C segmentation fault while using structures

Question

I'm trying to write a data structure with two elements, and then defining a variable of that type struct. However, after initializing the variable in the main function, I'm getting segmentation fault and I don't know why.

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

struct AnimalSizes {
    char stringName[50]; 
    double sizeLength; 
    } animalSizes[2]; 

int main()
{

    struct AnimalSizes *snakes; 
    strcpy(snakes[0].stringName,"Anaconda"); 
    snakes[0].sizeLength=3.7; 
    strcpy(snakes[1].stringName,"Python"); 
    snakes[1].sizeLength= 2.4; 
    printf("%c", *snakes[0].stringName); 
    printf("%lf", snakes[0].sizeLength); 
    printf("%c", *snakes[1].stringName); 
    printf("%lf", snakes[1].sizeLength);
    return 0;
}

vtronko · Accepted Answer · 2020-04-19 16:08:05Z

2

You try to strcpy to destination where is no allocated memory. That is undefined behavior. You should first allocate enough memory to hold two AnimalSizes instances:

struct AnimalSizes *snakes;
snakes = malloc(2 * sizeof(struct AnimalSizes));

Also, here printf("%c", snakes[0].stringName); you are trying to output the first character of stringName. I assume, what you rather want to do is to output whole string with %s.

answered Apr 19, 2020 at 16:08

vtronko

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1 Comment

unicorns101 Over a year ago

Hi, Thank you for your answer! I was wondering why we had to use dynamic memory allocation. I'm kind of lost in that topic and any help would be appreciated!

nanofarad · Accepted Answer · 2020-04-19 16:05:33Z

You've declared a pointer to a struct AnimalSizes, and you have declared an array struct AnimalSizes[2], but you have not made the pointer point to this array:

int main()
{

    struct AnimalSizes *snakes = &animalSizes[0]; 
...
}

Alternatively, you may choose to not declare a global variable, rather choosing to allocate memory in main:

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

struct AnimalSizes {
    char stringName[50]; 
    double sizeLength; 
    }; 

int main()
{

    struct AnimalSizes *snakes = (struct AnimalSizes*) malloc(2*sizeof(struct AnimalSizes)); 
    strcpy(snakes[0].stringName,"Anaconda"); 
    snakes[0].sizeLength=3.7; 
    strcpy(snakes[1].stringName,"Python"); 
    snakes[1].sizeLength= 2.4; 
    printf("%c", *snakes[0].stringName); 
    printf("%lf", snakes[0].sizeLength); 
    printf("%c", *snakes[1].stringName); 
    printf("%lf", snakes[1].sizeLength);
    free(snakes);
    return 0;
}

user3629249 · Accepted Answer · 2020-04-20 17:12:05Z

the following proposed code:

eliminates any need for malloc() and free()
performs the desired functionality
separates the definition of the struct from any instance of the struct.
inserts some spacing between the first letter of the snake name and the 'size' of the snake, for readability
applies certain other changes to the code for 'human' readability

and now the proposed code:

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

struct AnimalSizes 
{
    char stringName[50]; 
    double sizeLength; 
};


int main( void )
{
    struct AnimalSizes snakes[2]; 

    strcpy(snakes[0].stringName,"Anaconda"); 
    snakes[0].sizeLength=3.7; 

    strcpy(snakes[1].stringName,"Python"); 
    snakes[1].sizeLength= 2.4; 

    printf("%c  ",  snakes[0].stringName[0]); 
    printf("%lf\n", snakes[0].sizeLength); 

    printf("%c  ",  snakes[1].stringName[0]); 
    printf("%lf\n", snakes[1].sizeLength);
    return 0;
}

a run of the proposed code outputs:

A  3.700000
P  2.400000

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