19. c skill¶
在leetcode上提交代码经常会出现一下错误
- Heap-buffer-overflow 访问堆内存超出范围
- Heap-use-after-free 使用已经释放的堆内存
- Stack-buffer-overflow 非法访问栈空间，通常是数据越界
- Global-buffer-overflow 非常访问全局空间，通常是全局变量数组访问越界
复现方式1：
gcc -O -g -fsanitize=address test.c
./a.out
复现方式2：
在CMakeLists.txt添加
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -g -Wall -fsanitize=address")
19.1. 字符串数组¶
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char s1[10] = "This";
char *s2 = "This";
char s3[] = {'T','h','i','s','\0'};
char s4[5] ={0}; s4[0] = 'T'; s4[1]='h'; s4[2]='i'; s4[3]='s';
s4[4]='\0';s4[5]='f';s4[6]='g';
printf("s1 vs s2: %d\n",strcmp(s1,s2));
printf("s1 vs s3: %d\n",strcmp(s1,s3));
printf("s2 vs s3: %d\n",strcmp(s2,s3));
printf("s1 vs s4: %d\n",strcmp(s1,s4));
for(int i=0; i<6; i++)printf("%d:%c:%x ",i,s1[i],s1[i]);printf("\n");
for(int i=0; i<6; i++)printf("%d:%c:%x ",i,s2[i],s2[i]);printf("\n");
for(int i=0; i<6; i++)printf("%d:%c:%x ",i,s3[i],s3[i]);printf("\n");
for(int i=0; i<6; i++)printf("%d:%c:%x ",i,s4[i],s4[i]);printf("\n");
printf("\n");
return 0;
}
0 1 2 3 4 5 6
+---------------------------------
|'T' |'h' |'i' |'s' |'\0'| | |
+---------------------------------
这5个字符串定义等价，一共占用5个存储单元， strlen() = 4, strlen 车辆的数组长度不包含最后一个字符’0’。
在没有地址检查的时候，访问数组是可以随意越界的， 除非越界到系统的阻拦，否则是可以随意越界的。 在编译时使用编译选项
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -g -Wall -fsanitize=address")
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char s1[10] = "This";
char *s2 = "This"; //允许越界， 是否允许越界，和后续的变量定义有关系
char s3[] = {'T','h','i','s','\0'};
char s4[5] ={0}; s4[0] = 'T'; s4[1]='h'; s4[2]='i'; s4[3]='s';
s4[4]='\0';
printf("s1 vs s2: %d\n",strcmp(s1,s2));
printf("s1 vs s3: %d\n",strcmp(s1,s3));
printf("s2 vs s3: %d\n",strcmp(s2,s3));
printf("s1 vs s4: %d\n",strcmp(s1,s4));
for(int i=0; i<10; i++)printf("%d:%c:%x ",i,s1[i],s1[i]);printf("\n");
}
https://beginnersbook.com/2014/01/c-strings-string-functions/
19.2. C 变量类型和最大限制¶
在 glibc 的limit.h [1] 中定义有C语言类型的各种最大限制。
#include <stdio.h>
#include <limits.h>
int main()
{
printf("minmun char:%d maximun char:%d\n",CHAR_MIN, CHAR_MAX);
}
minmun char:0 maximun char:255
19.4. 问题记录¶
[ 7%] Building C object CMakeFiles/151.out.dir/151-reverse-words-in-a-string.c.o
/opt/rh/devtoolset-8/root/usr/bin/cc -g -Wall -fsanitize=address -o CMakeFiles/151.out.dir/151-reverse-words-in-a-string.c.o -c /home/user1/GoodCommand/source/src/leetcode/151-reverse-words-in-a-string.c
Linking C executable 151.out
/usr/bin/cmake -E cmake_link_script CMakeFiles/151.out.dir/link.txt --verbose=1
/opt/rh/devtoolset-8/root/usr/bin/cc -g -Wall -fsanitize=address CMakeFiles/151.out.dir/151-reverse-words-in-a-string.c.o -o 151.out -L/opt/rh/devtoolset-7/root/usr/lib/gcc/aarch64-redhat-linux/7 -rdynamic
/opt/rh/devtoolset-8/root/usr/libexec/gcc/aarch64-redhat-linux/8/ld: cannot find libasan_preinit.o: No such file or directory
/opt/rh/devtoolset-8/root/usr/libexec/gcc/aarch64-redhat-linux/8/ld: cannot find -lasan
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/151.out.dir/build.make:92: 151.out] Error 1
make[2]: Leaving directory '/home/user1/GoodCommand/source/src/leetcode/build'
make[1]: *** [CMakeFiles/Makefile2:67: CMakeFiles/151.out.dir/all] Error 2
make[1]: Leaving directory '/home/user1/GoodCommand/source/src/leetcode/build'
make: *** [Makefile:79: all] Error 2
[user1@centos build]$
| [1] | limits.h https://sourceware.org/git/?p=glibc.git;a=blob;f=include/limits.h;h=8195da78a4a6074d737ec45ba27b8fec6005543e;hb=HEAD |