Segfault while malloc in C structures

Question

I'm observing segmentation fault, when I try to allocate memory to a structure.

After memcpy() I'm trying to copy the contents of one more structure to the above structure.

Below is the code snippet :

struct student {
    char *username;
    char *id;
    int roll;
};

struct db {
    struct student *s1;
    struct student *s2;
};

void print_struct(struct student *);

int main (void) {
    struct student *student1, *student2;
    struct db *db1;
    char *name = "ram";
    char *id = "200ABCD";
    int roll = 34;

    student1 = (struct student *)malloc(sizeof(struct student));
    student1->username = name;
    student1->id = id;
    student1->roll = roll;

    printf("\nStudent 1\n");
    print_struct(student1);

    printf("\nStudent 2\n");
    student2 =student1;
    print_struct(student2);

    printf("\nDb of s1\n");
    db1->s1 = (struct student *)malloc(sizeof(struct student));  ===> segfault here
    db1->s1 = student1;
    print_struct(db1->s1);

    return 0;
}

void print_struct(struct student *s) {
    printf("Name: %s\n", s->username);
    printf("Id: %s\n", s->id);
    printf("R.No: %d\n", s->roll);
    return;
}

[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, May 05 '17 at 06:17

LPs · Answer 1 · 2017-05-05T06:50:42.207

4

At the top of your main function you declared db1 as:

struct db *db1;

It is a not initialized pointer, so you must allocate memory for the pointed struct db before dereference to its members:

db1 = malloc(sizeof(struct db));
if (db1 != NULL)
{
   db1->s1 = malloc(sizeof(struct student));
}

Moreover take notes that malloc & co. can fail: always check thier returned value

Lastly you are trying to allocate space for st1 in db1 struct, but just after that you are overwriting (memory leak) the pointer using:

db1->s1 = student1;

edited May 05 '17 at 06:50

answered May 05 '17 at 06:17

LPs

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I think you missed to mention about the "variable" at the very beginning. :) – Sourav Ghosh May 05 '17 at 06:33
@SouravGhosh Probably I need coffee: I'm not getting you.. – LPs May 05 '17 at 06:46
Sorry if I was not clear...`You must allocate the struct db` is not proper, you need to mention about the variable `db1` of type `struct db`..inn'it? – Sourav Ghosh May 05 '17 at 06:47
@SouravGhosh Got it. Hope my bad English it's clear enough ;) – LPs May 05 '17 at 06:51

score 4 · Answer 2 · edited May 23 '17 at 12:02

Your code is inconsistent. See for yourself.

In first case, you did

student1 = (struct student *)malloc(sizeof(struct student));

 /* Considering malloc success, 'student' points to valid memory */

student1->username = name;    //
student1->id = id;            //all the access are OK
student1->roll = roll;        //

but later, you did

db1->s1 = (struct student *)malloc(sizeof(struct student));  
db1->s1 = student1;

/* Wait, how do you know 'db' points to anything valid, at all? */

without allocating proper memory for db.

That said, if you're going to assign another pointer to a pointer, you don't need to reserve memory for the destination pointer. You're just making the pointer to point to something already valid (depending on the "validity" of the source pointer).

In case you're trying to use the memory location directly (read to or write from) pointed to by any pointer (example, destination of strcpy()) without any prior assigment, then you need to allocate memory first.

Obligatory suggestions:

Please see this discussion on why not to cast the return value of malloc() and family in C..
Always check the returned pointer against NULL to ensure malloc() is success before using the returned pointer.

score -1 · Answer 3 · edited May 05 '17 at 08:51

-1

You need to allocate memory for db1 (through malloc) first, then its elements.

edited May 05 '17 at 08:51

Jonas

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answered May 05 '17 at 06:16

Pras

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Segfault while malloc in C structures

3 Answers3