Why does my queue initializing function (c99) returns segmentation fault?

Question

/* Queue structure which holds all necessary data */

typedef struct queue {
    int head;
    int tail;
    void** vals;
    int len;
    int count;
} queue_t;

/* creates a new queue with a given size */
queue_t* create_queue(int capacity){
    queue_t* q = NULL;
    q->len = capacity;
    q->vals = malloc(sizeof(void*)*q->len);
    q->head = 0;
    q->tail = 0;
    q->count = 0;
    return q;
}

I can't see where could be problem. I tried to run my code with valgrind and it says:

==14818== Memcheck, a memory error detector
==14818== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==14818== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==14818== Command: ./main
==14818== 
==14818== Invalid write of size 4
==14818==    at 0x10918A: create_queue (in /mnt/c/Users/jakub/projects/bab36prga-hw6/main)
==14818==    by 0x10946F: main (in /mnt/c/Users/jakub/projects/bab36prga-hw6/main)
==14818==  Address 0x10 is not stack'd, malloc'd or (recently) free'd
==14818== 
==14818== 
==14818== Process terminating with default action of signal 11 (SIGSEGV)
==14818==  Access not within mapped region at address 0x10
==14818==    at 0x10918A: create_queue (in /mnt/c/Users/jakub/projects/bab36prga-hw6/main)
==14818==    by 0x10946F: main (in /mnt/c/Users/jakub/projects/bab36prga-hw6/main)
==14818==  If you believe this happened as a result of a stack
==14818==  overflow in your program's main thread (unlikely but
==14818==  possible), you can try to increase the size of the
==14818==  main thread stack using the --main-stacksize= flag.
==14818==  The main thread stack size used in this run was 8388608.
==14818== 
==14818== HEAP SUMMARY:
==14818==     in use at exit: 0 bytes in 0 blocks
==14818==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==14818== 
==14818== All heap blocks were freed -- no leaks are possible
==14818== 
==14818== For lists of detected and suppressed errors, rerun with: -s
==14818== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
Segmentation fault

I am not really sure what that means, because the only thing that could be wrong is malloc so... am I not allowed to malloc space for void** array?

I am trying to create circular queue with dynamic reallocation (so you can push in unlimited amount of elements) but I am stuck at basic initialization. I am new to valgring and I would appriciate if somebody halped me to interpret where is the problem.

queue_t* q = NULL; q->len = capacity; You are creating a null pointer and then try to access it ... — derpirscher
– derpirscher, Commented Apr 28, 2023 at 15:02
Than how do I initialize struct? Because without "= NULL" I can't compile. — Jakub Novák
– Jakub Novák, Commented Apr 28, 2023 at 15:09

Jakub Novák · Accepted Answer · 2023-04-28 15:29:19Z

0

Nevermind - I think I got it

queue_t* create_queue(int capacity){
    queue_t* q = malloc(sizeof(queue_t));
    q->len = capacity;
    q->vals = malloc(sizeof(void*)*q->len);
    q->head = 0;
    q->tail = 0;
    q->count = 0;
    return q;
}

answered Apr 28, 2023 at 15:29

Jakub Novák

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