unable to read from stdin inside a loop

Question

This code works when these lines are commented out as shown.But if those 4 lines are uncommented,there is a SIGSEV fault-string 's' does not get initialised . How is it that a function call works at its own but not in a loop ? when the loop has nothing to do with the function's data.

  #define yes 1
    #define no 0
    #include<stdio.h>
    #include<stdlib.h>
    #include<string.h>
    char *readline(FILE *f){
        char *buff=NULL,*newbuff=NULL,c;
        int buf_sz=1,len=0;
        int keep_going=yes;
        while(keep_going){
            c=fgetc(f);
            if(c==EOF || c=='\n'){
                keep_going=no;
                break;
            }
            if(len==buf_sz-1 && keep_going!=no){
                if(!buff){
                    buf_sz=512;
                    newbuff=malloc(buf_sz);
                }
                else{
                    buf_sz*=2;
                    newbuff=realloc(buff,buf_sz);
                }
                buff=newbuff;
            }
            buff[len++]=c;
        }
        return buff;
    }
    int main(){
        char *s;
        int l,left_mid,right_mid,lp,rp,change=no;
    //  int n;
    //  scanf("%d",&n);
    //  while(n--){
        s=readline(stdin);
        l=strlen(s);
            printf("%d",l);
        //}

            return 0;

          }

You fail to nul-terminate the buffer when jumping outof the loop. BTW: the loop can be simplified a lot. BTW: realloc(NULL, ...) works just like malloc, so you could omit that extra condition. — wildplasser
– wildplasser, Commented Jan 2, 2014 at 21:29
A blank line results in readline returning NULL, which then immediately crashes in strlen. If the first line with n has nothing else on it, that will be a blank line... — Chris Dodd
– Chris Dodd, Commented Jan 2, 2014 at 21:58

BLUEPIXY · Accepted Answer · 2014-01-03 10:57:47Z

1

buff needs to be null-terminated.
scanf("%d", &n); keeps the newline.
The buffer is not secured when only a newline is entered.

example to fix

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

char *readline(FILE *f){
    char *buff=NULL,*newbuff=NULL;
    int c;
    int buf_sz=512,len=0;
    buff=malloc(buf_sz);
    if(!buff) return NULL;
    while(EOF != (c=fgetc(f)) && c != '\n'){
        if(len == buf_sz -1){
            buf_sz*=2;
            newbuff=realloc(buff,buf_sz);
            if(newbuff==NULL){
                free(buff);
                return NULL;
            }
            buff=newbuff;
        }
        buff[len++]=c;
    }
    buff[len]='\0';

    return buff;
}

int main(){
    char *s;
    int n, l;
    scanf("%d%*c",&n);
    while(n--){
        s=readline(stdin);//check return value
        l=strlen(s);
        printf("%d\n", l);
        free(s);
    }
    return 0;

}

edited Jan 3, 2014 at 10:57

answered Jan 2, 2014 at 21:48

BLUEPIXY

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3 Comments

piyukr Over a year ago

could you elaborate please?

BLUEPIXY Over a year ago

@piyukr newline are remaining to be consumed when you enable the scanf. So, when readline is called, a state in which you enter a new line only.

BLUEPIXY Over a year ago

@piyukr NULL is returned in the buffer is not secured so exit the loop immediately when readline is performed on the new line only. strlen (NULL) causes an exception.

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