How can I capture a string from a function in C that returns string?

Question

I'm new to C and I have created the following code, but when run the program crashes. Why? How do I stop it crashing?

char *get()
{
    char *n;
    printf("\n user name : ");
    scanf("%s", &n);
    return n;
}

int main()
{
    char *c = get();

    printf("%s", c);

    return 0;
}

You need a buffer where to write. And c-faq.com/stdio/scanfprobs.html — Artefacto
– Artefacto, Commented Sep 13, 2011 at 19:29
Don't use scanf() (linked by @Artefacto), use fgets() instead ideone.com/oUAbk — jfs
– jfs, Commented Sep 14, 2011 at 1:03

Blindy · Accepted Answer · 2011-09-13 19:30:58Z

4

In C it is typical for the parent to handle memory allocation since there's no garbage collector to clean up after yourself, so it has to do that anyway:

void get(char *n)
{
    printf("\n user name : ");
    scanf("%s", n);
}

int main()
{
    char c[200];
    get(c);
    printf("%s", c);

    return 0;
}

http://ideone.com/Tw347

answered Sep 13, 2011 at 19:30

Blindy

68k10 gold badges96 silver badges141 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Keith Thompson · Accepted Answer · 2011-09-13 20:02:36Z

scanf is reading characters from stdin and storing them in the memory that n points to. You haven't initialized n to point to anything, so scanf is probably trying to store your input in some arbitrary location in memory. You're lucky that it crashed rather than quietly acting as if it were working correctly.

Returning a string from a C function is more complex than you might expect.

There are (at least) three general approaches:

Require the caller to pass in a pointer to (the first element of) an array into which the string is to be stored, along with another argument telling the function how big the array is. This can require error handling if the array isn't big enough to hold the result.
Declare a static array inside the function and return a pointer to it. (You can't return a pointer to a non-static local array, since the array ceases to exist when the function returns.) Problems: Multiple calls use the same storage space (particularly problematic in the presence of threading), and the allocated size is fixed.
Allocate the result inside the function using malloc(). This requires the caller to free() the result.

Recommended reading: the comp.lang.c FAQ. In particular, question 7.5b is nearly a direct answer to your question (and would have saved me some typing if I'd realized it earlier). (I don't usually link to individual questions because I like to encourage people to browse.)

EDIT: Also, scanf with an unqualified "%s" format is inherently unsafe. It will attempt to store however many characters are entered in the array; there's no way to avoid buffer overruns (say, if your cat sits on the keyboard). @Artefacto referred to this problem in the link in the comment.

codablank · Accepted Answer · 2011-09-13 19:41:15Z

1

First of all you musst allocate memory for your string

you can dynamically doing it by using malloc;

for example with a size of 200 :

#include <stdlib.h>

char (*n)[200] = malloc(sizeof *n);

don't forget to liberate the pointer with

free(n);

answered Sep 13, 2011 at 19:41

codablank

1812 silver badges9 bronze badges

Collectives™ on Stack Overflow

How can I capture a string from a function in C that returns string?

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related