使用ld的wrap选项替换已有库函数

很多时候，可能需要替换已有库函数，或者对其库函数进行修改。
为了避免对那些静态链接库或者动态链接库文件大动干戈，我们可以使用ld提供的–wrap选项。

例如，想把所有的malloc函数都作修改，以便让malloc出的内存都是32字节对齐的。
我们可以给ld传选项“­­wrap=malloc”， 告诉ld，我们将替换名称为malloc的函数。
接着再如下定义一个新的malloc函数:

<span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">void</span> * <span style="font-size:12px;color:#00ff;margin:0px; padding:0px; border:0px; vertical-align:baseline;">__wrap_malloc</span>( <span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">size_t</span> <span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">size</span>) {  <span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">void</span>* <span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">result</span>;  result=memalign(32, size);  <span style="font-size:12px;color:#a020f0;margin:0px; padding:0px; border:0px; vertical-align:baseline;">return</span> result;}

可以看到，程序中有个类似库函数名称的__wrap_malloc。
ld在遇到__wrap选项后，会使用__wrap_malloc函数名替换所有对malloc的调用。
并以此实现替换的作用。

那麽，如果还向调用原函数怎么办呢？
可以使用__real_malloc，这个函数名称就对应了原来的malloc。
每次调用malloc时都打印生成的指针地址。

<span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">void</span> * <span style="font-size:12px;color:#00ff;margin:0px; padding:0px; border:0px; vertical-align:baseline;">__wrap_malloc</span>( <span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">size_t</span> <span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">size</span>) {  <span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">void</span>* <span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">result</span>;  result= __real_malloc( size);  printf(<span style="font-size:12px;color:#bc8f8f;margin:0px; padding:0px; border:0px; vertical-align:baseline;">"Malloc: allocate %d byte mem, start at %p"</span>, size, result);  <span style="font-size:12px;color:#a020f0;margin:0px; padding:0px; border:0px; vertical-align:baseline;">return</span> result;}

附完整程序：

<span style="font-size:12px;color:#da70d6;margin:0px; padding:0px; border:0px; vertical-align:baseline;">#include</span> <span style="font-size:12px;color:#bc8f8f;margin:0px; padding:0px; border:0px; vertical-align:baseline;"><stdio.h></span><span style="font-size:12px;color:#da70d6;margin:0px; padding:0px; border:0px; vertical-align:baseline;">#include</span> <span style="font-size:12px;color:#bc8f8f;margin:0px; padding:0px; border:0px; vertical-align:baseline;"><stdlib.h></span><span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">void</span> * <span style="font-size:12px;color:#00ff;margin:0px; padding:0px; border:0px; vertical-align:baseline;">__wrap_malloc</span>( <span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">size_t</span> <span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">size</span>) {  <span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">void</span>* <span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">result</span>;  <span style="font-size:12px;color:#b22222;margin:0px; padding:0px; border:0px; vertical-align:baseline;">//</span><span style="font-size:12px;color:#b22222;margin:0px; padding:0px; border:0px; vertical-align:baseline;">result= __real_malloc( size); </span>  result = memalign(32, size);  printf(<span style="font-size:12px;color:#bc8f8f;margin:0px; padding:0px; border:0px; vertical-align:baseline;">"Malloc: allocate %d byte mem, start at %p"</span>, size, result);  <span style="font-size:12px;color:#a020f0;margin:0px; padding:0px; border:0px; vertical-align:baseline;">return</span> result;}<span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">int</span> <span style="font-size:12px;color:#00ff;margin:0px; padding:0px; border:0px; vertical-align:baseline;">main</span> (){  <span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">int</span> <span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">i</span>,<span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">n</span>;  <span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">char</span> * <span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">buffer</span>;  printf (<span style="font-size:12px;color:#bc8f8f;margin:0px; padding:0px; border:0px; vertical-align:baseline;">"How long do you want the string? "</span>);  <span style="font-size:12px;color:#b22222;margin:0px; padding:0px; border:0px; vertical-align:baseline;">//</span><span style="font-size:12px;color:#b22222;margin:0px; padding:0px; border:0px; vertical-align:baseline;">scanf ("%d", &i); </span>  i = 200;  buffer = (<span style="font-size:12px;color:#228b22;margin:0px; padding:0px; border:0px; vertical-align:baseline;">char</span>*) malloc (i+1);  <span style="font-size:12px;color:#a020f0;margin:0px; padding:0px; border:0px; vertical-align:baseline;">if</span> (buffer==<span style="font-size:12px;color:#5f9ea0;margin:0px; padding:0px; border:0px; vertical-align:baseline;">NULL</span>) exit (1);  <span style="font-size:12px;color:#a020f0;margin:0px; padding:0px; border:0px; vertical-align:baseline;">for</span> (n=0; n<i; n++)    buffer[n]=rand()%26+<span style="font-size:12px;color:#bc8f8f;margin:0px; padding:0px; border:0px; vertical-align:baseline;">'a'</span>;  buffer[i]=<span style="font-size:12px;color:#bc8f8f;margin:0px; padding:0px; border:0px; vertical-align:baseline;">'\0'</span>;  printf (<span style="font-size:12px;color:#bc8f8f;margin:0px; padding:0px; border:0px; vertical-align:baseline;">"Random string: %s\n"</span>,buffer);  free (buffer);  <span style="font-size:12px;color:#a020f0;margin:0px; padding:0px; border:0px; vertical-align:baseline;">return</span> 0;}

编译选项：

$<span style="font-size:12px;color:#b886b;margin:0px; padding:0px; border:0px; vertical-align:baseline;">gcc</span> test_malloc.c -Wl,--wrap=malloc