xalloc, xallocz, xspanalloc, xfree, xsummary – basic memory management


void* xalloc(ulong size)

void* xallocz(ulong size, int clr)

void* xspanalloc(ulong size, int align, ulong span)

void xfree(void *p)

void xsummary(void)


These are primitives used by higher-level memory allocators in the kernel, such as malloc(9). They are not intended for use directly by most kernel routines. The main exceptions are routines that permanently allocate large structures, or need the special alignment properties guaranteed by xspanalloc.

Xalloc returns a pointer to a range of size bytes of memory. The memory will be zero filled and aligned on a 8 byte (BY2V) address. If the memory is not available, xalloc returns a null pointer.

Xmallocz will clear the memory after allocation if clr is set to a value other than zero. Since it is used by xmalloc, the same diagnostics apply.

Xspanalloc allocates memory given alignment and spanning constraints. The block returned will contain size bytes, aligned on a boundary that is 0 mod align, in such a way that the memory in the block does not span an address that is 0 mod span. Xspanalloc is intended for use allocating hardware data structures (eg, page tables) or I/O buffers that must satisfy specific alignment restrictions. If xspanalloc cannot allocate memory to satisfy the given constraints, it will panic(9). The technique it uses can sometimes cause memory to be wasted. Consequently, xspanalloc should be used sparingly.

Xfree frees the block of memory at p, which must be an address previously returned by xalloc (not xspanalloc).

Xsummary dumps memory allocation statistics to the console. The output includes the total free space, the number of free holes, and a summary of active holes. Each line shows ‘address top size’.