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| -rw-r--r-- | doc/info/chap/memory-allocation.texinfo | 75 |
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. |