realloc一个有意思的测试

百度百科：

      realloc 先判断当前的指针是否有足够的连续空间，如果有，扩大mem_address指向的地址，并且将mem_address返回，如果空间不够，先按照newsize指定的大小分配空间，将原有数据从头到尾拷贝到新分配的内存区域，而后释放原来mem_address所指内存区域（注意：原来指针是自动释放，不需要使用free），同时返回新分配的内存区域的首地址。即重新分配存储器块的地址。 

在本人的电脑上做的测试，gcc版本

[email protected]:~/sources/workbox/0001_app/telproxy_server$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 7.4.0-1ubuntu1~18.04.1' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 7.4.0 (Ubuntu 7.4.0-1ubuntu1~18.04.1)

 百度了一下gcc版本，发现7 .4 .0也不是很老，2018年12月6号发布。

GCC 9.2 released [2019-08-12]
PRU support [2019-06-12]
GCC support for TI PRU I/O processors has been added.
GCC 9.1 released [2019-05-03]
GNU Tools Cauldron 2019 [2019-04-15]
Will be held in Montréal, Canada, September 12-15 2019
GCC 8.3 released [2019-02-22]
AMD GCN support [2019-01-17]
GCC support for AMD GCN Fiji and Vega GPUs has been added. This back end was contributed by Mentor Graphics.
GCC 7.4 released [2018-12-06]
D front end added [2018-10-29]
The D programming language front end has been added to GCC. This front end was contributed by Iain Buclaw.
GCC 6.5 released [2018-10-26]

编译过程 gcc test.c 

测试一： 

#include<stdio.h>
#include<stdlib.h>

int main(int argc, char** argv)
{

        char* str = (char*)malloc (16);
        snprintf(str, 16, "%s", "yangpan");

        printf ("ptr:%p   %s\n",str,  str);

        char* nstr = realloc(str, 1024*1024);
        printf ("ptr:%p   %s\n",nstr,  nstr);

        free (str);
        free (str);
        free (str);
        free (str);
        free (str);
        free (nstr);

        printf ("ptr:%p   %s\n",str,  str);
        snprintf(str, 64, "%s", "hbxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
        printf ("ptr:%p   %s\n",str,  str);
        return 0;
}

居然我的电脑编译后能正常运行，不会蹦掉。哈哈。下面是运行结果

ptr:0x55f589948260   yangpan
ptr:0x7f6be433c010   yangpan
ptr:0x55f589948260   ` 
ptr:0x55f589948260   hbxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

测试二：

再进行测试，简单点，敲代码的方式简单的

int main(int argc, char** argv)
{

        char* str = (char*)malloc (16);
        if(!str)
        {
            printf("malloc err\n");
        }
        free (str);
        free (str);
        free (str);
        free (str);
        free (str);
        return 0;
}

上面代码运行正常运行。怎么才能让malloc崩掉呢？很烦恼。

再进行测试三：

#include<stdio.h>
#include<stdlib.h>


int main(int argc, char** argv)
{

        char* str = (char*)malloc (1024+8);

        snprintf(str, 64, "%s", "hbxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
        printf ("ptr:%p   %s\n",str,  str);

        str[1024+80] = 'y';
        printf ("str[1024+80]:%c\n", str[1024+80]);

        free (str);
        free (str);
        free (str);
        free (str);
        free (str);
        return 0;
}

诶，终于崩掉了，运行结果

[email protected]:~/sources/workbox/0001_app/telproxy_server$ ./a.out 
ptr:0x55af8c7f0260   hbxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
str[1024+80]:y
double free or corruption (!prev)
Aborted (core dumped)

再进行测试四：

#include<stdio.h>
#include<stdlib.h>


int main(int argc, char** argv)
{

        char* str = (char*)malloc (1024+8);

        snprintf(str, 64, "%s", "hbxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
        printf ("ptr:%p   %s\n",str,  str);

        str[1024+80] = 'y';
        printf ("str[1024+80]:%c\n", str[1024+80]);

        free (str);
        free (str);
        free (str);
        free (str);
        free (str);
        return 0;
}

运行有不崩了，运行结果

[email protected]:~/sources/workbox/0001_app/telproxy_server$ ./a.out 
ptr:0x55e4cb149260   hbxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
str[1024+80]:y

看来1024+8是一个坎。待续.....