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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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curs_instr(3X)															    curs_instr(3X)

NAME
instr, innstr, winstr, winnstr, mvinstr, mvinnstr, mvwinstr, mvwinnstr - get a string of characters from a curses window SYNOPSIS
#include <curses.h> int instr(char *str); int innstr(char *str, int n); int winstr(WINDOW *win, char *str); int winnstr(WINDOW *win, char *str, int n); int mvinstr(int y, int x, char *str); int mvinnstr(int y, int x, char *str, int n); int mvwinstr(WINDOW *win, int y, int x, char *str); int mvwinnstr(WINDOW *win, int y, int x, char *str, int n); DESCRIPTION
These routines return a string of characters in str, extracted starting at the current cursor position in the named window. Attributes are stripped from the characters. The four functions with n as the last argument return a leading substring at most n characters long (exclu- sive of the trailing NUL). RETURN VALUE
All of the functions return ERR upon failure, or the number of characters actually read into the string. X/Open defines no error conditions. In this implementation, if the window parameter is null or the str parameter is null, a zero is returned. NOTES
Note that all routines except winnstr may be macros. PORTABILITY
SVr4 does not document whether a length limit includes or excludes the trailing NUL. The ncurses library extends the XSI description by allowing a negative value for n. In this case, the functions return the string ending at the right margin. SEE ALSO
curses(3X). curs_instr(3X)
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