.bss
Encyclopedia
In computer programming
, the name .bss or bss is used by many compiler
s and linkers for a part of the data segment
containing statically-allocated variables
represented solely by zero-valued bit
s initially (i.e., when execution begins). It is often referred to as the "bss section" or "bss segment".
In C
, statically-allocated variables without an explicit initializer are initialized to zero (for arithmetic types) or a null pointer (for pointer types). Implementations of C typically represent zero values and null pointer values using a bit pattern consisting solely of zero-valued bits (though this is not required by the C standard). Hence, the bss section typically includes all uninitialized variables declared at the file level (i.e., outside of any function) as well as uninitialized local variable
s declared with the
Typically, the program loader initializes the memory allocated for the bss section when it loads the program. Operating systems may use a technique called zero-fill-on-demand to efficiently implement the bss segment . In embedded software, the bss segment is mapped into memory that is initialized to zero by the C run-time system
before
Some application binary interface
s also support an sbss segment for "small data". Typically, these data items can be accessed by leaner code using instructions that can only access a certain range of addresses.
Historically, BSS (from Block Started by Symbol) was a pseudo-operation in UA-SAP (United Aircraft Symbolic Assembly Program), the assembler developed in the mid-1950s for the IBM 704
by Roy Nutt, Walter Ramshaw, and others at United Aircraft Corporation
. The BSS keyword was later incorporated into FAP (FORTRAN Assembly Program), IBM
's standard assembler for its 709 and 7090/94 computers. It defined a label (i.e. symbol) and reserved a block of uninitialized space for a given number of words .
A noted C programmer and author says "Some people like to remember it as "Better Save Space." Since the BSS segment only holds variables that don't have any value yet, it doesn't actually need to store the image of these variables. The size that BSS will require at runtime is recorded in the object file, but BSS (unlike the data segment) doesn't take up any actual space in the object file."
Computer programming
Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, the name .bss or bss is used by many compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
s and linkers for a part of the data segment
Data segment
A data segment is a portion of virtual address space of a program, which contains the global variables and static variables that are initialized by the programmer...
containing statically-allocated variables
Static variable
In computer programming, a static variable is a variable that has been allocated statically — whose lifetime extends across the entire run of the program...
represented solely by zero-valued bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s initially (i.e., when execution begins). It is often referred to as the "bss section" or "bss segment".
In C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
, statically-allocated variables without an explicit initializer are initialized to zero (for arithmetic types) or a null pointer (for pointer types). Implementations of C typically represent zero values and null pointer values using a bit pattern consisting solely of zero-valued bits (though this is not required by the C standard). Hence, the bss section typically includes all uninitialized variables declared at the file level (i.e., outside of any function) as well as uninitialized local variable
Local variable
In computer science, a local variable is a variable that is given local scope. Such a variable is accessible only from the function or block in which it is declared. In programming languages with only two levels of visibility, local variables are contrasted with global variables...
s declared with the
static keyword. An implementation may also assign statically-allocated variables initialized with a value consisting solely of zero-valued bits to the bss section.Typically, the program loader initializes the memory allocated for the bss section when it loads the program. Operating systems may use a technique called zero-fill-on-demand to efficiently implement the bss segment . In embedded software, the bss segment is mapped into memory that is initialized to zero by the C run-time system
Run-time system
A run-time system is a software component designed to support the execution of computer programs written in some computer language...
before
main is entered.Some application binary interface
Application binary interface
In computer software, an application binary interface describes the low-level interface between an application program and the operating system or another application.- Description :...
s also support an sbss segment for "small data". Typically, these data items can be accessed by leaner code using instructions that can only access a certain range of addresses.
Historically, BSS (from Block Started by Symbol) was a pseudo-operation in UA-SAP (United Aircraft Symbolic Assembly Program), the assembler developed in the mid-1950s for the IBM 704
IBM 704
The IBM 704, the first mass-produced computer with floating point arithmetic hardware, was introduced by IBM in 1954. The 704 was significantly improved over the IBM 701 in terms of architecture as well as implementations which were not compatible with its predecessor.Changes from the 701 included...
by Roy Nutt, Walter Ramshaw, and others at United Aircraft Corporation
United Aircraft and Transport Corporation
The United Aircraft and Transport Corporation was formed in 1929, when William Boeing of the Boeing firms teamed up with Frederick Rentschler of Pratt & Whitney to form a large, amalgamated firm, uniting business interests in all aspects of aviation—a combination of aircraft engine and airframe...
. The BSS keyword was later incorporated into FAP (FORTRAN Assembly Program), IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
's standard assembler for its 709 and 7090/94 computers. It defined a label (i.e. symbol) and reserved a block of uninitialized space for a given number of words .
A noted C programmer and author says "Some people like to remember it as "Better Save Space." Since the BSS segment only holds variables that don't have any value yet, it doesn't actually need to store the image of these variables. The size that BSS will require at runtime is recorded in the object file, but BSS (unlike the data segment) doesn't take up any actual space in the object file."
See also
- Linker (computing)
- Data segmentData segmentA data segment is a portion of virtual address space of a program, which contains the global variables and static variables that are initialized by the programmer...
- Code segmentCode segmentIn computing, a code segment, also known as a text segment or simply as text, is one of the sections of a program in an object file or in memory, which contains executable instructions....
- Uninitialized variableUninitialized variableIn computing, an uninitialized variable is a variable that is declared but is not set to a definite known value before it is used. It will have some value, but not a predictable one. As such it is a programming error and a common source of bugs in software....