Where in memory are my variables stored?


Variables can be stored in several places in memory, depending on their lifetime. Variables that are defined outside any function (whether of global or file static scope), and variables that are defined inside a function as static variables, exist for the lifetime of the program's execution. These variables are stored in the "data segment." The data segment is a fixed-size area in memory set aside for these variables. The data segment is subdivided into two parts, one for initialized variables and another for uninitialized variables.

Variables that are defined inside a function as auto variables (that are not defined with the keyword static) come into existence when the program begins executing the block of code (delimited by curly braces {}) containing them, and they cease to exist when the program leaves that block of code.

Variables that are the arguments to functions exist only during the call to that function. These variables are stored on the "stack". The stack is an area of memory that starts out small and grows automatically up to some predefined limit. In DOS and other systems without virtual memory, the limit is set either when the program is compiled or when it begins executing. In UNIX and other systems with virtual memory, the limit is set by the system, and it is usually so large that it can be ignored by the programmer.

The third and final area doesn't actually store variables but can be used to store data pointed to by variables. Pointer variables that are assigned to the result of a call to the malloc() function contain the address of a dynamically allocated area of memory. This memory is in an area called the "heap." The heap is another area that starts out small and grows, but it grows only when the programmer explicitly calls malloc() or other memory allocation functions, such as calloc(). The heap can share a memory segment with either the data segment or the stack, or it can have its own segment. It all depends on the compiler options and operating system. The heap, like the stack, has a limit on how much it can grow, and the same rules apply as to how that limit is determined.


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