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    <p><code>malloc</code> always returns memory that is set to the maximum alignment required by any of the primitive types. This allows <code>malloc</code>'d memory to store any type you may need. My understanding of the description of <code>posix_memalign</code>, is that it returns a memory location who's address will be a multiple of whatever you specify as the alignment.</p> <p>Im not sure how useful this would be when writing a custom memory pool, but I have had a go at providing an example of how this could be implemented. The difference is with my example, anything allocated with <code>malloc_aligned</code> has to be freed with <code>free_aligned</code>; however, with <code>posix_memalign</code> you can use <code>free</code>.</p> <pre><code>#include &lt;stdlib.h&gt; #include &lt;stdio.h&gt; void *malloc_aligned(size_t alignment, size_t bytes) { // we need to allocate enough storage for the requested bytes, some // book-keeping (to store the location returned by malloc) and some extra // padding to allow us to find an aligned byte. im not entirely sure if // 2 * alignment is enough here, its just a guess. const size_t total_size = bytes + (2 * alignment) + sizeof(size_t); // use malloc to allocate the memory. char *data = malloc(sizeof(char) * total_size); if (data) { // store the original start of the malloc'd data. const void * const data_start = data; // dedicate enough space to the book-keeping. data += sizeof(size_t); // find a memory location with correct alignment. the alignment minus // the remainder of this mod operation is how many bytes forward we need // to move to find an aligned byte. const size_t offset = alignment - (((size_t)data) % alignment); // set data to the aligned memory. data += offset; // write the book-keeping. size_t *book_keeping = (size_t*)(data - sizeof(size_t)); *book_keeping = (size_t)data_start; } return data; } void free_aligned(void *raw_data) { if (raw_data) { char *data = raw_data; // we have to assume this memory was allocated with malloc_aligned. // this means the sizeof(size_t) bytes before data are the book-keeping // which points to the location we need to pass to free. data -= sizeof(size_t); // set data to the location stored in book-keeping. data = (char*)(*((size_t*)data)); // free the memory. free(data); } } int main() { char *ptr = malloc_aligned(7, 100); printf("is 5 byte aligned = %s\n", (((size_t)ptr) % 5) ? "no" : "yes"); printf("is 7 byte aligned = %s\n", (((size_t)ptr) % 7) ? "no" : "yes"); free_aligned(ptr); return 0; } </code></pre>
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    1. POWhat does posix_memalign/memalign do
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    1. COThanks for this! Very useful piece of code as a replacement for memalign, which is somehow still missing from mingw.
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