Getting string with C function

Question

I need to get strings dynamically but as I need to get more than one string, I need to use functions. So far I wrote this (I put //**** at places i think might be wrong)

char* getstring(char *str);

int main() {

    char *str;
    strcpy(str,getstring(str));//*****
    printf("\nString: %s", str);
    return 0;
}

char* getstring(char str[]){//*****
    //this part is copy paste from my teacher lol
    char c;
    int i = 0, j = 1;
    str = (char*) malloc (sizeof(char));
    printf("Input String:\n ");
    while (c != '\n') {//as long as c is not "enter" copy to str
        c = getc(stdin);
        str = (char*)realloc(str, j * sizeof(char));
        str[i] = c;
        i++;
        j++;
    }
    str[i] = '\0';//null at the end
    printf("\nString: %s", str);
    return str;//******
}

printf in the function is working but not back in main function. I tried returning void, getting rid of *s or adding, making another str2 and tring to strcpy there or not using strcpy at all. Nothing seems to working. Am I misssing something? Or maybe this is not possible at all //Thank you so much for your answers

We had to struggle in year 1 in here. The easy stuff in C is never easy for more than some 5% of people. — Paul Stelian, Dec 24 '16 at 18:12
@coderredoc The casting is optional? I'd take itas self-discipline, if nothing else. — Paul Stelian, Dec 24 '16 at 18:13
http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — user2736738, Dec 24 '16 at 18:13
The memory is 1 too few for the `'\0'` terminator. Move `j++;` above the `realloc` line. — Weather Vane, Dec 24 '16 at 18:14
@coderredoc I see, thanks. I'll now criticize his teacher for using getc instead of fgetc (or unlike with gets there is a valid reason?) — Paul Stelian, Dec 24 '16 at 18:15
Consider using [getline(3)](http://man7.org/linux/man-pages/man3/getline.3.html) if it is available on your system. — Basile Starynkevitch, Dec 24 '16 at 18:16
@PaulStelian the first time `realloc` is called `j` is 1 so the memory allocation changes from 1 to 1. No room for terminator, as I said. — Weather Vane, Dec 24 '16 at 18:17
@Basile I think we all are taught to be portable, and getline definitely is not available on Windows. — Paul Stelian, Dec 24 '16 at 18:17
@PaulStelian indeed `i` is index, `j` is length (to include terminator). — Weather Vane, Dec 24 '16 at 18:19
@PaulStelian: but then we can always ask that same teacher to stop using `getline` :) — Jongware, Dec 25 '16 at 11:13

score 3 · Answer 1 · edited May 23 '17 at 12:01

3

Getting the string part can be taken from this answer. Only put a \n as input to the getline funtion.

char * p = getline('\n');

Three things :- don't cast malloc, check if malloc/realloc is successful and sizeof is not a function.

edited May 23 '17 at 12:01

Community

1
1

answered Dec 24 '16 at 18:19

user2736738

30,591
5
42
56

`sizeof` may be not a function but does obey the syntax of a function! Just like C# typeof() builtin keyword – Paul Stelian Dec 25 '16 at 11:21
1

@PaulStelian.: yep – user2736738 Dec 25 '16 at 11:22

score 3 · Answer 2 · answered Dec 24 '16 at 18:19

The problem is not with the function that you are using, but with the way you try copying its result into an uninitialized pointer.

Good news is that you don't have to copy - your function already allocates a string in dynamic memory, so you can copy the pointer directly:

char *str = getstring(str);

This should fix the crash. A few points to consider to make your function better:

main needs to free(str) when it is done in order to avoid memory leak
Store realloc result in a temporary pointer, and do a NULL check to handle out-of-memory situations properly

Tyler Durden · Answer 3 · 2016-12-25T11:03:59.890

There are two things to take away from the lesson as it stands now:

(1) You should have one way of returning the reference to the new string, either as an argument passed by reference to the function OR as a return value; you should not be implementing both.

(2) Because the subroutine your teacher gave you allocates memory on the heap, it will be available to any part of your program and you do not have to allocate any memory yourself. You should study the difference between heap memory, global memory, and automatic (stack) memory so you understand the differences between them and know how to work with each type.

(3) Because the memory is already allocated on the heap there is no need to copy the string.

Given these facts, your code can be simplified to something like the following:

int main() {

    char *str = getstring();
    printf( "\nString: %s", str );
    return 0;
}

char* getstring(){
   .... etc

Going forward, you want to think about how you de-allocate memory in your programs. For example, in this code the string is never de-allocated. It is a good habit to think about your strategy for de-allocating any memory that you allocate.

mim.ms · Answer 4 · 2016-12-24T19:26:33.617

1

Let's simplify the code a bit:

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

char* getstring()
{
    char c = 0;
    int i = 0, j = 2;
    char *str = NULL;

    if ((str = (char*) malloc(sizeof(char))) == NULL)
        return NULL;

    printf("Input String: ");
    while (c = getc(stdin)) {
        if (c == '\n') break;

        str = (char*) realloc(str, j * sizeof(char));
        str[i++] = c;
        j++;
    }
    str[i] = '\0';
    printf("getstring() String: %s\n", str);
    return str;
}

int main()
{
    char *str = getstring();
    printf("main() String: %s\n", str);
    free(str);
    return 0;
}

Then execute:

$ make teststring && ./teststring 
cc     teststring.c   -o teststring
Input String: asdfasfasdf
getstring() String: asdfasfasdf
main() String: asdfasfasdf

edited Dec 24 '16 at 19:26

answered Dec 24 '16 at 18:23

mim.ms

112
4

1

You do not seem to have taken on the earlier comments about `j`. – Weather Vane Dec 24 '16 at 18:28
2

... and `while (c != '\n')` is undefined behaviour the first time - uninitialised . – Weather Vane Dec 24 '16 at 18:29
The input character string contains a newline. – BLUEPIXY Dec 24 '16 at 18:51

Getting string with C function

4 Answers4