Difference between static and dynamic memory allocation in operating system
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There are three types of allocation — static, automatic, and dynamic.
Static Allocation means, that the memory for your variables is allocated when the program starts. The size is fixed when the program is created. It applies to global variables, file scope variables, and variables qualified with static defined inside functions.
Automatic memory allocation occurs for (non-static) variables defined inside functions, and is usually stored on the stack (though the C standard doesn't mandate that a stack is used). You do not have to reserve extra memory using them, but on the other hand, have also limited control over the lifetime of this memory. E.g: automatic variables in a function are only there until the function finishes.
void func() {
int i; /* `i` only exists during `func` */
}
Dynamic memory allocation is a bit different. You now control the exact size and the lifetime of these memory locations. If you don't free it, you'll run into memory leaks, which may cause your application to crash, since at some point of time, system cannot allocate more memory.
int* func() {
int* mem = malloc(1024);
return mem;
}
int* mem = func(); /* still accessible */
In the upper example, the allocated memory is still valid and accessible, even though the function terminated. When you are done with the memory, you have to free it:
Dynamic memory allocation
Is memory allocated at runtime using calloc(), malloc() and friends. It is sometimes also referred to as 'heap' memory, although it has nothing to do with the heap data-structure ref.
int * a = malloc(sizeof(int));
Heap memory is persistent until free() is called. In other words, you control the lifetime of the variable.
Automatic memory allocation
This is what is commonly known as 'stack' memory, and is allocated when you enter a new scope (usually when a new function is pushed on the call stack). Once you move out of the scope, the values of automatic memory addresses are undefined, and it is an error to access them.
int a = 43;
Note that scope does not necessarily mean function. Scopes can nest within a function, and the variable will be in-scope only within the block in which it was declared. Note also that where this memory is allocated is not specified. (On a sane system it will be on the stack, or registers for optimisation)
Static memory allocation
Is allocated at compile time, and the lifetime of a variable in static memory is the lifetime of the program.
In C, static memory can be allocated using the static keyword. The scope is the compilation unit only.
Things get more interesting when the extern keyword is considered. When an extern variable is defined the compiler allocates memory for it. When an extern variable is declared, the compiler requires that the variable be defined elsewhere. Failure to declare/define extern variables will cause linking problems, while failure to declare/define static variables will cause compilation problems.
Static Allocation means, that the memory for your variables is allocated when the program starts. The size is fixed when the program is created. It applies to global variables, file scope variables, and variables qualified with static defined inside functions.
Automatic memory allocation occurs for (non-static) variables defined inside functions, and is usually stored on the stack (though the C standard doesn't mandate that a stack is used). You do not have to reserve extra memory using them, but on the other hand, have also limited control over the lifetime of this memory. E.g: automatic variables in a function are only there until the function finishes.
void func() {
int i; /* `i` only exists during `func` */
}
Dynamic memory allocation is a bit different. You now control the exact size and the lifetime of these memory locations. If you don't free it, you'll run into memory leaks, which may cause your application to crash, since at some point of time, system cannot allocate more memory.
int* func() {
int* mem = malloc(1024);
return mem;
}
int* mem = func(); /* still accessible */
In the upper example, the allocated memory is still valid and accessible, even though the function terminated. When you are done with the memory, you have to free it:
Dynamic memory allocation
Is memory allocated at runtime using calloc(), malloc() and friends. It is sometimes also referred to as 'heap' memory, although it has nothing to do with the heap data-structure ref.
int * a = malloc(sizeof(int));
Heap memory is persistent until free() is called. In other words, you control the lifetime of the variable.
Automatic memory allocation
This is what is commonly known as 'stack' memory, and is allocated when you enter a new scope (usually when a new function is pushed on the call stack). Once you move out of the scope, the values of automatic memory addresses are undefined, and it is an error to access them.
int a = 43;
Note that scope does not necessarily mean function. Scopes can nest within a function, and the variable will be in-scope only within the block in which it was declared. Note also that where this memory is allocated is not specified. (On a sane system it will be on the stack, or registers for optimisation)
Static memory allocation
Is allocated at compile time, and the lifetime of a variable in static memory is the lifetime of the program.
In C, static memory can be allocated using the static keyword. The scope is the compilation unit only.
Things get more interesting when the extern keyword is considered. When an extern variable is defined the compiler allocates memory for it. When an extern variable is declared, the compiler requires that the variable be defined elsewhere. Failure to declare/define extern variables will cause linking problems, while failure to declare/define static variables will cause compilation problems.
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