Visit Our UNIX and Linux User Community

Linux and UNIX Man Pages

Test Your Knowledge in Computers #29
Difficulty: Medium
Computer hard disks were first introduced in 1956.
True or False?
Linux & Unix Commands - Search Man Pages

malloc(2) [plan9 man page]

MALLOC(2)							System Calls Manual							 MALLOC(2)

NAME
malloc, free, realloc, calloc - memory allocator SYNOPSIS
#include <u.h> #include <libc.h> void* malloc(long size) void free(void *ptr) void* realloc(void *ptr, long size) void* calloc(long nelem, long elsize) DESCRIPTION
Malloc and free provide a simple memory allocation package. Malloc returns a pointer to a new block of at least size bytes. The block is suitably aligned for storage of any type of object. No two active pointers from malloc will have the same value. The argument to free is a pointer to a block previously allocated by malloc; this space is made available for further allocation. It is legal to free a null pointer; the effect is a no-op. Realloc changes the size of the block pointed to by ptr to size bytes and returns a pointer to the (possibly moved) block. The contents will be unchanged up to the lesser of the new and old sizes. The call realloc(0, size) means the same as Calloc allocates space for an array of nelem elements of size elsize. The space is initialized to zeros. Free frees such a block. Alef Except for calloc, these routines are available from Alef; they use the same arena as alloc. Malloc and realloc execute a check when they fail, rather than return nil. Memory returned by malloc is cleared. Realloc does not guarantee new memory is cleared unless ptr is nil. SOURCE
/sys/src/libc/port/malloc.c SEE ALSO
brk(2) DIAGNOSTICS
Malloc, realloc and calloc return 0 if there is no available memory. Errstr is likely to be set. BUGS
The different specification of calloc is bizarre. User errors can corrupt the storage arena. The most common gaffes are (1) freeing an already freed block, (2) storing beyond the bounds of an allocated block, and (3) freeing data that was not obtained from the allocator. When malloc and free detect such corruption, they abort. MALLOC(2)

Check Out this Related Man Page

MALLOC(3)						     Library Functions Manual							 MALLOC(3)

NAME
malloc, free, realloc, calloc - main memory allocator SYNOPSIS
char *malloc(size) unsigned size; free(ptr) char *ptr; char *realloc(ptr, size) char *ptr; unsigned size; char *calloc(nelem, elsize) unsigned nelem, elsize; DESCRIPTION
Malloc and free provide a simple general-purpose memory allocation package. Malloc returns a pointer to a block of at least size bytes beginning on a word boundary. The argument to free is a pointer to a block previously allocated by malloc; this space is made available for further allocation, but its contents are left undisturbed. Needless to say, grave disorder will result if the space assigned by malloc is overrun or if some random number is handed to free. Malloc allocates the first big enough contiguous reach of free space found in a circular search from the last block allocated or freed, coalescing adjacent free blocks as it searches. It calls sbrk (see break(2)) to get more memory from the system when there is no suitable space already free. Realloc changes the size of the block pointed to by ptr to size bytes and returns a pointer to the (possibly moved) block. The contents will be unchanged up to the lesser of the new and old sizes. Realloc also works if ptr points to a block freed since the last call of malloc, realloc or calloc; thus sequences of free, malloc and realloc can exploit the search strategy of malloc to do storage compaction. Calloc allocates space for an array of nelem elements of size elsize. The space is initialized to zeros. Each of the allocation routines returns a pointer to space suitably aligned (after possible pointer coercion) for storage of any type of object. DIAGNOSTICS
Malloc, realloc and calloc return a null pointer (0) if there is no available memory or if the arena has been detectably corrupted by stor- ing outside the bounds of a block. Malloc may be recompiled to check the arena very stringently on every transaction; see the source code. BUGS
When realloc returns 0, the block pointed to by ptr may be destroyed. MALLOC(3)

Featured Tech Videos