C语言函数指针详解

#include <stdio.h>
#define  GET_MAX 	0
#define  GET_MIN 	1
int get_max(int i,int j)
{
	return i>j?i:j;
}
int get_min(int i,int j)
{
	return i>j?j:i;
}
int compare(int i,int j,int flag)
{
	int ret;
	//这里定义了一个函数指针，就可以根据传入的flag，灵活地决定其是指向求大数或求小数的函数
	//便于方便灵活地调用各类函数
	int (*p)(int,int);
	if(flag == GET_MAX)
		p = get_max;
	else
		p = get_min;
	ret = p(i,j);
	return ret;
}
int main()
{
	int i = 5,j = 10,ret;
	ret = compare(i,j,GET_MAX);
	printf("The MAX is %d\n",ret);
	ret = compare(i,j,GET_MIN);
	printf("The MIN is %d\n",ret);
	return 0 ;
}

#include <stdio.h>
#include <string.h>
void check(char *a,char *b,int (*cmp)(const char *,const char *));
main()
{
    char s1[80],s2[80];
    int (*p)(const char *,const char *);
	//将库函数strcmp的地址赋值给函数指针p
    p=strcmp;
    printf("Enter two strings.\n");
    gets(s1);
    gets(s2);
    check(s1,s2,p);
}
void check(char *a,char *b,int (*cmp)(const char *,const char *))
{
    printf("Testing for equality.\n");
	//表达式(*cmp)(a,b)调用strcmp，由cmp指向库函数strcmp()，由a和b作调用strcmp()的参数。
	//调用时，与声明的情况类似，必须在*cmp周围使用一对括号，使编译程序正确操作，
	//同时这也是一种良好的编码风格，指示函数是通过指针调用的，而不是函数名。
    if((*cmp)(a,b)==0)
        printf("Equal\n");
    else
        printf("Not Equal\n");
}

#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include <stdio.h>
// example of qsort using function pointer for comparison function (*compar)()
// see man qsort
char *num[] = {
	"000000001",
	"1", 
	"1000", 
	"  100 ", 
	"1     ", 
};

// compare p1 & p2 as strings
// call with the address of the array element, e.g. &(char *) = char **
int string_comp(const void *p1, const void *p2) 
{
	// be careful that address of element is passed so there is an
	// extra * needed here given that it is already (char *)
	// return strcmp((char *) p1, (char *) p2); /* a bug as needs deref */
	return strcmp(*((char **) p1), *((char **) p2));
}
// compare p1 & p2 as integers
int int_comp(const void *p1, const void *p2) 
{
	int i1, i2;
	// i1 = atoi((char *) p1); /* bug: same reason as line in string_comp() */
	i1 = atoi(*((char **) p1)); /* correct */
	// i2 = atoi((char *) p2); /* bug: same reason as line in string_comp() */
	i2 = atoi(*((char **) p2)); /* correct */
	// printf("comp(%s,%s)\n", p1, p2); /* bug: for debugging */
	// printf("comp(\"%s\", \"%s\")\n", *(char **) p1, *(char **) p2); /* correct: for debugging */
	if (i1 < i2) return -1;
	else if (i1 == i2) return 0;
	else return 1;
}
void print_array(char *a[], int n)
{
	int i;
	for (i=0; i < n; i++)
		printf("\"%s\" ", a[i]);
}
int main()
{
	printf("Original array\n"); print_array(num, 5); printf("\n");
	qsort(num, 5, sizeof(char *), int_comp);
	printf("sorted array as int\n"); print_array(num, 5); printf("\n");
	qsort(&num, 5, sizeof(char *), string_comp);
	printf("sorted array as string\n"); print_array(num, 5); printf("\n");
	return(0);
}

#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <errno.h>
#include <sys/mman.h>
#define L (32)
int addnum(int a)
{
	return a+255;
}
int main(int argc, char *argv[], char *envp[])
{
	int a, (*f)(int), *code_buf;
	char *p, data[]={0x0f, 0x0b};
	/* Try uncommenting out */
	/*
	f = NULL;
	a = f(10);
	printf("0: f(10)=%d f=%p\n\n", a, f);
	*/
	f = addnum;
	a = f(10); // LINE1
	printf("1: f(10)=%d f=%p\n\n", a, f);
	code_buf = (int *) mmap(0, 4096, PROT_EXEC|PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
	/* Try swapping the two mmap lines */
	// code_buf = (int *) mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
	printf("code_buf %p\n", code_buf);
	memcpy(code_buf, addnum, L);
	f = (int (*)(int)) code_buf;
	a = f(10); // LINE2
	printf("2: f(10)=%d f=%p\n\n", a, f);
	p = index((char *) code_buf, 0xff);
	printf("before *p=%hhx\n", *p);
	*p = 100;
	printf("after *p=%hhx\n", *p);
	a = f(10); // LINE3
	printf("3: f(10)=%d\n\n", a);
	*((char *) code_buf) = 0xc3;
	a = f(10); // LINE4
	printf("4: f(10)=%d\n\n", a);
	memcpy(code_buf, data, 2);
	printf("before last call\n");
	a = f(10); // LINE5
	return 0;
}

#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <errno.h>
#include <sys/mman.h>
int encrypt(int a) // simple encryption with a (secret) constant
{
	return a ^ 255;
}
int main(int argc, char *argv[], char *envp[])
{
	int (*f)(int);
	int secret, x, y;
	// test encrypt
	f = encrypt;
	printf("1: enter test data\n");
	scanf("%d", &x);
	y = f(x);
	printf("1: test original encrypt(): f(%x)=%x\n", x, y);
	printf("enter new key\n");
	scanf("%d", &secret);
	// create a new function by using encrypt's code-bytes as a template
	// which is similar to encrypt() except that it is xor with the secret
	// key which has been input
	// let f point to the new function created by the code below
	// LINE1
	/* your code goes here */
	// LINE2
	secret = 255; // erase secret, set it back to the original key 
	// test if it works
	printf("2: value to encrypt\n");
	scanf("%d", &x);
	y = f(x); // should use the input secret key above
	printf("2: f(): f(%x)=%x\n", x, y);
	printf("erased secret %d\n", secret);
	// test if f() is modifiable
	*((int *)f) = 0; // LINE3: must segfault here
	return 0;
}