Question

请教一道c语言书上的题目

// 12.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <stdio.h>
#include <string.h>

#define MAXLINES 5000 /* max #lines to be sorted */

char *lineptr[MAXLINES]; /* pointers to text lines */
int readlines(char *lineptr[], int nlines);
void writelines(char *lineptr[], int nlines);

void qsort(char *lineptr[], int left, int right);
/* sort input lines */

main()
{
int nlines; /* number of input lines read */
if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
qsort(lineptr, 0, nlines-1);
writelines(lineptr, nlines);
return 0;
} else {
printf("error: input too big to sort\n");
return 1;
}
}

#define MAXLEN 1000 /* max length of any input line */
int getline(char *, int);
char *alloc(int);
/* readlines: read input lines */
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'; /* delete newline */
strcpy(p, line);
lineptr[nlines++] = p;
}
return nlines;
}

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

/* qsort: sort v[left]...v[right] into increasing order */
void qsort(char *v[], int left, int right)
{
int i, last;
void swap(char *v[], int i, int j);
if (left >= right) /* do nothing if array contains */
return; /* fewer than two elements */
swap(v, left, (left + right)/2);
last = left;
for (i = left+1; i <= right; i++)
if (strcmp(v[i], v[left]) < 0)
swap(v, ++last, i);
swap(v, left, last);
qsort(v, left, last-1);
qsort(v, last+1, right);
}
/* swap: interchange v[i] and v[j] */
void swap(char *v[], int i, int j)
{
char *temp;
temp = v[i];
v[i] = v[j];
v[j] = temp;
}

int getline(char s[],int lim)
{
int c,i;

for(i=0;i<lim-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;
}
请问 alloc 这里有什么用 alloc函数出现在readlines函数的中间

Answer 1

alloc函数的作用是再缓冲区中申请n个字符大小的空间。

执行过程：判断缓冲区中allocp指针后是否还有n个字符的空间，如果存在则修改allocp指针（将allocp指针向后移动n个位置）并返回该函数执行前的allocp位置。否则返回0.

Answer 2

分配一块空间，并且返回首地址