I saw quite a few posts about making a dynamic array so I tried making my own for learning purposes.

Here it is:

#ifndef VECTOR_H
#define VECTOR_H

#include <stdlib.h>
#include <stdbool.h>
#include <string.h>



typedef struct
        int size_of_each;
        int reserved;
        int used;
        void* data;
} vector_t;

bool vector_init(vector_t *, int);
void vector_append(vector_t *, void *);
void* vector_get(vector_t *, int );
void vector_set(vector_t *, int, void *);
void vector_free(vector_t *);


bool vector_init(vector_t *p, int s)
        p->size_of_each = s;
        p->reserved = VECTOR_RESERVE_SIZE;
        p->used = 0;
        p->data = malloc(VECTOR_RESERVE_SIZE * p->size_of_each);

        if (p->data != NULL) { return false; }
        else { return true; }

void vector_append(vector_t *p, void *d)
        if (p->used == p->reserved)
                p->data = realloc(p->data, VECTOR_RESERVE_SIZE);
                //printf("reallocated \n");
                p->reserved += VECTOR_RESERVE_SIZE;
        memcpy((void*)((char*)p->data + p->size_of_each * p->used),  // copy to back of the vector
                        d, p->size_of_each);

        p->used += 1;

void* vector_get(vector_t *p, int i)
        if (!(i > p->used))
                return (void*)((char*)p->data + p->size_of_each * i);
        else { return NULL; }

void vector_set(vector_t *p, int i, void *d)
        if (!(i > p->used))
                memcpy((void*)((char*)p->data + p->size_of_each * i), d, p->size_of_each);

void vector_free(vector_t *p)


Here is an example of its usage:

#include <stdio.h>

#include "vector.h"

int main()
        int a = 321;
        int b = 45;
        int c = 21;
        int d = 31;
        int e = 71;

        vector_t v;
        vector_init(&v, sizeof (int));

        vector_append(&v, &a);
        vector_append(&v, &b);
        vector_append(&v, &c);
        vector_append(&v, &d);
        vector_append(&v, &e);

        printf("1st element is  %d \n", *(int*)vector_get(&v, 0));
        printf("2nd element is  %d \n", *(int*)vector_get(&v, 1));
        printf("3th element is  %d \n", *(int*)vector_get(&v, 2));
        printf("4th element is  %d \n", *(int*)vector_get(&v, 3));
        printf("5th element is  %d \n\n", *(int*)vector_get(&v, 4));

        vector_set(&v, 4, &a);
        printf("4th element after setting is  %d \n\n", *(int*)vector_get(&v, 4));

        printf("size of int is: %d \n", sizeof (int));
        printf("address of 1st element: %p \n", vector_get(&v, 0));
        printf("address of 2nd element: %p \n", vector_get(&v, 1));
        printf("address of 3rd element: %p \n", vector_get(&v, 2));
        printf("address of 4th element: %p \n", vector_get(&v, 3));
        printf("address of 5th element: %p \n", vector_get(&v, 4));


I've written some C++ before and this was one of my first few projects in C.

I'd like to know overall how good my code is and whether I'm using good practices / conventions.


this: p->data = realloc(p->data, VECTOR_RESERVE_SIZE);

should be this: p->data = realloc(p->data, (p->reserved + VECTOR_RSERVE_SIZE) * P->size_of_each);

Edit: Maybe it's too late but, if people are still reviewing this I would really like to know how does this approach compare to std::vector.


2 Answers 2


Self document

Consider users do not want to see or may not have access to the implementation and are left to declarations.

These deserve parameters names and at least a comment about what each function does and how to use. (e.g. Call vector_init() first. It overwrites all vector_t members.)

// Deserve for documentation here.
bool vector_init(vector_t *, int);
void vector_append(vector_t *, void *);
void* vector_get(vector_t *, int );
void vector_set(vector_t *, int, void *);
void vector_free(vector_t *);

Think big

Why limit to sizes up to INT_MAX when C readily handles sizes up to SIZE_MAX. That could be billions times more.

Use size_t for array indexing and sizing.

    // int size_of_each;
    // int used;
    size_t size_of_each;
    size_t used;

// vector_get(vector_t *, int );
vector_get(vector_t *, size_t);

Design: Smaller empty impact

Consider an app making 100s or 1,000,000s instance of this vector (e.g. a vector of vectors). Many of them will could be empty and never used. This is no advantage in doing that first malloc() at vector_init() time and a wasted allocation for all those unused instances. Allocate when needed. IMO, vector_init() should not allocate anything - it does make then that function error free - an important attribute to an initialization function.

Design: Slow Linear growth

Rather than linearly grow the array (incurring O(n*n) re-allocation time), scale the new size by maybe doubling (or some factor).

Design: Error handling

Detect and report (re-)allocation failures.

// void vector_append(vector_t *p, void *d)
bool vector_append(vector_t *p, void *d)  //return true on error

Design: range check

vector_get(vector_t *p, int i), vector_set(vector_t *p, int i, void *d) perform no range check on i. Live as dangerously as you please, yet I'd definitely, at least, include parameters check on vector_set().

Design: const

I'd expect const to show that the data state has not changed.

// void* vector_get(vector_t *p, int i)
void* vector_get(const vector_t *p, int i)

Bug: Code in header file

Code is apparently designed for different VECTOR_RESERVE_SIZE depending on which .c includes it.

Unfortunately, if 2 .c files each include this, the there will be 2 vector_init(), vector_free(), etc, creating linker conflict.

Instead, divide code into vector.h and vector.c files. I suspect though this causes trouble with the VECTOR_RESERVE_SIZE scheme. IMO, VECTOR_RESERVE_SIZE is unnecessary.

Corner bug

No need to assume vector_free() is not called again. Leave data in a diminished, but correct state.

void vector_free(vector_t *p) {
    p->data = NULL; // add
    p->used = 0; // add

else not needed

    if (!(i > p->used))
            return (void*)((char*)p->data + p->size_of_each * i);
    // else { return NULL; }
    return NULL;

Odds & Ends

p->size_of_each * p->used may overflow int math leading to UB. Best to ensure at least size_t math.

void * not needed:

// memcpy((void*)((char*)p->data + p->size_of_each * p->used), d, p->size_of_each);
memcpy((char*)p->data + (size_t)1 * p->size_of_each * p->used, d, p->size_of_each);

// return (void*)((char*)p->data + p->size_of_each * i);
return (char*)p->data + (size_t)1 * p->size_of_each * i;

New functions

int vector_apply(vector, int (*function)(void *state, void *element), void *state) ⟶ Apply the function to each element in the vector. Return early if not zero returned from function.

vector_right_size(vector) ⟶ Shrink the vector to the minimum needed size.

  • 1
    \$\begingroup\$ It might be worth emphasising that the documentation should indicate what the return values mean - I certainly had trouble inferring that from the code. Also, vector_right_size() could be vector_shrink_to_fit to help anyone familiar with C++ standard containers. \$\endgroup\$ Mar 24, 2021 at 7:57

In C, as in C++, object pointers can be freely converted to void*, so quite a few casts can be removed.

It's probably convenient to store data as a char*, since that's how we use it. We should still use void* as external interface, of course.

This is a dangerous anti-pattern:

            p->data = realloc(p->data, VECTOR_RESERVE_SIZE);

If the realloc() fails, it returns a null pointer. By overwriting p->data before we check for null, we leak the old value of p->data, which is no longer accessible. And the null-check is also missing, so subsequent access is Undefined Behaviour.

Style-wise, I think I'd find (!(i > p->used)) easier to read as (i <= p->used). And if (p->data != NULL) { return false; } else { return true; } as return !p->data;.

The example code should set a good example, and check the return value of vector_init() (and also vector_append() once you realise it needs to indicate success/failure) and react appropriately. Otherwise, you'll find you're encouraging slapdash usage.

  • \$\begingroup\$ Thanks for the feedback,I have problem when I'm trying to implement those safety checks at vector_append(), I get a segfault when i try to just assert the reallocated pointer and i can't figure out why \$\endgroup\$
    – user237990
    Mar 22, 2021 at 15:50
  • \$\begingroup\$ It's obviously wrong to assert, since we know the pointer might be null. \$\endgroup\$ Mar 22, 2021 at 16:49
  • 1
    \$\begingroup\$ You need something more like char *ptr = realloc(p->data, new_size); if (!ptr) return false; p->data = ptr; …; return true;. \$\endgroup\$ Mar 22, 2021 at 17:32
  • 1
    \$\begingroup\$ @Roland, you're probably thinking of conversion from void*, which is allowed in C, but requires a cast in C++. The conversion to void* is a widening conversion and fine in both languages. \$\endgroup\$ Mar 23, 2021 at 10:09
  • 1
    \$\begingroup\$ Thanks @chux - edited. \$\endgroup\$ Mar 24, 2021 at 7:51

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