In computer programming, the block starting symbol (abbreviated to .bss or bss) is the portion of an object file, executable, or assembly language code that contains statically allocated variables that are declared but have not been assigned a value yet. It is often referred to as the "bss section" or "bss segment".
Typically only the length of the bss section, but no data, is stored in the object file. The program loader allocates memory for the bss section when it loads the program. By placing variables with no value in the .bss section, instead of the .data or .rodata section which require initial value data, the size of the object file is reduced.
On some platforms, some or all of the bss section is initialized to zeroes. Unix-like systems and Windows initialize the bss section to zero, allowing C and C++ statically allocated variables initialized to values represented with all bits zero to be put in the bss segment. Operating systems may use a technique called zero-fill-on-demand to efficiently implement the bss segment. [1] In embedded software, the bss segment is mapped into memory that is initialized to zero by the C run-time system before main()
is entered. Some C run-time systems may allow part of the bss segment not to be initialized; C variables must explicitly be placed into that portion of the bss segment. [2]
On some computer architectures, the application binary interface also supports an sbss segment for "small data". Typically, these data items can be accessed using shorter instructions that may only be able to access a certain range of addresses. Architectures supporting thread-local storage might use a tbss section for uninitialized, static data marked as thread-local. [3]
Historically, BSS (from Block Started by Symbol) is 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. [4] [5] The BSS keyword was later incorporated into FORTRAN Assembly Program [6] (FAP) and Macro Assembly Program [7] (MAP), IBM's standard assemblers 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. [8] In this situation BSS served as a shorthand in place of individually reserving a number of separate smaller data locations. Some assemblers support a complementary or alternative directive BES, for Block Ended by Symbol, where the specified symbol corresponds to the end of the reserved block. [9]
In C, statically allocated objects 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 (despite filling bss with zero is not required by the C standard, all variables in .bss are required to be individually initialized to some sort of zeroes according to Section 6.7.8 of C ISO Standard 9899:1999 or section 6.7.9 for newer standards). Hence, the BSS segment typically includes all uninitialized objects (both variables and constants) declared at file scope (i.e., outside any function) as well as uninitialized static local variables (local variables declared with the static
keyword); static local constants must be initialized at declaration, however, as they do not have a separate declaration, and thus are typically not in the BSS section, though they may be implicitly or explicitly initialized to zero. An implementation may also assign statically-allocated variables and constants initialized with a value consisting solely of zero-valued bits to the BSS section.
Peter van der Linden, a 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." [10]
In Fortran, common block variables are allocated in this segment. [11] Some compilers may, for 64-bit instruction sets, limit offsets, in instructions that access this segment, to 32 bits, limiting its size to 2 GB or 4 GB. [12] [13] [14] Also, note that Fortran does not require static data to be initialized to zero. On those systems where the bss segment is initialized to zero, putting common block variables and other static data into that segment guarantees that it will be zero, but for portability, programmers should not depend on that.
C is a general-purpose computer programming language. It was created in the 1970s by Dennis Ritchie, and remains very widely used and influential. By design, C's features cleanly reflect the capabilities of the targeted CPUs. It has found lasting use in operating systems, device drivers, and protocol stacks, but its use in application software has been decreasing. C is commonly used on computer architectures that range from the largest supercomputers to the smallest microcontrollers and embedded systems.
Fortran is a general-purpose, compiled imperative programming language that is especially suited to numeric computation and scientific computing.
In computing, a linker or link editor is a computer system program that takes one or more object files and combines them into a single executable file, library file, or another "object" file.
The Netwide Assembler (NASM) is an assembler and disassembler for the Intel x86 architecture. It can be used to write 16-bit, 32-bit (IA-32) and 64-bit (x86-64) programs. It is considered one of the most popular assemblers for Linux and x86 chips.
In computer science, imperative programming is a programming paradigm of software that uses statements that change a program's state. In much the same way that the imperative mood in natural languages expresses commands, an imperative program consists of commands for the computer to perform. Imperative programming focuses on describing how a program operates step by step, rather than on high-level descriptions of its expected results.
An object file is a computer file containing object code, that is, machine code output of an assembler or compiler. The object code is usually relocatable, and not usually directly executable. There are various formats for object files, and the same machine code can be packaged in different object file formats. An object file may also work like a shared library.
OpenMP is an application programming interface (API) that supports multi-platform shared-memory multiprocessing programming in C, C++, and Fortran, on many platforms, instruction-set architectures and operating systems, including Solaris, AIX, FreeBSD, HP-UX, Linux, macOS, and Windows. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.
C dynamic memory allocation refers to performing manual memory management for dynamic memory allocation in the C programming language via a group of functions in the C standard library, namely malloc, realloc, calloc, aligned_alloc and free.
In computer science, a pointer is an object in many programming languages that stores a memory address. This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware. A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on that page. The actual format and content of a pointer variable is dependent on the underlying computer architecture.
In compiler construction, name mangling is a technique used to solve various problems caused by the need to resolve unique names for programming entities in many modern programming languages.
In computer programming, a static variable is a variable that has been allocated "statically", meaning that its lifetime is the entire run of the program. This is in contrast to shorter-lived automatic variables, whose storage is stack allocated and deallocated on the call stack; and in contrast to objects, whose storage is dynamically allocated and deallocated in heap memory.
In computing, a data segment is a portion of an object file or the corresponding address space of a program that contains initialized static variables, that is, global variables and static local variables. The size of this segment is determined by the size of the values in the program's source code, and does not change at run time.
In computer programming, thread-local storage (TLS) is a memory management method that uses static or global memory local to a thread.
In computer programming, an automatic variable is a local variable which is allocated and deallocated automatically when program flow enters and leaves the variable's scope. The scope is the lexical context, particularly the function or block in which a variable is defined. Local data is typically invisible outside the function or lexical context where it is defined. Local data is also invisible and inaccessible to a called function, but is not deallocated, coming back in scope as the execution thread returns to the caller.
In Linux, the System.map file is a symbol table used by the kernel.
Interprocedural optimization (IPO) is a collection of compiler techniques used in computer programming to improve performance in programs containing many frequently used functions of small or medium length. IPO differs from other compiler optimizations by analyzing the entire program as opposed to a single function or block of code.
In the C programming language, an external variable is a variable defined outside any function block. On the other hand, a local (automatic) variable is a variable defined inside a function block.
As an alternative to automatic variables, it is possible to define variables that are external to all functions, that is, variables that can be accessed by name by any function. Because external variables are globally accessible, they can be used instead of argument lists to communicate data between functions. Furthermore, because external variables remain in existence permanently, rather than appearing and disappearing as functions are called and exited, they retain their values even after the functions that set them have returned.
In computer programming, a variable-length array (VLA), also called variable-sized or runtime-sized, is an array data structure whose length is determined at run time . In C, the VLA is said to have a variably modified type that depends on a value.
Absoft Fortran Compilers are set of Fortran compilers for Microsoft Windows, Apple Macintosh, and Linux produced by Absoft Corporation. The compilers are source code compatible across platforms.
The GOFF specification was developed for IBM's MVS operating system to supersede the IBM OS/360 Object File Format to compensate for weaknesses in the older format.