UCX Features

Allocator String Buffer Memory Pool
Iterator Collection List Map


Header file: allocator.h

The UCX allocator provides an interface for implementing an own memory allocation mechanism. Various function in UCX provide an additional alternative signature that takes an allocator as argument. A default allocator implementation using the stdlib memory management functions is available via the global symbol cxDefaultAllocator.

If you want to define your own allocator, you need to initialize the CxAllocator structure with a pointer to an allocator class (containing function pointers for the memory management functions) and an optional pointer to an arbitrary memory region that can be used to store state information for the allocator. An example is shown below:

struct my_allocator_state {
    size_t total;
    size_t avail;
    char mem[];

static cx_allocator_class my_allocator_class = {
        my_realloc_impl,   // all these functions are somewhere defined

CxAllocator create_my_allocator(size_t n) {
    CxAllocator alloc;
    alloc.cl = &my_allocator_class;
    alloc.data = calloc(1, sizeof(struct my_allocator_state) + n);
    return alloc;

void free_my_allocator(CxAllocator *alloc) {


Header file: string.h

UCX strings come in two variants: immutable (cxstring) and mutable (cxmutstr). The functions of UCX are designed to work with immutable strings by default but in situations where it is necessary, the API also provides alternative functions that work directly with mutable strings. Functions that change a string in-place are, of course, only accepting mutable strings.

When you are using UCX functions, or defining your own functions, you are sometimes facing the “problem”, that the function only accepts arguments of type cxstring but you only have a cxmutstr at hand. In this case you should not introduce a wrapper function that accepts the cxmutstr, but instead you should use the cx_strcast() function to cast the argument to the correct type.

In general, UCX strings are not necessarily zero-terminated. If a function guarantees to return zero-terminated string, it is explicitly mentioned in the documentation of the respective function. As a rule of thumb, you should not pass the strings of a UCX string structure to another API without explicitly ensuring that the string is zero-terminated.


Header file: buffer.h

Instances of this buffer implementation can be used to read from or write to memory like you would do with a stream. This allows the use of cx_stream_copy() (see Utilities) to copy contents from one buffer to another, or from a file or network streams to the buffer and vice-versa.

More features for convenient use of the buffer can be enabled, like automatic memory management and automatic resizing of the buffer space.

Since UCX 3.0, the buffer also supports automatic flushing of contents to another stream (or buffer) as an alternative to automatically resizing the buffer space. Please refer to the API doc for the fields prefixed with flush_ to learn more.

Memory Pool

Header file: mempool.h

A memory pool is providing an allocator implementation that automatically deallocates the memory upon its destruction. It also allows you to register destructor functions for the allocated memory, which are automatically called before the memory is deallocated. Additionally, you may also register independent destructor functions within a pool in case some external library allocated memory for you, which should be destroyed together with this pool.

Many UCX features support the use of an allocator. The strings, for instance, provide several functions suffixed with _a that allow specifying an allocator. You can use this to keep track of the memory occupied by dynamically allocated strings and cleanup everything with just a single call to cxMempoolDestroy().

The following code illustrates this on the example of reading a CSV file into memory.

#include <stdio.h>
#include <cx/mempool.h>
#include <cx/linked_list.h>
#include <cx/string.h>
#include <cx/buffer.h>
#include <cx/utils.h>

typedef struct {
    cxstring column_a;
    cxstring column_b;
    cxstring column_c;
} CSVData;

int main(void) {
    CxMempool* pool = cxBasicMempoolCreate(128);

    FILE *f = fopen("test.csv", "r");
    if (!f) {
        perror("Cannot open file");
        return 1;
    // close the file automatically at pool destruction
    cxMempoolRegister(pool, f, (cx_destructor_func) fclose);

    // create a buffer using the memory pool for destruction
    CxBuffer *content = cxBufferCreate(NULL, 256, pool->allocator, CX_BUFFER_AUTO_EXTEND);

    // read the file into the buffer and turn it into a string
    cx_stream_copy(f, content, (cx_read_func) fread, (cx_write_func) cxBufferWrite);
    cxstring contentstr = cx_strn(content->space, content->size);

    // split the string into lines - use the mempool for allocating the target array
    cxstring* lines;
    size_t lc = cx_strsplit_a(pool->allocator, contentstr,
                              CX_STR("\n"), SIZE_MAX, &lines);

    // skip the header and parse the remaining data into a linked list
    // the nodes of the linked list shall also be allocated by the mempool
    CxList* datalist = cxLinkedListCreate(pool->allocator, NULL, sizeof(CSVData));
    for (size_t i = 1 ; i < lc ; i++) {
        if (lines[i].length == 0) continue;
        cxstring fields[3];
        size_t fc = cx_strsplit(lines[i], CX_STR(";"), 3, fields);
        if (fc != 3) {
            fprintf(stderr, "Syntax error in line %zu.\n", i);
            return 1;
        CSVData* data = cxMalloc(pool->allocator, sizeof(CSVData));
        data->column_a = fields[0];
        data->column_b = fields[1];
        data->column_c = fields[2];
        cxListAdd(datalist, data);

    // iterate through the list and output the data
    CxIterator iter = cxListIterator(datalist);
    cx_foreach(CSVData*, data, iter) {
        printf("Column A: %.*s | "
               "Column B: %.*s | "
               "Column C: %.*s\n",
               (int)data->column_a.length, data->column_a.ptr,
               (int)data->column_b.length, data->column_b.ptr,
               (int)data->column_c.length, data->column_c.ptr

    // cleanup everything, no manual free() needed 

    return 0;


Header file: iterator.h

In UCX 3 a new feature has been introduced to write own iterators, that work with the cx_foreach macro. In previous UCX releases there were different hard-coded foreach macros for lists and maps that were not customizable. Now, creating an iterator is as simple as creating a CxIterator struct and setting the fields in a meaningful way.

You do not always need all fields in the iterator structure, depending on your use case. Sometimes you only need the index (for example when iterating over simple lists), and other times you will need the slot and kv_data fields (for example when iterating over maps).

Usually an iterator is not mutating the collection it is iterating over. In some programming languages it is even disallowed to change the collection while iterating with foreach. But sometimes it is desirable to remove an element from the collection while iterating over it. This is, what the CxMutIterator is for. The only differences are, that the mutating flag is true and the src_handle is not const. On mutating iterators it is allowed to call the cxFlagForRemoval() function, which instructs the iterator to remove the current element from the collection on the next call to cxIteratorNext() and clear the flag afterward. If you are implementing your own iterator, it is up to you to implement this behavior in your next method, or make the implementation of the flag_removal method always return false.


Header file: collection.h

Collections in UCX 3 have several common features. If you want to implement an own collection data type that uses the same features, you can use the CX_COLLECTION_MEMBERS macro at the beginning of your struct to roll out all members a usual UCX collection has.

struct my_fancy_collection_s {
    struct my_collection_data_s *data;

Based on this structure, this header provides some convenience macros for invoking the destructor functions that are part of the basic collection members. The idea of having destructor functions within a collection is that you can destroy the collection and the contents with one single function call. When you are implementing a collection, you are responsible for invoking the destructors at the right places, e.g. when removing (and deleting) elements in the collection, clearing the collection, or - the most prominent case - destroying the collection.

You can always look at the UCX list and map implementations if you need some inspiration.


Header file: list.h

This header defines a common interface for all list implementations, which is basically as simple as the following structure.

struct cx_list_s {
    CX_COLLECTION_MEMBERS       // size, capacity, etc.
    cx_list_class const *cl;    // The list class definition

The actual structure contains one more class pointer that is used when wrapping a list into a pointer aware list with cxListStorePointers(). What this means, is that - if you want to implement your own list structure - you only need to cover the case where the list is storing copies of your objects.

UCX comes with two common list implementations (linked list and array list) that should cover most use cases. But if you feel the need to implement an own list, the only thing you need to do is to define a struct where struct cx_list_s, and set an appropriate list class that implements the functionality. It is strongly recommended that this class is shared among all instances of the same list type, because otherwise the cxListCompare function cannot use the optimized implementation of your class and will instead fall back to using iterators to compare the contents element-wise.

Linked List

Header file: linked_list.h

On top of implementing the list interface, this header also defines several low-level functions that work with arbitrary structures. Low-level functions, in contrast to the high-level list interface, can easily be recognized by their snake-casing. The function cx_linked_list_at, for example, implements a similar functionality like cxListAt, but operates on arbitrary structures. The following snippet shows how it is used. All other low-level functions work similarly.

struct node {
    node *next;
    node *prev;
    int data;

const ptrdiff_t loc_prev = offsetof(struct node, prev);
const ptrdiff_t loc_next = offsetof(struct node, next);
const ptrdiff_t loc_data = offsetof(struct node, data);

struct node a = {0}, b = {0}, c = {0}, d = {0};
cx_linked_list_link(&a, &b, loc_prev, loc_next);
cx_linked_list_link(&b, &c, loc_prev, loc_next);
cx_linked_list_link(&c, &d, loc_prev, loc_next);

cx_linked_list_at(&a, 0, loc_next, 2); // returns pointer to c

Array List

Header file: array_list.h

Since low-level array lists are just plain arrays, there is no need for such many low-level functions as for linked lists. However, there is one extremely powerful function that can be used for several complex tasks: cx_array_copy. The full signature is shown below:

enum cx_array_copy_result cx_array_copy(
        void **target,
        size_t *size,
        size_t *capacity,  // optional
        size_t index,
        void const *src,
        size_t elem_size,
        size_t elem_count,
        struct cx_array_reallocator_s *reallocator // optional

The target argument is a pointer to the target array pointer. The reason for this additional indirection is that - given that you provide a reallocator - this function writes back the pointer to the possibly reallocated array. THe next two arguments are pointers to the size and capacity of the target array. Tracking the capacity is optional. If you do not specify a pointer for the capacity, automatic reallocation of the array is entirely disabled (i.e. it does not make sense to specify a reallocator then). In this case, the function cannot copy more than size-index elements and if you try, it will return CX_ARRAY_COPY_REALLOC_NOT_SUPPORTED and do nothing.

On a successful invocation, the function copies elem_count number of elements, each of size elem_size from src to *target and uses the reallocator to extend the array when necessary. Finally, the size, capacity, and the pointer to the array are all updated and the function returns CX_ARRAY_COPY_SUCCESS.

The third, but extremely rare, return code is CX_ARRAY_COPY_REALLOC_FAILED and speaks for itself.

A few things to note: * *target and src can point to the same memory region, effectively copying elements within the array with memmove * *target does not need to point to the start of the array, but size and capacity always start counting from the position, *target points to - in this scenario, specifying a reallocator is forbidden for obvious reasons * index does not need to be within size of the current array, if capacity is specified * index does not even need to be within the capacity of the array, if reallocator is specified


Header file: map.h

Similar to the list interface, the map interface provides a common API for implementing maps. There are some minor subtle differences, though.

First, the remove method is not always a destructive removal. Instead, the last argument is a Boolean that indicates whether the element shall be destroyed or returned.

void *(*remove)(CxMap *map, CxHashKey key, bool destroy);

When you implement this method, you are either supposed to invoke the destructors and return NULL, or just remove the element from the map and return it.

Secondly, the iterator method is a bit more complete. The signature is as follows:

CxIterator (*iterator)(CxMap const *map, enum cx_map_iterator_type type);

There are three map iterator types: for values, for keys, for pairs. Depending on the iterator type requested, you need to create an iterator with the correct methods that return the requested thing. There are no automatic checks to enforce this - it’s completely up to you. If you need inspiration on how to do that, check the hash map implementation that comes with UCX.

Hash Map

Header file: hash_map.h

UCX provides a basic hash map implementation with a configurable amount of buckets. If you do not specify the number of buckets, a default of 16 buckets will be used. You can always rehash the map with cxMapRehash() to change the number of buckets to something more efficient, but you need to be careful, because when you use this function you are effectively locking into using this specific hash map implementation, and you would need to remove all calls to this function when you want to exchange the concrete map implementation with something different.


Header file: utils.h

UCX provides some utilities for routine tasks. Most of them are simple macros, like e.g. the cx_for_n() macro, creating a for loop counting from zero to (n-1) which is extremely useful to traverse the indices of an array.

But the most useful utilities are the stream copy functions, which provide a simple way to copy all - or a bounded amount of - data from one stream to another. Since the read/write functions of a UCX buffer are fully compatible with stream read/write functions, you can easily transfer data from file or network streams to a UCX buffer or vice-versa.

The following example shows, how easy it is to read the contents of a file into a buffer:

FILE *inputfile = fopen(infilename, "r");
if (inputfile) {
    CxBuffer fbuf;
    cxBufferInit(&fbuf, NULL, 4096, NULL, CX_BUFFER_AUTO_EXTEND);
    cx_stream_copy(inputfile, &fbuf,
                   (cx_read_func) fread,
                   (cx_write_func) cxBufferWrite);
    // ... do something meaningful with the contents ...
} else {
    perror("Error opening input file");
    if (fout != stdout) {

Printf Functions

Header file: printf.h

In this utility header you can find printf()-like functions that can write the formatted output to an arbitrary stream (or UCX buffer, resp.), or to memory allocated by an allocator within a single function call. With the help of these convenience functions, you do not need to snprintf your string to a temporary buffer anymore, plus you do not need to worry about too small buffer sizes, because the functions will automatically allocate enough memory to contain the entire formatted string.

Compare Functions

Header file: compare.h

This header file contains a collection of compare functions for various data types. Their signatures are designed to be compatible with the cx_compare_func function pointer type.