/**
 * @file    libsbrk.c
 * @brief   Naive implementation of sbrk()
 *
 * QNX doesn't support sbrk(), and heap-allocated data can reside anywhere in
 * the virtual address space. However, Emacs depdends on sbrk() for its own
 * allocator implementation, as well as for dumping a pre-loaded image.
 * This implementation reserves a non-backed region in the virtual address
 * space. A call to sbrk() that moves the break point causes the sub range
 * between the old and new break points to be backed by virtual memory.
 */


#include <sys/mman.h>
#include <stdint.h>
#include <errno.h>

#define SEGMENT_SIZE    1024 * 1024 * 1024

static void         *base_addr = MAP_FAILED;
static uintptr_t    break_addr;
static uintptr_t    max_addr;

/**
 * Moves the break point of the emulated data segment.
 * @param   increment   Amount of memory to add to the data segment
 * @return  Previous break point, if successful, -1 on failure
 */
void *
sbrk(intptr_t increment)
{
    if (base_addr == MAP_FAILED) {
        // Allocate a non-backed region of the address space.
        base_addr = mmap(0, SEGMENT_SIZE, PROT_NONE, MAP_PRIVATE | MAP_ANON,
                         -1, 0);
        if (base_addr == MAP_FAILED) {
            errno = ENOMEM;
            return (void *)-1;
        }

        break_addr = (uintptr_t)base_addr;
        max_addr = (uintptr_t)base_addr + SEGMENT_SIZE;
    }

    // Round up to a page size.
    increment = (increment + __PAGESIZE - 1) & ~(__PAGESIZE - 1);

    // Check for overflow.
    uintptr_t   new_addr = break_addr + increment;
    if (new_addr >= max_addr) {
        errno = ENOMEM;
        return (void *)-1;
    }

    // Only support incrementing the break point (though it's not hard to use
    // mprotect(PROT_NONE) to handle decrementing as well).
    if (new_addr <= break_addr) {
        return (void *)break_addr;
    }

    // Back the new sub-range.
    if (mmap((void *)break_addr, increment, PROT_READ | PROT_WRITE,
             MAP_FIXED | MAP_PRIVATE | MAP_ANON, -1, 0) == MAP_FAILED) {
        errno = ENOMEM;
        return (void *)-1;
    }

    void    *result = (void *)break_addr;
    break_addr = new_addr;
    return result;
}