Stack vs heap vs data segment1/21/2024 The stack and heap are traditionally located at opposite ends of the process's virtual address space. When some more memory need to be allocated using malloc and calloc function, heap grows upward as shown in above diagram. In C language dynamic memory allocation is done by using malloc and calloc functions. Heap segment is also part of RAM where dynamically allocated variables are stored. Memory layout of a C program Heap Segment When a new stack frame needs to be added (as a result of a newly called function), the stack grows downward (See the figure 1).įig 1. Data is added or removed in a last-in-first-out manner to stack. Also all recursive function calls are added to stack. Local variables do not appear in data or bss segment. Local variables have a scope to the block which they are defined in they are created when control enters into the block. Stack segment is used to store all local variables and is used for passing arguments to the functions along with the return address of the instruction which is to be executed after the function call is over. bss sections is to think of "bss" as an abbreviation for "Better Save Space!" A simple way to remember the difference between the. It was originally an acronym for the "Block Storage Start" instruction from the IBM 704 assembly language (circa 1957) and the acronym has stuck. bss to denote uninitialized data is universal. Bryant explains in his famous book on Computer Systems: A Programmer's Perspective, Why is uninitialized data called. Object file formats distinguish between initialized and uninitialized variables for space efficiency uninitialized variables do not have to occupy any actual disk space in the object file. This section occupies no actual space in the object file it is merely a place holder. Global, external, and static variable are by default initialized to zero. bss segment stores all uninitialized global, static, and external variables (declared with extern keyword). Initialized data or simply data segment stores all global, static, constant, and external variables (declared with extern keyword) that are initialized beforehand.Ĭontrary to initialized data segment, uninitialized data or. bss data segment, stack, and heap) to store variables depending upon if they are local or global, and initialized or uninitialized. Data segment is further divided into four sub-data segments (initialized data segment, uninitialized or. This data could be in form of initialized or uninitialized variables, and it could be local or global. The text segment of an executable object file is often read-only segment that prevents a program from being accidentally modified.ĭata segment stores program data. These segments can be described as follows:Ĭode segment, also known as text segment contains machine code of the compiled program. Modern systems use a single text segment to store program instructions, but more than one segment for data, depending upon the storage class of the data being stored there. The memory is logically divided into text and data segments. For a running program both the machine instructions (program code) and data are stored in the same memory space. This article focus on the concepts of code and data segments of a program and does not take any specific platform into account. However, the actual layout of a program's in-memory image is left entirely up to the operating system, and often the program itself as well. pe (Portable Executable), and elf (Executable and Linkable Format). Object files have a specific format, however this format may vary from system to system. Shared object file: These special type of relocatable object files are loaded into memory and linked dynamically, at either load time or run time.Executable object file: These are executable files contain binary code and data in a form that can be copied directly into memory and executed.Relocatable object files contain binary code and data in a form that can be combined with other relocatable object files at compile time to create an executable object file. Static linkers such as the Unix ld program take collection of relocatable object files and command line arguments as input and generate a fully linked executable object file as output that can be loaded into memory and run. Relocatable object file: These are static library files.Technically an object file is a sequence of bytes stored on disk in a file. BSS Segmentsīefore talking of memory layout of a C program and its various segments to store data and code instructions we should first understand that a compiler driver (that invokes the language preprocessor, compiler, assembler, and linker, as needed on behalf of the user) can generate three types of object files depending upon the options supplied to the compiler driver.
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