Thread: get input string

>how can i get an input string with an unknown size ?
You have to be careful about it when it comes to unknown sized strings. The most commonly used way is to dynamically allocate memory for a string and if you run out of memory, realloc it:

Code:

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

#define BASE 256
#define STEP ( BASE + 256 ) /* Or whatever suits you */

static char *readline ( FILE *f )
{
  size_t size, nread;
  char *buf, *save, *read;
  
  if ( ( buf = malloc ( BASE ) ) == NULL )
    return NULL;

  size = nread = BASE;
  read = buf;
  
  if ( fgets ( read, nread, f ) == NULL ) {
    free ( buf );
    return NULL;
  }

  while ( ( save = strchr ( read, '\n' ) ) == NULL ) {
    nread = STEP + 1;
    size += STEP;

    if ( ( save = realloc ( buf, size ) ) == NULL )
      return buf;

    buf = save;
    read = buf + ( size - STEP - 1 );
    
    if ( fgets ( read, nread, f ) == NULL ) {
      free ( buf );
      return NULL;
    }
  }
  
  *save = '\0';
  nread = save - buf;

  if ( ( save = realloc ( buf, nread + 1 ) ) == NULL )
    return buf;
  
  return save;
}

int main ( void )
{
  char *p = readline ( stdin );
  
  puts ( p );
  free ( p );
  
  return 0;
}

-Prelude

Use realloc , or use a linked list
One possible solution

Code:

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

char *addc(char *s,char c,int i)
{
        char *tmp;
        if(!s) 
        {
                s=malloc(1);
                if(!s)
                {
                        perror(NULL);
                        exit(1);
                }
                s[0]=c;
                return s;
        }
        tmp=realloc(s,i+1);
        if(tmp)
        {
             s=tmp;
             s[i]=c;
        }
        else
        {
             perror(NULL);
             exit(1);
        }
        i++;
        return s;
}

int main(void)
{
        char *str=0,c,*tmp;
        int i=0;
        while( (c=getchar()) != EOF)
        {
                str=addc(str,c,i++);
        }
        str=addc(str,0,i);
        printf("str=%s",str);
        return 0;
}

$echo 11223 | ./ a.out
$str=11223

@pinko_liberal
A couple of comments:

malloc'ing one byte of memory at a time is a relatively expensive way of doing business. It would be better to malloc in chunks in a real app (ie not a demo, like this one)

>>i++;
This achieves nothing, as you don't use i after incrementing it.

>>char c;
>>while( (c=getchar()) != EOF)
getchar() returns an int, not a char.

You are right, I wonder how I could have declared c a char, reallocing in chucks is always better of course, but this was just a quick and dirty program.

Originally posted by Hammer
@pinko_liberal
A couple of comments:

malloc'ing one byte of memory at a time is a relatively expensive way of doing business. It would be better to malloc in chunks in a real app (ie not a demo, like this one)

>>i++;
This achieves nothing, as you don't use i after incrementing it.

>>char c;
>>while( (c=getchar()) != EOF)
getchar() returns an int, not a char.