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NAME

malloc, free — kernel memory allocator

SYNOPSIS

#include <sys/types.h>
#include <sys/malloc.h>

void *
malloc(unsigned long size, int type, int flags);

void
free(void *addr, int type);

DESCRIPTION

The malloc() function allocates uninitialized memory in kernel address space for an object whose size is specified by size. free() releases memory at address addr that was previously allocated by malloc() for re-use. If addr is a null pointer, no action occurs.

The flags argument further qualifies malloc's operational characteristics as follows:

The type argument broadly identifies the kernel subsystem for which the allocated memory was needed, and is commonly used to maintain statistics about kernel memory usage. These statistics can be examined using vmstat(8) or systat(1) if either of the kernel options(4) KMEMSTATS or DEBUG are enabled.

The following types are currently defined:

RETURN VALUES

malloc() returns a kernel virtual address that is suitably aligned for storage of any type of object.

DIAGNOSTICS

A kernel compiled with the DIAGNOSTIC configuration option attempts to detect memory corruption caused by such things as writing outside the allocated area and unbalanced calls to the malloc() and free() functions. Failing consistency checks will cause a panic or a system console message:

• panic: “malloc - bogus type”
• panic: “malloc: out of space in kmem_map”
• panic: “malloc: allocation too large”
• panic: “malloc: wrong bucket”
• panic: “malloc: lost data”
• panic: “free: unaligned addr”
• panic: “free: duplicated free”
• panic: “free: multiple frees”
• panic: “kmeminit: minbucket too small/struct freelist too big”
• “multiply freed item ⟨addr⟩”
• “Data modified on freelist: ⟨data object description⟩”

DEBUGGING

A kernel compiled with the MALLOC_DEBUG option allows for more extensive debugging of memory allocations. The debug_malloc_type, debug_malloc_size, debug_malloc_size_lo and debug_malloc_size_hi variables choose which allocation to debug. debug_malloc_type should be set to the memory type and debug_malloc_size should be set to the memory size to debug. 0 can be used as a wildcard. debug_malloc_size_lo and debug_malloc_size_hi can be used to specify a range of sizes if the exact size to debug is not known. When those are used, debug_malloc_size needs to be set to the wildcard. M_DEBUG can also be specified as an allocation type to force allocation with debugging.

Every call to malloc() with a memory type and size that matches the debugged type and size will allocate two virtual pages. The pointer returned will be aligned so that the requested area will end at the page boundary and the second virtual page will be left unmapped. This way we can catch reads and writes outside the allocated area.

Every call to free() with memory that was returned by the debugging malloc will cause the memory area to become unmapped so that we can catch dangling reads and writes to freed memory.

There are no special diagnostics if any errors are caught by the debugging malloc. The errors will look like normal access to unmapped memory. On a memory access error, the show malloc command in ddb(4) can be invoked to see what memory areas are allocated and freed. If the faulting address is within two pages from an address on the allocated list, there was an access outside the allocated area. If the faulting address is within two pages from an address on the free list, there was an access to freed memory.

Care needs to be taken when using the MALLOC_DEBUG option: the memory consumption can run away pretty quickly and there is a severe performance degradation when allocating and freeing debugged memory types.

SEE ALSO

systat(1), vmstat(8)