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MALLOC(9) BSD Kernel Developer's Manual MALLOC(9) NAME malloc, MALLOC, free, FREE -- kernel memory management routines SYNOPSIS #include <sys/types.h> #include <sys/malloc.h> void * malloc(unsigned long size, struct malloc_type *type, int flags); MALLOC(space, cast, unsigned long size, struct malloc_type *type, int flags); void free(void *addr, struct malloc_type *type); FREE(void *addr, struct malloc_type *type); void * realloc(void *addr, unsigned long size, struct malloc_type *type, int flags); void * reallocf(void *addr, unsigned long size, struct malloc_type *type, int flags); 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. The memory is not zeroed. If addr is NULL, then free() does nothing. The realloc() function changes the size of the previously allocated mem- ory referenced by addr to size bytes. The contents of the memory are un- changed up to the lesser of the new and old sizes. Note that the re- turned value may differ from addr. If the requested memory cannot be al- located, NULL is returned and the memory referenced by addr is valid and unchanged. If addr is NULL, the realloc() function behaves identically to malloc() for the specified size. The reallocf() function call is identical to the realloc function call, except that it will free the passed pointer when the requested memory cannot be allocated. The MALLOC() macro variant is functionally equivalent to (space) = (cast)malloc((u_long)(size), type, flags) and the FREE() macro variant is equivalent to free((addr), type) Unlike its standard C library counterpart (malloc(3)), the kernel version takes two more arguments. The flags argument further qualifies malloc()'s operational characteristics as follows: M_NOWAIT Causes malloc(), realloc(), or reallocf() to return NULL if the request cannot be immediately fulfilled due to resource shortage. Otherwise, the current process may be put to sleep to wait for resources to be released by other processes. If this flag is set, malloc() will return NULL rather then block. Note that M_WAITOK is defined to be 0, meaning that blocking operation is the default. Also note that M_NOWAIT is required when running in an interrupt context. M_ASLEEP Causes malloc(), realloc(), or reallocf() to call asleep() if the request cannot be immediately fulfilled due to a resource short- age. M_ASLEEP is not useful alone and should always be or'd with M_NOWAIT to allow the function to call asleep() and return NULL immediately. It is expected that the caller will at some point call await() and then retry the allocation. Depending on the routine in question, the caller may decide to propagate the tem- porary failure up the call chain and actually have some other higher level routine block on the async wait that the function queued. M_WAITOK Indicates that it is Ok to wait for resources. It is unconve- niently defined as 0 so care should be taken never to compare against this value directly or try to AND it as a flag. The de- fault operation is to block until the memory allocation succeeds. malloc(), realloc(), and reallocf() can only return NULL if M_NOWAIT is specified. M_USE_RESERVE Indicates that the system can dig into its reserve in order to obtain the requested memory. This option used to be called M_KERNEL but has been renamed to something more obvious. This option has been deprecated and is slowly being removed from the kernel, and so should not be used with any new programming. The type argument is used to perform statistics on memory usage, and for basic sanity checks. The statistics can be examined by `vmstat -m'. A type is defined using the malloc_type_t typedef via the MALLOC_DECLARE() and MALLOC_DEFINE() macros. /* sys/something/foo_extern.h */ MALLOC_DECLARE(M_FOOBUF); /* sys/something/foo_main.c */ MALLOC_DEFINE(M_FOOBUF, "foobuffers", "Buffers to foo data into the ether"); /* sys/something/foo_subr.c */ ... MALLOC(buf, struct foo_buf *, sizeof *buf, M_FOOBUF, M_NOWAIT); RETURN VALUES malloc(), realloc(), and reallocf() return a kernel virtual address that is suitably aligned for storage of any type of object, or NULL if the re- quest could not be satisfied (implying that M_NOWAIT was set). If M_ASLEEP was set and the function returns NULL, it will call asleep() as a side effect. IMPLEMENTATION NOTES The memory allocator allocates memory in chunks that have size a power of two for requests up to the size of a page of memory. For larger re- quests, one or more pages is allocated. While it should not be relied upon, this information may be useful for optimizing the efficiency of memory use. SEE ALSO vmstat(8) DIAGNOSTICS A kernel compiled with the DIAGNOSTIC configuration option attempts to detect memory corruption caused by such things as writing outside the al- located area and imbalanced calls to the malloc() and free() functions. Failing consistency checks will cause a panic or a system console mes- sage: +o panic: "malloc: bogus type" +o panic: "malloc: allocation too large" +o panic: "malloc: wrong bucket" +o panic: "malloc: lost data" +o panic: "free: address 0x%x out of range" +o panic: "free: type %d out of range" +o panic: "free: unaligned addr <description of object>" +o panic: "free: item modified" +o panic: "free: multiple free[s]" +o "Data modified on freelist: <description of object>" BSD June 16, 1996 BSD
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | IMPLEMENTATION NOTES | SEE ALSO | DIAGNOSTICS
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