Enter a string into a struct: segmentation fault

Question

So far I've been able to answer all all my questions by a dutiful search, but this one has me stumped.

So. I've reduced this code to the minimum necessary to produce the error. Here it is:

#include <studio.h>  

struct string {
      char *data;
} s;

int main(int argc, char *argv[])
{
    printf("Enter a string. ");
    scanf("%s\n", &s.data);
    printf("%s", s.data);

    return 0;
}

I'm using gcc 4.6.3 and compiling with -Wall -g using classic defensive programming tactics.

I posted this from my phone, so there may be typos and autocorrect madness.

Answer 1

When using scanf, to read as a string using an unallocated pointer, specify the 'm' directive forcing scanf to allocate memory as needed while skipping newline. You are responsible for freeing the memory allocated to the string. scanf expects the pointer provided to be type char**.

scanf ("%m[^\n]%*c", &s.data);

The trailing %*c reads and discards the trailing newline. (be aware of this if you simply press [enter])

Answer 2

Point 1. allocate memeory to the pointer before using it

s.data = calloc(DATASIZE, sizeof (char));

^{Recommended Alternative: malloc()}

Point 2. Use scanf() to take the input and store it.

scanf("%s", s.data);

^{Recommended Alternative: fgets()}

Please find below is a corrected version of your code.

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

#define DATASIZE 128

struct string {
      char *data;
} s;

int main(int argc, char *argv[])
{
    s.data = calloc(DATASIZE, sizeof (char));
    printf("Enter a string. \n");
    scanf("%s", s.data);
    printf("Input is : %s\n", s.data);

    return 0;
}

Answer 3

You need allocated the memory by using malloc first.

s.data = malloc(100);

Answer 4

The fact that it's in a struct isn't the problem. The problem is that the pointer data is not initialized, and there's no memory for the stuff to be copied into.

This solves the problem:

struct string 
{
 char data [SOMEBIGNUMBER]
} s;

and so does this: keep string as it is, but allocate the space for data in main using malloc.

Answer 5

It seems that in this statement

("%s", s.data);

there is a typo. I think you mean

scanf("%s", s.data);

Otherwise

("%s", s.data);

is an expression with the comma operator that simply returns s.data.

The first problem with your code that you have to allocate memory where you are going to write enetered data.

For example

s.data = malloc( 256 * sizeof( char ) );

Instead of 256 you may use nay value as you like.

The second problem is that you should use fgets instead of scanf because using the last is unsafe.

So the main can look like

int main( void )
{
    const size_t N = 256;

    s.data = malloc( N * sizeof( char ) );

    s.data[0] = '\0';

    printf("Enter a string (no more than %zu characters). ", N );
    fgets( s.data, N, stdin );

    printf("%s\n", s.data);

    free( s.data );

    return 0;
}

Also you may remove the new line character that will be stored in data.s after fgets.

For example

size_t len = strlen( s.data );
if ( len && s.data[len - 1] == '\n' ) s.data[len - 1] = '\0';

Answer 6

In this use malloc for memory allocation of structure object and char pointer. below is corrected code....

#include <stdio.h>
#include <malloc.h>

struct string {
  char *data;
} *s;
int main(int argc, char *argv[])
{
   s = (struct string*)malloc(sizeof(struct string));
   s->data = (char*)malloc(sizeof(char) * 100);

   printf("Enter a string : ");
   scanf("%s", s->data);
   printf("%s\n", s->data);

   return 0;
}