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Full Discussion: C: inputting string of unknown length
Top Forums Programming C: inputting string of unknown length Post 303040985 by DevuanFan on Monday 11th of November 2019 03:54:15 PM
Old 11-11-2019
C: inputting string of unknown length
I realize this general issue (inputting strings of variable length in C) has been addressed in myriad locations before, but I'm interested in knowing why my specific approach is not working. (BTW I'm intentionally keeping the size increments small so that I can more easily follow what's going on. After it works on a small scale, I can increase the size to something more reasonable. The main motivation for this approach is that I want to increase the size by a fixed increment, not by doubling the allocated memory each time.)

Here is the code:
Code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define SIZE 4

int main(void)
{
	int mem = SIZE;
	char *str = malloc(mem); // let's keep str pointing to beginning of string...
	char *next_read = str; // ...and next_read pointing to where next character should go

	fgets(next_read, mem, stdin);
	next_read--; // so that after we add SIZE to pointer, it points to current '\0'

	while(str[strlen(str)-1] != '\n') // if we got whole string, the last char will be '\n'
	{
		mem += SIZE;
		str = realloc(str, mem); 
		next_read += SIZE;
		fgets(next_read, SIZE+1, stdin); // read the rest (hopefully) of the line into the new space
		printf("str is now %s\n", str);
	}

	printf("final str is %s", str);
	// free(str);
	return 0;
}

The code works fine for short strings, but stops working (program seems to get stuck) if string is longer:
Code:
bruno@thinkpad:~/Desktop434$ gcc getstring.c 
bruno@thinkpad:~/Desktop436$ ./a.out 
I love linux
str is now I love 
str is now I love linu
str is now I love linux

final str is I love linux
bruno@thinkpad:~/Desktop436$ ./a.out 
I love linux and the C programming language
str is now I love 
str is now I love linu
str is now I love linux an
str is now I love linux and th
str is now I love linux and the C 
str is now I love linux and the C 
str is now I love linux and the C 
str is now I love linux and the C 
str is now I love linux and the C 
str is now I love linux and the C 
str is now I love linux and the C

I'm a newbie in C and would like to learn something by debugging this.
Last edited by DevuanFan; 11-11-2019 at 05:00 PM..
 

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LEARN ABOUT OPENSOLARIS

indicator

indicator(1F)							   FMLI Commands						     indicator(1F)

NAME

       indicator - display application specific alarms and/or the "working" indicator

SYNOPSIS

       indicator [-b [n]] [-c column] [-l length] [-o] [-w]
	    [string]...

DESCRIPTION

       The  indicator  function  displays  application specific alarms or the "working" indicator, or both, on the  FMLI banner line. The argument
       string is a string to be displayed on the banner line, and should always be the last argument given. Note that string is not  automatically
       cleared from the banner line.

OPTIONS

       -bn	    The -b option rings the terminal bell n times, where n is an integer from 1 to 10. The default value is 1. If the terminal has
		    no bell, the screen is flashed instead, if possible.

       -c column    The -c option defines the column of the banner line at which to start the indicator string. The argument column   must  be	an
		    integer from 0 to DISPLAYW-1. If the -c option is not used, column defaults to 0 .

       -l length    The -l option defines the maximum length of the string displayed. If string is longer than length characters, it will be trun-
		    cated. The argument length must be an integer from 1 to DISPLAYW. If the -l option is not used, length defaults  to  DISPLAYW.
		    Note that if string doesn't fit it will be truncated.

       -o	    The -o option causes indicator to duplicate its output to stdout .

       -w	    The -w option turns on the "working" indicator.

EXAMPLES

       Example 1 A sample output of the indicator command.

       When the value entered in a form field is invalid, the following use of indicator will ring the bell three times and display the word WRONG
       starting at column 1 of the banner line.

	 invalidmsg=`indicator -b 3 -c 1 "WRONG"`

       To clear the indicator after telling the user the entry is wrong:

	 invalidmsg=`indicator -b 9 -c 1 "WRONG"; sleep 3;
		 indicator -c 1 "     "`

       In this example the value of invalidmsg (in this case the default value Input is not valid), still appears on the  FMLI message line.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWcsu			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       attributes(5)

SunOS 5.11							    5 Jul 1990							     indicator(1F)
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