Getting string with C function

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.

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

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.

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