# 桩函数

在做单元测试时，经常需要将特定函数替换成测试需要模拟的函数，以便产生特定的输入或输出。这个过程称为打桩（stub），替换后的函数称为桩函数。

打桩的方式有多种，这里给出的方式比较常用：

stub.h：

```c
#ifndef __STUB_H__
#define __STUB_H__

#ifdef __cplusplus
exern "C"
{
#endif

#define CODESIZE 5U

struct func_stub
{
	void *fn;
	unsigned char code_buf[CODESIZE];
};

int stub_init();
void stub_set(struct func_stub *pstub, void *fn, void *fn_stub);
void stub_reset(struct func_stub *pstub);

#ifdef __cplusplus
}
#endif

#endif
```

stub.c：

```c
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <errno.h>
#include <limits.h>
#include <sys/mman.h>

#include "stub.h"

static long pagesize = -1;

static inline void *pageof(const void* p)
{
	return (void *)((unsigned long)p & ~(pagesize - 1));
}

void stub_set(struct func_stub *pstub, void *fn, void *fn_stub)
{
	pstub->fn = fn;
	memcpy(pstub->code_buf, fn, CODESIZE);

	if (-1 == mprotect(pageof(fn), pagesize * 2, PROT_READ | PROT_WRITE | PROT_EXEC))
	{
		perror("mprotect to w+r+x faild");
		exit(errno);
	}

	*(unsigned char *)fn = (unsigned char)0xE9;
	*(unsigned int *)((unsigned char *)fn + 1) = (unsigned char *)fn_stub - (unsigned char *)fn - CODESIZE;

	if (-1 == mprotect(pageof(fn), pagesize * 2, PROT_READ | PROT_EXEC))
	{
		perror("mprotect to r+x failed");
		exit(errno);
	}

	return;
}

void stub_reset(struct func_stub *pstub)
{
	if (NULL == pstub->fn)
	{
		return;
	}

	if (-1 == mprotect(pageof(pstub->fn), pagesize * 2, PROT_READ | PROT_WRITE | PROT_EXEC))
	{
		perror("mprotect to w+r+x faild");
		exit(errno);
	}

	memcpy(pstub->fn, pstub->code_buf, CODESIZE);

	if (-1 == mprotect(pageof(pstub->fn), pagesize * 2, PROT_READ | PROT_EXEC))
	{
		perror("mprotect to r+x failed");
		exit(errno);
	}

	memset(pstub, 0, sizeof(struct func_stub));

	return;
}
 
int stub_init(void)
{
	int ret;

	pagesize = sysconf(_SC_PAGE_SIZE);

	ret = 0;
	if (pagesize < 0)
	{
		perror("get system _SC_PAGE_SIZE configure failed");
		ret = -1;
	}

	return ret;
}
```

main.c：

```c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include "stub.h"

void f1()
{
	printf("f1\n");
	return;
}

void f2()
{
	printf("f2\n");
	return;
}

void *_memset(void *s, int ch, size_t n)
{
	printf("-memset\n");
	return s;
}

int main()
{
	char ac[10] = {1};
	struct func_stub stub;

	if (-1 == stub_init())
	{
		printf("faild to init stub\n");
		return 0;
	}

	// stub memset
	stub_set(&stub, (void *)memset, (void *)_memset);
	memset(ac, 0, 10);
	stub_reset(&stub);

	memset(ac, 0, 10);
	printf("ac[0] = %hhu\n", ac[0]);

	// stub f1
	stub_set(&stub, (void *)f1, (void *)f2);
	f1();
	stub_reset(&stub);
	f1();

	return 0;
}
```

# 参考

* [C语言的3种打桩方法总结](C语言的3种打桩方法总结)
* [cpp-stub](https://github.com/coolxv/cpp-stub)
* [stub in DianH3C](https://github.com/DianH3C/INFO/blob/master/lifan/Project/INFO/ut/stub/stub.c)