05-28-2009
The problem with your latter version is that the variable in main() won't be modified at all. In the subroutine, only the ptr local to the subroutine will be modified. To allow the ptr in main() to be modified, you are required to take a pointer to that variable, and thus requiring '&' on the calling side and an additional '*' on the callee side. This is true regardless of the type of the variable. It just happens that the type this time is void*. If the variable is an "int", then the parameter in the subroutine signature will be "int*".
One thing you need to keep in mind, is that parameters are passed by value.
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LEARN ABOUT ULTRIX
alloca
malloc(3) Library Functions Manual malloc(3)
Name
malloc, free, realloc, calloc, alloca - memory allocator
Syntax
char *malloc(size)
unsigned size;
free(ptr)
void *ptr;
char *realloc(ptr, size)
void *ptr;
unsigned size;
char *calloc(nelem, elsize)
unsigned nelem, elsize;
char *alloca(size)
int size;
Description
The and subroutines provide a simple general-purpose memory allocation package. The subroutine returns a pointer to a block of at least
size bytes beginning on a word boundary.
The argument to is a pointer to a block previously allocated by 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 is overrun or if some random number is handed to
The subroutine maintains multiple lists of free blocks according to size, allocating space from the appropriate list. It calls to get more
memory from the system when there is no suitable space already free. For further information, see
The subroutine changes the size of the block pointed to by ptr to size bytes and returns a pointer to the (possibly moved) block. The con-
tents will be unchanged up to the lesser of the new and old sizes.
In order to be compatible with older versions, also works if ptr points to a block freed since the last call of or Sequences of and were
previously used to attempt storage compaction. This procedure is no longer recommended.
The subroutine allocates space for an array of nelem elements of size elsize. The space is initialized to zeros.
The subroutine allocates size bytes of space associated with the stack frame of the caller. This temporary space is available for reuse
when the caller returns. On MIPS machines, calling reclaims all available storage. On VAX machines, the space is automatically freed on
return.
Each of the allocation routines returns a pointer to space suitably aligned (after possible pointer coercion) for storage of any type of
object.
Restrictions
When returns 0, the block pointed to by ptr may be destroyed.
Currently, the allocator is unsuitable for direct use in a large virtual environment where many small blocks are kept, since it keeps all
allocated and freed blocks on a circular list. Just before more memory is allocated, all allocated and freed blocks are referenced.
Because the subroutine is machine dependent, its use should be avoided.
Diagnostics
The and subroutines return a null pointer (0) if there is no available memory or if the arena has been detectably corrupted by storing out-
side the bounds of a block.
RISC malloc(3)