶段（gcc hello.o）
[root@localhost hello]# gcc hello.o
[root@localhost hello]# ./a.out
Hello World!!!
[root@localhost hello]# file a.out //检测可执行文件的信息
a.out: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux 2.6.9, dynamically linked (uses shared libs), for GNU/Linux 2.6.9, not stripped
[root@localhost hello]# ldd a.out //检查用到的库文件
linux-gate.so.1 => (0x002ab000)
libc.so.6 => /lib/libc.so.6 (0x00b3a000)
/lib/ld-linux.so.2 (0x00b17000)
结束
