Exercise 5.7. Rewrite readlines to store lines in an array supplied by main, rather than calling alloc to maintain storage.

Exercise 5.7. Rewrite readlines to store lines in an array supplied by main, rather than
calling alloc to maintain storage.

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

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

char *lineptr[MAXLINES]; //pointers to text lines

int readlines(char *lineptr[], char *buf, 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
    char buf[BUFSIZE];  
    if((nlines = readlines(lineptr, buf, MAXLINES)) >= 0)
    {
        printf("%d\n", nlines);
        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 lenght of any input line


int getlin(char *, int);

/* readlines: read input lines */
int readlines(char *lineptr[], char *buf, int maxlines)
{
    int len, nlines;

    char line[MAXLEN];
    
    char *p = buf;     
    char *bufstop = buf + BUFSIZE;
    
    nlines = 0;
    while((len = getlin(line, MAXLEN)) > 0)
    {
        if(nlines >= maxlines || p + len > bufstop)
            return -1;
        else
        {
            line[len - 1] = '\0'; //delete newline
            strcpy(p, line);
            lineptr[nlines++] = p;
            p += len;
        }
    }
    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]);
}

int getlin(char *l, int lim)
{
    int c;
    char *tmp = l;
    for(; --lim > 0 && (c=getchar()) != EOF && c!='\n'; l++)
        *l = c;
    if(c=='\n')
        *l++ = c;
    *l = '\0';
    return l - tmp;
}


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

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

Result: