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authorMattias Andrée <maandree@operamail.com>2015-10-17 19:56:40 +0200
committerMattias Andrée <maandree@operamail.com>2015-10-17 19:56:40 +0200
commit9772e31215aaee31858af3e3862b4789e4c9d193 (patch)
tree3e363149c9fd0d14b802d0d16e62ca6bad34b5a8 /doc
parentinfo: sections for the memory allocation chapter (diff)
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Signed-off-by: Mattias Andrée <maandree@operamail.com>
Diffstat (limited to 'doc')
-rw-r--r--doc/info/chap/memory-allocation.texinfo75
1 files changed, 70 insertions, 5 deletions
diff --git a/doc/info/chap/memory-allocation.texinfo b/doc/info/chap/memory-allocation.texinfo
index b5b5094..3d59634 100644
--- a/doc/info/chap/memory-allocation.texinfo
+++ b/doc/info/chap/memory-allocation.texinfo
@@ -1,17 +1,31 @@
@node Memory allocation
@chapter Memory allocation
+@cpindex Memory allocation
+@cpindex Allocate memory
The ability to allocate memory of on at compile-time
unknown amount is an important feature for most
programs.
+@cpindex Virtual address space
+@cpindex Virtual memory-address
+@cpindex Memory, virtual address space
+@cpindex @sc{RAM}
+@cpindex Swap space
On modern operating systems, processes do not have
direct access to the memory. Only the operating system
kernel does. Instead, the process have virtual
-memory address, that the kernel maps to either,
-real @sc{RAM} memory, swap space (disc backed),
-file segments, or zeroes.
-
+memory-addresses, that the kernel maps to either,
+real @sc{RAM}, swap space (disc backed), file segments,
+or zeroes.
+
+@cpindex Forks
+@cpindex Exec:s
+@cpindex Process image
+@cpindex Memory sharing, private memory
+@cpindex Private memory sharing
+@cpindex Memory deduplication
+@cpindex Deduplication memory
Memory for a process is either allocated programmatically,
or when the process forks (is created) or exec:s
(changes process image.) The operating system kernel is
@@ -23,21 +37,47 @@ memory content diverges. It is also possible to allocate
memory in such a way that processes can share it. so that
updates from one process influences the other processes.
+@cpindex Virtual address space segments
+@cpindex Memory segments
+@cpindex Segments, virtual address space
A process' virtual address space is divided into segments.
There are three important segments.
@table @i
@item text segment
+@cpindex Segment, text
+@cpindex Text segment
+@cpindex @code{.text}
+@cpindex Instructions
+@cpindex Static constants
+@cpindex Constants, static
+@cpindex Literals
+@cpindex Strings, literals
When a process exec:s this segment is allocated.
It contains instructions, static constants, and
literals.
@item BSS segment
+@cpindex Segment, @sc{BSS}
+@cpindex @sc{BSS} segment
+@cpindex @code{.bss}
+@cpindex Block Started by Symbol
+@cpindex Uninitialised variables
+@cpindex Zero variables
+@cpindex Global variables
+@cpindex Static variables
When a process exec:s this segment is allocated.
It contains all global and static variables that are
initialised to zero or lacks explicit initialisation.
On some systems this segment is merged into the data
-segment.
+segment. @sc{BSS} is an acronym: `Block Started by Symbol'.
@item data segment
+@cpindex Segment, data
+@cpindex Data segment
+@cpindex @code{.data}
+@cpindex Heap
+@cpindex Memory, heap
+@cpindex Global variables
+@cpindex Static variables
When a process exec:s this segment is allocated.
It is filled all global and static variables that
are not covered by the @sc{BSS} segment.
@@ -46,6 +86,11 @@ is unfixed; it can be resized. Any part of the segment
that is a result of it being resized, is referred to
as the heap.
@item stack segment
+@cpindex Segment, stack
+@cpindex Stack segment
+@cpindex Memory, stack
+@cpindex Call-stack
+@cpindex Automatic variables
This segment contains a@footnote{Program stack's can
created programmtically, hence `a' rather than `the'.}
program stack. A program stack contains information
@@ -73,6 +118,8 @@ extent that it would overlap with the stack segment.
In C there are multiple ways to allocate memory.
@itemize @bullet{}
@item
+@cpindex Global variables
+@cpindex Static allocations
Variables that are declared outside functions are
called global variables@footnote{Even if that are
declared with @code{static}. In this context,
@@ -86,6 +133,9 @@ their content is. In the data segment (or in the
@sc{BSS} segment if the elements are zeroes.) These
allocations are known as @i{static allocations}.
@item
+@cpindex @code{static}
+@cpindex Static variables
+@cpindex Static allocations
Variables that are declared inside functions, with
@code{static}, are stored just like global variables.
These are called static variables, and remain unchanged
@@ -93,6 +143,11 @@ between function calls, and only change in statements
other than the declaration statement. These allocations
are known as @i{static allocations}.
@item
+@cpindex @code{auto}
+@cpindex Local variables
+@cpindex Automatic variables
+@cpindex Automatic allocations
+@cpindex Stack-allocations
Variables that are declared inside functions (these
are known as local variables), without @code{static},
but with @code{auto}, are called automatic variables.
@@ -105,10 +160,14 @@ add @code{register}. These allocations are known as
@i{automatic allocations}@footnote{Known in some other
programming languages as stack-allocations.}.
@item
+@cpindex @code{register}
+@cpindex Register variables
Variables that are declared with @code{register}
are not stored in any segment. They lack addresses
and stored as CPU registers.
@item
+@cpindex Dynamic allocation
+@cpindex Heap-allocation
Using system calls, a heap can be created, where
new allocations are stored. These are often allocated
with the function @code{malloc}, and are known as
@@ -116,10 +175,15 @@ with the function @code{malloc}, and are known as
programming languages as heap-allocations.}.
@end itemize
+@cpindex Dynamic allocation
+@cpindex Automatic allocations
+@cpindex Stack-allocations
Some compilers, including @sc{GCC}, provide two
additional ways to allocate memory.
@table @asis
@item @i{Variable-length arrays}
+@cpindex Variable-length arrays
+@cpindex Arrays, variable-length
A simple example of variable-length arrays, available
with some compilers, is
@example
@@ -146,6 +210,7 @@ void function(size_t n)
@end example
@item @code{alloca}
+@fnindex alloca
@code{alloca} is a special function that is implement
by the compiler. It increases the stack and returns
a pointer to the beginning of the new part of the stack.