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A set of general-purpose dynamic array macros for C structures are included with uthash in utarray.h. To use these macros in your own C program, just copy utarray.h into your source directory and use it in your programs.

#include "utarray.h"

The dynamic array supports basic operations such as push, pop, and erase on the array elements. These array elements can be any simple datatype or structure. The array operations are based loosely on the C++ STL vector methods.

Internally the dynamic array contains a contiguous memory region into which the elements are copied. This buffer is grown as needed using realloc to accommodate all the data that is pushed into it.


To download the utarray.h header file, follow the links on to clone uthash or get a zip file, then look in the src/ sub-directory.

BSD licensed

This software is made available under the revised BSD license. It is free and open source.


The utarray macros have been tested on:

  • Linux,

  • Mac OS X,

  • Windows, using Visual Studio 2008 and Visual Studio 2010



The array itself has the data type UT_array, regardless of the type of elements to be stored in it. It is declared like,

UT_array *nums;

New and free

The next step is to create the array using utarray_new. Later when you’re done with the array, utarray_free will free it and all its elements.

Push, pop, etc

The central features of the utarray involve putting elements into it, taking them out, and iterating over them. There are several operations to pick from that deal with either single elements or ranges of elements at a time. In the examples below we will use only the push operation to insert elements.


Support for dynamic arrays of integers or strings is especially easy. These are best shown by example:


This example makes a utarray of integers, pushes 0-9 into it, then prints it. Lastly it frees it.

Integer elements
#include <stdio.h>
#include "utarray.h"

int main() {
  UT_array *nums;
  int i, *p;

  for(i=0; i < 10; i++) utarray_push_back(nums,&i);

      p=(int*)utarray_next(nums,p)) {


  return 0;

The second argument to utarray_push_back is always a pointer to the type (so a literal cannot be used). So for integers, it is an int*.


In this example we make a utarray of strings, push two strings into it, print it and free it.

String elements
#include <stdio.h>
#include "utarray.h"

int main() {
  UT_array *strs;
  char *s, **p;


  s = "hello"; utarray_push_back(strs, &s);
  s = "world"; utarray_push_back(strs, &s);
  p = NULL;
  while ( (p=(char**)utarray_next(strs,p))) {


  return 0;

In this example, since the element is a char*, we pass a pointer to it (char**) as the second argument to utarray_push_back. Note that "push" makes a copy of the source string and pushes that copy into the array.

About UT_icd

Arrays be made of any type of element, not just integers and strings. The elements can be basic types or structures. Unless you’re dealing with integers and strings (which use pre-defined ut_int_icd and ut_str_icd), you’ll need to define a UT_icd helper structure. This structure contains everything that utarray needs to initialize, copy or destruct elements.

typedef struct {
    size_t sz;
    init_f *init;
    ctor_f *copy;
    dtor_f *dtor;
} UT_icd;

The three function pointers init, copy, and dtor have these prototypes:

typedef void (ctor_f)(void *dst, const void *src);
typedef void (dtor_f)(void *elt);
typedef void (init_f)(void *elt);

The sz is just the size of the element being stored in the array.

The init function will be invoked whenever utarray needs to initialize an empty element. This only happens as a byproduct of utarray_resize or utarray_extend_back. If init is NULL, it defaults to zero filling the new element using memset.

The copy function is used whenever an element is copied into the array. It is invoked during utarray_push_back, utarray_insert, utarray_inserta, or utarray_concat. If copy is NULL, it defaults to a bitwise copy using memcpy.

The dtor function is used to clean up an element that is being removed from the array. It may be invoked due to utarray_resize, utarray_pop_back, utarray_erase, utarray_clear, utarray_done or utarray_free. If the elements need no cleanup upon destruction, dtor may be NULL.

Scalar types

The next example uses UT_icd with all its defaults to make a utarray of long elements. This example pushes two longs, prints them, and frees the array.

long elements
#include <stdio.h>
#include "utarray.h"

UT_icd long_icd = {sizeof(long), NULL, NULL, NULL };

int main() {
  UT_array *nums;
  long l, *p;
  utarray_new(nums, &long_icd);

  l=1; utarray_push_back(nums, &l);
  l=2; utarray_push_back(nums, &l);

  while( (p=(long*)utarray_next(nums,p))) printf("%ld\n", *p);

  return 0;


Structures can be used as utarray elements. If the structure requires no special effort to initialize, copy or destruct, we can use UT_icd with all its defaults. This example shows a structure that consists of two integers. Here we push two values, print them and free the array.

Structure (simple)
#include <stdio.h>
#include "utarray.h"

typedef struct {
    int a;
    int b;
} intpair_t;

UT_icd intpair_icd = {sizeof(intpair_t), NULL, NULL, NULL};

int main() {

  UT_array *pairs;
  intpair_t ip, *p;

  ip.a=1;  ip.b=2;  utarray_push_back(pairs, &ip);
  ip.a=10; ip.b=20; utarray_push_back(pairs, &ip);

      p=(intpair_t*)utarray_next(pairs,p)) {
    printf("%d %d\n", p->a, p->b);

  return 0;

The real utility of UT_icd is apparent when the elements of the utarray are structures that require special work to initialize, copy or destruct.

For example, when a structure contains pointers to related memory areas that need to be copied when the structure is copied (and freed when the structure is freed), we can use custom init, copy, and dtor members in the UT_icd.

Here we take an example of a structure that contains an integer and a string. When this element is copied (such as when an element is pushed into the array), we want to "deep copy" the s pointer (so the original element and the new element point to their own copies of s). When an element is destructed, we want to "deep free" its copy of s. Lastly, this example is written to work even if s has the value NULL.

Structure (complex)
#include <stdio.h>
#include <stdlib.h>
#include "utarray.h"

typedef struct {
    int a;
    char *s;
} intchar_t;

void intchar_copy(void *_dst, const void *_src) {
  intchar_t *dst = (intchar_t*)_dst, *src = (intchar_t*)_src;
  dst->a = src->a;
  dst->s = src->s ? strdup(src->s) : NULL;

void intchar_dtor(void *_elt) {
  intchar_t *elt = (intchar_t*)_elt;
  if (elt->s) free(elt->s);

UT_icd intchar_icd = {sizeof(intchar_t), NULL, intchar_copy, intchar_dtor};

int main() {
  UT_array *intchars;
  intchar_t ic, *p;
  utarray_new(intchars, &intchar_icd);

  ic.a=1; ic.s="hello"; utarray_push_back(intchars, &ic);
  ic.a=2; ic.s="world"; utarray_push_back(intchars, &ic);

  while( (p=(intchar_t*)utarray_next(intchars,p))) {
    printf("%d %s\n", p->a, (p->s ? p->s : "null"));

  return 0;


This table lists all the utarray operations. These are loosely based on the C++ vector class.


utarray_new(UT_array *a, UT_icd *icd)

allocate a new array

utarray_free(UT_array *a)

free an allocated array

utarray_init(UT_array *a,UT_icd *icd)

init an array (non-alloc)

utarray_done(UT_array *a)

dispose of an array (non-allocd)

utarray_reserve(UT_array *a,int n)

ensure space available for n more elements

utarray_push_back(UT_array *a,void *p)

push element p onto a

utarray_pop_back(UT_array *a)

pop last element from a

utarray_extend_back(UT_array *a)

push empty element onto a

utarray_len(UT_array *a)

get length of a

utarray_eltptr(UT_array *a,int j)

get pointer of element from index

utarray_eltidx(UT_array *a,void *e)

get index of element from pointer

utarray_insert(UT_array *a,void *p, int j)

insert element p to index j

utarray_inserta(UT_array *a,UT_array *w, int j)

insert array w into array a at index j

utarray_resize(UT_array *dst,int num)

extend or shrink array to num elements

utarray_concat(UT_array *dst,UT_array *src)

copy src to end of dst array

utarray_erase(UT_array *a,int pos,int len)

remove len elements from a[pos]..a[pos+len-1]

utarray_clear(UT_array *a)

clear all elements from a, setting its length to zero

utarray_sort(UT_array *a,cmpfcn *cmp)

sort elements of a using comparison function

utarray_find(UT_array *a,void *v, cmpfcn *cmp)

find element v in utarray (must be sorted)

utarray_front(UT_array *a)

get first element of a

utarray_next(UT_array *a,void *e)

get element of a following e (front if e is NULL)

utarray_prev(UT_array *a,void *e)

get element of a before e (back if e is NULL)

utarray_back(UT_array *a)

get last element of a


  1. utarray_new and utarray_free are used to allocate a new array and free it, while utarray_init and utarray_done can be used if the UT_array is already allocated and just needs to be initialized or have its internal resources freed.

  2. utarray_reserve takes the "delta" of elements to reserve (not the total desired capacity of the array-- this differs from the C++ STL "reserve" notion)

  3. utarray_sort expects a comparison function having the usual strcmp -like convention where it accepts two elements (a and b) and returns a negative value if a precedes b, 0 if a and b sort equally, and positive if b precedes a. This is an example of a comparison function:

    int intsort(const void *a, const void *b) {
        int _a = *(const int *)a;
        int _b = *(const int *)b;
        return (_a < _b) ? -1 : (_a > _b);
  4. utarray_find uses a binary search to locate an element having a certain value according to the given comparison function. The utarray must be first sorted using the same comparison function. An example of using utarray_find with a utarray of strings is included in tests/test61.c.

  5. A pointer to a particular element (obtained using utarray_eltptr or utarray_front, utarray_next, utarray_prev, utarray_back) becomes invalid whenever another element is inserted into the utarray. This is because the internal memory management may need to realloc the element storage to a new address. For this reason, it’s usually better to refer to an element by its integer index in code whose duration may include element insertion.