Example in K&R accessing local variable outside its function?

Question

From this question I learned that you indeed should not export a local variable's address and use it outside the function in which it was declared.

However, it seems to me that K&R are breaking this rule in the program shown below taken from their book, p. 108.

I'm looking at the line lineptr[nlines++] = p; inside the function readlines. Why is it here OK to "export" p and use it later outside readlines?

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

#define MAXLINES 5000

char *lineptr[MAXLINES];

int readlines(char *lineptr[], int nlines);
void writelines(char *lineptr[], int nlines);

void qsort(char *lineptr[], int left, int right);

int main(int argc, char *argv[])
{
     int nlines;

     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
int getline(char *, int);
char *alloc(int);

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)
{
     while(nlines -- > 0)
         printf("%s\n", *lineptr++);
}

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;   
  }
  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;
}

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

     temp = v[i];
     v[i] = v[j];
     v[j] = temp;
}

void qsort(char *v[], int left, int right) {

    int i, last;

    if(left >= right) 
       return;

    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);
}

I'm 99.9% sure that `system("PAUSE");` _never_ appeared in K&R's book. — CB Bailey, Jun 18 '13 at 07:27
Where do you see the address of a local variable being taken? — Jim Balter, Jun 18 '13 at 07:56
I seem to have misunderstood what it exactly is that is being taken/exported. Nikos C.'s explanation below made everything clear. — Rasmus Rønn Nielsen, Jun 18 '13 at 08:04
My point is that, in the question you cited, &a is the problem, whereas there's no &p here ... that alone should make it clear that your citation isn't relevant. — Jim Balter, Jun 18 '13 at 08:16

Nikos C. · Accepted Answer · 2013-06-18T07:43:22.053

In:

lineptr[nlines++] = p;

the value of p is stored, not its address. The address is &p. Of course, the value of p happens to be an address, since p is a pointer, and the value of a pointer represents an address. But that has no consequence here. The rule is still being followed; no address of any local variable has been stored anywhere outside the function, and the value of p is not the address of a local variable.

If you follow the call chain you can determine that the value of p can be either 0, or be an address somewhere inside allocbuf. And allocbuf is not a local variable. It's a static variable at file scope.

Aaaaah, of course! Now I can get on with my life :) Thank you so much. — Rasmus Rønn Nielsen, Jun 18 '13 at 07:58

Example in K&R accessing local variable outside its function?

1 Answers1