Issue with for loop in C when using struct

Question

I did the following code to read values from a text file. Text file looks like this.(in code it's named as annotation.txt)

Here's the code.

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

typedef struct object
{
    char obj_name[20];
    int x_min;
    int y_min;
    int x_max;
    int y_max;

} Object;

typedef struct annotation
{
    char img_name[20];
    int width;
    int height;
    int num_obj;

    Object objects[];
} Annotation;

void readAnnotation(char *fileName, Annotation *annot);

int main()
{
    Annotation annot;
    readAnnotation("annotation.txt", &annot);

    printf("%s\n", annot.img_name);
    printf("%d\n", annot.width);
    printf("%d\n", annot.height);
    printf("%d\n", annot.num_obj);
    printf("0th object name for the 1st time %s\n", annot.objects[0].obj_name); // here it can be seen as 'dog'

    for (int i = 0; i < 3; i++)
    {
        printf("%s\n", annot.objects[i].obj_name); // oth one disappear
        printf("%d\n", annot.objects[i].x_min);
        printf("%d\n", annot.objects[i].y_min);
        printf("%d\n", annot.objects[i].x_max);
        printf("%d\n", annot.objects[i].y_max);
    };
    printf("0th object name for the 2nd time %s\n", annot.objects[0].obj_name); //here it disappear
};

void readAnnotation(char *fileName, Annotation *annot)
{
    FILE *fp;
    fp = fopen(fileName, "r");
    if (fp == NULL)
    {
        printf("\nError: Cannot open file");
        exit(1);
    };

    fscanf(fp, "%s", annot->img_name);
    fscanf(fp, "%d", &(annot->width));
    fscanf(fp, "%d", &(annot->height));
    fscanf(fp, "%d", &(annot->num_obj));

    for (int i = 0; i < annot->num_obj; i++)
    {
        fscanf(fp, "%s", annot->objects[i].obj_name);
        fscanf(fp, "%d", &(annot->objects[i].x_min));
        fscanf(fp, "%d", &(annot->objects[i].y_min));
        fscanf(fp, "%d", &(annot->objects[i].x_max));
        fscanf(fp, "%d", &(annot->objects[i].y_max));
    };

    fclose(fp);
};

When I run the code, inside the for loop written in main() function makes the annot.objects[0].obj_name to disappear. When I check it before the for loop the value can be seen as 'dog'. But within the for loop and after the for loop, that value disappears. Can anyone tell me the issue with my code and give me a proper answer to solve this.

tadman · Accepted Answer · 2020-05-22 03:50:27Z

1

objects is never initialized. You're just writing to junk memory. Object objects[]; does not create an array, it actually creates a pointer to an array, and you're required to allocate memory for it. If you don't you're just exercising an uninitialized pointer and your program may or may not work or might crash. It's undefined behaviour.

Normally this is declared as Objects* objects to make it clear it's really a pointer and to remember to allocate. Arrays and pointers can often be interchanged, so no other code changes are required.

Before your readAnnotation loop:

annot->objects = calloc(annot->num_obj, sizeof(Object));

answered May 22, 2020 at 3:50

tadman

212k23 gold badges237 silver badges266 bronze badges

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

2 Comments

isrnick Over a year ago

A flexible array as the last member of a struct has special behavior that it can be resized at memory allocation. If annot were allocated dynamically you could do something like annot = realloc(annot, sizeof(Annotation) + annot->num_obj * sizeof(Object)); and then access each object by their index normally: annot->objects[index] .

tadman Over a year ago

@isrnick That's true, I forgot about that because it's not often you see this used that way. In C I often see Object objects[0] to make it clear it's a stub.

user3629249 · Accepted Answer · 2020-05-22 04:33:28Z

0

regarding:

typedef struct annotation
{
    char img_name[20];
    int width;
    int height;
    int num_obj;

    Object objects[];
} Annotation;

The above only allocates room for the first 4 fields then a pointer, however, No room is allocated for an array 0f object Suggest: either use malloc() and/or realloc() to allocate memory for the instances of Object

Otherwise any reference to objects[x] is undefined behavior can can lead to a seg fault event

answered May 22, 2020 at 4:33

user3629249

16.6k1 gold badge18 silver badges17 bronze badges

Collectives™ on Stack Overflow

Issue with for loop in C when using struct

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related