Using a pointer in a struct

Question

While I was inputting "stuList[1].name", an error occurred and the program crashed. How do I fix this?

#include <stdio.h>
#include <conio.h>

typedef struct student
    {
        int id;
        char *name;
        float percentage;
    } student;

int main()
{
    student stuList[3];
    stuList[0].name = "vermidonhapic"; 
    stuList[2].name = "didiervermiyer";

    scanf("%s\n",&stuList[1].name);

    printf(" name is: %s \n", stuList[0].name);
    printf(" name is: %s \n", stuList[2].name);  
    printf(" name is: %s \n", stuList[1].name);

    system("PAUSE");
}

scanf() with a "%s" format is extremely dangerous, even if the pointer points to properly allocated memory. It will read an arbitrarily long string into the destination buffer; there's no way to prevent it from writing past the end of your array. — Keith Thompson
– Keith Thompson, Commented Jan 19, 2012 at 21:30

amit · Accepted Answer · 2012-01-19 20:52:20Z

2

In this command: scanf("%s\n",&stuList[1].name);

You are scanning string from input without actually allocating the required memory.

This results in trying to write the data in an unkown address, which is not yours, which results in a segmentation fault.

To fix it, first allocate memory: stuList[1].name = malloc(MAX_LENGTH);, and only after: scan the string from stdin.

answered Jan 19, 2012 at 20:52

amit

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user325117 · Accepted Answer · 2012-01-19 20:52:24Z

2

name is just a pointer that isn't pointing to memory you own, so writing through it will corrupt memory. You need to allocate some memory on it with malloc(), or make it an array rather than a pointer.

answered Jan 19, 2012 at 20:52

user325117

Comments

ouah · Accepted Answer · 2012-01-19 21:02:58Z

2

stuList[1].name is an invalid pointer as it has not been initialized to point to a valid object.

You should dynamically allocate memory (through malloc function) for stuList[1].name so scanf could write to an allocated object.

stuList[1].name = malloc(size_enough_to_store_your_string);

Moreover, stuList[1].name is a pointer to char so you should use stuList[1].name and not &stuList[1].name (pointer to a pointer to char) in your scanf function call.

edited Jan 19, 2012 at 21:02

answered Jan 19, 2012 at 20:51

ouah

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1 Comment

ouah Over a year ago

@amit thanks, I misread the string literal pointers assignments. Fixed.

David Heffernan · Accepted Answer · 2012-01-19 20:51:48Z

1

You forgot to allocate memory for stuList[1].name. You need to allocate it from the heap with malloc or stack allocate it.

stuList[1].name = malloc(MAX_NAME_LENGTH);//heap allocation
//use stuList[1].name
free(stuList[1].name);

char buffer[MAX_NAME_LENGTH];//stack allocation
stuList[1].name = buffer;

I've omitted all error checking, protection against over-running buffers etc.

Since stuList[1].name is a pointer, you are incorrect in taking its address in your scanf.

answered Jan 19, 2012 at 20:51

David Heffernan

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