指向函数的指针

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

#define MAXLEN 1000
#define MAXLINES 5000

char *lineptr[MAXLINES];
int readlines(char *lineptr[], int nlines); 
void writelines(char *lineptr[], int nlines); 
void qsort(void *lineptr[], int left, int right, int (*comp)(void *, void *)); 
int numcmp(char *, char *); 
int getline(char *, int);
char *alloc(int);

int main(int argc, char *argv[]) 
{ 
	int nlines; 
	int numeric = 0;
	if (argc > 1 && strcmp(argv[1], "-n") == 0) 
		numeric = 1; 
	if ((nlines = readlines(lineptr, MAXLINES)) >= 0) { 
		qsort((void**) lineptr, 0, nlines-1, (int (*)(void*,void*))(numeric ? numcmp : strcmp)); 
		writelines(lineptr, nlines); 
		return 0; 
	} else { 
		printf("input too big to sort\n"); 
		return 1; 
	} 
}

void qsort(void *v[], int left, int right, 
		   int (*comp)(void *, void *)) 
{ 
	int i, last; 
	void swap(void *v[], int, int); 
	if (left >= right) 
		return; 
	swap(v, left, (left + right)/2); 
	last = left; 
	for (i = left+1; i <= right; i++) 
		if ((*comp)(v[i], v[left]) < 0) 
			swap(v, ++last, i); 
		swap(v, left, last); 
		qsort(v, left, last-1, comp); 
		qsort(v, last+1, right, comp); 
}

int numcmp(char *s1, char *s2) 
{ 
	double v1, v2; 
	v1 = atof(s1); 
	v2 = atof(s2); 
	if (v1 < v2) 
		return -1; 
	else if (v1 > v2) 
		return 1; 
	else 
		return 0; 
}

void swap(void *v[], int i, int j) 
{ 
	void *temp; 
	temp = v[i]; 
	v[i] = v[j]; 
	v[j] = temp; 
}

int readlines(char *lineptr[], int maxlines)
{
	int len, nlines;
	char *p, line[MAXLEN];
	
	nlines = 0;
	while((len = getline(line, MAXLEN)) > 0)
		if (nlines >= maxlines || (p = alloc(len)) == NULL)
			return -1;
		else
		{
			line[len-1] = '\0';
			strcpy(p, line);
			lineptr[nlines++] = p;
		}
		return nlines;
}

void writelines(char *lineptr[], int nlines)
{
	int i;
	
	for (i=0; i<nlines; i++)
	{
		printf("%s\n", lineptr[i]);
	}
}

int getline(char *s, int max)
{
	int i;
	int c;
	for (i=0; i<max-1 && (c = getchar()) != EOF && c != '\n'; i++)
	{
		s[i] = c;
	}
	if (c == '\n')
	{
		s[i] = c;
		++i;
	}
	s[i] = '\0';
	return i;
}

#define ALLOCSIZE 10000
static char allocbuf[ALLOCSIZE];
static char *allocp = allocbuf;

char *alloc(int n)
{
	if (allocbuf + ALLOCSIZE - allocp >= n)
	{
		allocp += n;
		return allocp - n;
	}
	else
	{
		return 0;
	}
}