K&Rcbookastripped-downversionofUnixsort

在K&R c书籍中，有一个示例程序是实现Unix sort命令的功能，它被称为'裸露的Unix sort”。这个程序的实现过程展示了使用C语言处理字符串和文件的基本方法。

下面是该程序的代码（摘自K&R c书籍）：

/* sort input lines */
#include <stdio.h>
#include <string.h>
#define MAXLINES    5000    /* max #lines to be sorted */
#define MAXLEN      1000    /* max length of any input line */
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 */
int 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);
/* 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 = malloc(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