Styjun

this the 1-17 problem from kernighan & ritchie, it 's quite a simple problem but when i tested with the value MAXLEN=1000 the complier displays segmentation fault and i dont quite understand why because 1000 seems like quite a resonable value , it's seems to work 10000 but why?

#include <stdio.h>
#define MINL 80
#define MAXLEN 1000
int get_line (char s[] , int lim )

 int c,i,j;
 for (i=0; (c=getchar())!=EOF && c!='n';i++)
 
 if (i<lim-2)
 
 s[j]=c;
 ++j;
 
 if (c=='n')

 
 s[j]=c;
 ++i;
 ++j;
 
 s[j]='';

 return i;


int main ()

 int len;
 char line[MAXLEN];
 while ((len=get_line(line,MAXLEN))>0)
 
 if (len>MAXLEN)
 
 printf("%s",line);
 

 

 return 0;

As said in a remark your variable j is non initialized, so when you set s[j] the behavior is undefined and in your case you have a segmentation fault because you write out of the array.

In fact you do not need that variable :

#include <stdio.h>

#define MAXLEN 10

int get_line (char s[] , int lim )

 int c, i = 0;

 lim -= 1;

 while ((c=getchar()) != EOF)
 
 if (i < lim)
 
 s[i] = c;
 

 i += 1;

 if (c == 'n')
 break;
 

 s[(i < lim) ? i : lim] = '';

 return i;


int main ()

 int len;
 char line[MAXLEN];

 while ((len = get_line(line,MAXLEN)) > 0)
 
 /*if (len>MAXLEN)*/
 
 printf("%d '%s'n", len, line);
 
 

 return 0;

I changed the value of MAXLEN to easily test the case the input string is greater, and also the printf to see the result in all cases

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -pedantic -Wextra -Wall g.c
pi@raspberrypi:/tmp $ ./a.out
aze
4 'aze
'
123456789
10 '123456789'
12345678
9 '12345678
'
1234567890
11 '123456789'
^C

To test the EOF case :

pi@raspberrypi:/tmp $ echo aze | ./a.out
4 'aze
'
pi@raspberrypi:/tmp $ 

You should turn on warnings for your compiler. It would show you that j is uninitialized:

/home/user/main.c:11:15: warning: variable 'j' is uninitialized when used here [-Wuninitialized]
 s[j]=c;
 ^
/home/user/main.c:6:14: note: initialize the variable 'j' to silence this warning
 int c,i,j;
 ^
 = 0

the following proposed code:

1. cleanly compiles


2. performs the desired functionality


3. eliminates confusion in the code layout


4. eliminates unused #define statements


5. eliminates unneeded variables

And now, the proposed code:

#include <stdio.h>


#define MAXLEN 1000


int get_line (char s[] , int lim )

 int i;

 for ( i=0; i<(lim - 2); i++ )
 
 int c = getchar();

 if( c == EOF )
 
 break;
 

 s[i] = (char)c;

 if( c =='n' )
 
 break;
 
 

 s[i] = '';

 return i;



int main ( void )

 int len;
 char line[MAXLEN];
 while ((len=get_line(line,MAXLEN))>0)
 
 printf("%s",line);
 

 return 0;