c - 如何用自定义代码包装库函数？

Question

我有一个库实现了malloc。我想用自定义 malloc 函数重写此函数，该函数执行某些操作，然后调用库的 malloc 函数。

如何在不丢失库函数的情况下重新定义符号 malloc？

Answer 1

GNU 链接器提供了 --wrap symbol 标志来将自定义函数包装在现有函数周围。

如您所见，最后一个标志:http://ieee.uwaterloo.ca/coldfire/gcc-doc/docs/ld_3.html#SEC3

--wrap symbol

Use a wrapper function for symbol. Any undefined reference to symbol will be resolved to __wrap_symbol. Any undefined reference to __real_symbol will be resolved to symbol. This can used to provide a wrapper for a system function. The wrapper function should be called __wrap_symbol. If it wishes to call the system function, it should call __real_symbol. Here is a trivial example:

void *
__wrap_malloc (int c) {
    printf ("malloc called with %ld\n", c);
    return __real_malloc (c);
}

If you link other code with this file using --wrap malloc, then all calls to malloc will call the function __wrap_malloc instead. The call to __real_malloc in __wrap_malloc will call the real malloc function. You may wish to provide a __real_malloc function as well, so that links without the --wrap option will succeed. If you do this, you should not put the definition of __real_malloc in the same file as __wrap_malloc; if you do, the assembler may resolve the call before the linker has a chance to wrap it to malloc.