Problems with array of pointers to struct

Question

After defining the type student (which is a struct made of two arrays of characters and an int), I've created an array of pointers to student, which I need in order to modify its content inside of a series of functions.

int main(void) 
{
    student* students[NUMBER_OF_STUDENTS];

    strcpy(students[0]->name, "test");
    strcpy(students[0]->surname, "test");
    students[0]->grade = 18;

    return EXIT_SUCCESS;
}

My problem is that this simple piece of code returns -1 as exit status after running. Why is that?

jxh · Accepted Answer · 2019-05-17 22:14:09Z

3

The pointer students[0] is uninitialized. Dereferencing it results in undefined behavior.

Initialize it with the address of a valid object before attempting to access it.

student test;
students[0] = &test;

strcpy(students[0]->name, "test");
strcpy(students[0]->surname, "test");
students[0]->grade = 18;

answered May 17, 2019 at 22:14

jxh

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Comments

0___________ · Accepted Answer · 2019-05-17 22:22:59Z

3

Because it is UB. You have only pointer without the actual structs allocated.

students[x] = malloc(sizeof(*students[0]));

or statically

student s;
students[x] = &s;

or

 students[x] = &(student){.name = "test", .surname ="test", .grade = 18};

edited May 17, 2019 at 22:22

answered May 17, 2019 at 22:14

0___________

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Comments

Akhil Batra · Accepted Answer · 2019-05-17 23:00:21Z

1

The pointers are pointing to nowhere since you have not allocated any memory for them to point to.

int main(void) 
{
    student* students = (student*)malloc(sizeof(student)*[NUMBER_OF_STUDENTS]); \\malloc dynamically allocate heap memory during runtime

strcpy(students[0]->name, "test");
strcpy(students[0]->surname, "test");
students[0]->grade = 18;

return EXIT_SUCCESS;

}

*Note Edit by marko -- Strictly the pointers are pointing to whatever was last in the stack location or register holding it - it may be nothing, or something you actually care about. The joys of UB

edited May 17, 2019 at 23:00

answered May 17, 2019 at 22:15

Akhil Batra

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1 Comment

marko Over a year ago

Strictly the pointers are pointing to whatever was last in the stack location or register holding it - it may be nothing, or something you actually care about. The joys of UB

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