diff options
Diffstat (limited to 'unix/os/doc/zmaloc.hlp')
| -rw-r--r-- | unix/os/doc/zmaloc.hlp | 71 |
1 files changed, 71 insertions, 0 deletions
diff --git a/unix/os/doc/zmaloc.hlp b/unix/os/doc/zmaloc.hlp new file mode 100644 index 00000000..7863741b --- /dev/null +++ b/unix/os/doc/zmaloc.hlp @@ -0,0 +1,71 @@ +.help zmaloc May84 "System Interface" +.ih +NAME +zmaloc -- allocate memory +.ih +SYNOPSIS +.nf +zmaloc (buffer, nbytes, status) + +int buffer # address of buffer +int nbytes # size of buffer +int status +.fi +.ih +DESCRIPTION +An uninitialized region of memory at least \fInbytes\fR in size is dynamically +allocated. The address of the newly allocated buffer in units of SPP chars +is returned in \fIbuffer\fR. +.ih +RETURN VALUE +XERR is returned in \fIstatus\fR if the buffer cannot be allocated. +XOK is returned if the operation is successful. +.ih +NOTES +The integer \fIbuffer\fR is a memory address in SPP char units with an +arbitrary zero point, i.e., the type of address returned by \fBzlocva\fR. +The high level code converts the buffer address into an offset into \fBMem\fR, +i.e., into an SPP pointer. + +.nf + char_pointer_into_Mem = buffer - zlocva(Memc) + 1 + Memc[char_pointer] = first char of buffer +.fi + +Since the buffer address is returned in char units the buffer must be aligned +to at least the size of a char; no greater degree of alignment is guaranteed +nor required. See the specifications of \fBzlocva\fR for additional information +about addresses and address arithmetic. + +If the host system does not provide buffer management primitives (heap +management facilities), but can dynamically allocate memory to a process, +it will be necessary to build a memory allocator. This is normally done +by dynamically changing the top of the process address space. The region +between the highest address allocated at process creation time and the +current top of the process address space is the region used by the heap. +A simple and adequate heap management technique is to implement the heap +as a circular singly linked list of buffers. Each buffer is preceded by +a pointer to the next buffer and a flag telling whether or not the buffer +is currently allocated. Successive unused buffers are periodically collected +together into a single large buffer to minimize fragmentation. A buffer is +allocated by searching around the circular list for either the first fit +or the best fit. If an unused buffer of sufficient size is not found, +additional physical memory is allocated to the process and linked into the +list. + +On a system which cannot dynamically allocate memory to a process it will be +necessary to statically allocate a large \fBMem\fR common. The heap +management algorithm described above will work just as effectively for a +static array as for a dynamic region. If a heap manager has to be coded for +more than one machine we should add a semi-portable version to the system +(all current IRAF target machines provide heap management facilities at the +host level so we have not coded a portable memory allocator). + +Dynamic memory allocation may be used in the kernel implementation as well +as in the portable system and applications code. In general it is necessary +to use the same memory allocator in both the kernel and the high level +code to avoid trashing memory. +.ih +SEE ALSO +zmfree, zraloc, zlocva +.endhelp |