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It suffices to

The iterator expects that no elements are added or removed to the circular buffer during the iteration. A possible solution to detect such a programming error would be to add a generation number to struct circularBuffer which is incremented with every modification. Then copy that generation number to struct circularBufferIterator in circularBufferIteratorPrepare() and verify it to be unchanged in circularBufferIteratorNext(). I would add such a check at least if the program is compiled in DEBUG mode.

It suffices to

There is an asymmetry in your APIs, for example in

You can replace calloc() by malloc() if the initialization with zero bytes is not needed.

There is an asymmetry in your APIs, as can for example be seen in

You can replace calloc() by malloc() if the initialization with zero bytes is not needed.

(If you are planning to work with – implementation-defined – "over-aligned" types then the required alignment must be passed as an additional parameter to the creation function, and more work is necessary to ensure that the address of the data field is property aligned.)

It suffices to

#include <stddef.h> // max_align_t in source

in the implementation file "circularBuffer.c", it is not needed in the header file "circularBuffer.h".

Your calculations of alignElementSize seem unnecessary to me. Here are some quotes of the C11 standard (taken from http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf):

It is then not necessary to separate between alignElementSize and originalElementSize and the test and allocation simplifies to

if ((SIZE_MAX - sizeof(struct circularBuffer)) / elementSize < capacity) {
 return NULL;
}
struct circularBuffer *buf = calloc(1, sizeof(struct circularBuffer) + elementSize * capacity);

Your calculations of alignElementSize seem unnecessary to me (and causes to more memory to be allocated than necessary). Here are some quotes of the C11 standard (taken from http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf):

It is then not necessary to separate between alignElementSize and originalElementSize and the buffer creation simplifies to

struct circularBuffer *circularBufferCreate(size_t capacity, size_t elementSize) {
 
 if (elementSize == 0) {
 return NULL; 
 }
 if ((SIZE_MAX - sizeof(struct circularBuffer)) / elementSize < capacity) {
 return NULL;
}

struct circularBuffer *buf = calloc(1, sizeof(struct circularBuffer) + elementSize * capacity);
 if (!buf) {
 return NULL;
 }
 
 buf->capacity = capacity;
 buf->cardinality = 0;
 buf->elementSize = elementSize;
 
 buf->headOffset = 0;
 buf->tailOffset = 0;
 
 return buf;
}

You can replace calloc() by malloc() if the initialization with zero bytes is not needed.