Knowledge about memory allocation: Global, local, and static variables

Prerequisites-program memory allocation The memory occupied by a C/C ++ compiled program is divided into the following parts: 1. STACK: the stack zone is automatically allocated and released by the compiler, and stores function parameter values and local variable values. The operation method is similar to the stack in the data structure. 2. Heap-generally assigned and released by the programmer. If the programmer does not release the heap, it may be recycled by the OS at the end of the program. Note that it is different from the heap in the data structure. The allocation method is similar to the linked list. 3. Global (static)-the storage of global variables and static variables is put together, and the initialized global variables and static variables are in one area, uninitialized global variables and uninitialized static variables are in another adjacent area. -The system is released after the program ends. 4. Text Constant Area-constant strings are placed here. The program is released by the System 5. program code area-stores the binary code of the function body. A normal program is usually divided into program segments, data terminals, and stacks in the memory. The program segment contains the program's machine code and read-only data. This segment is usually read-only and Its write operations are illegal. The data segment contains static data in the program. Dynamic data is stored through stacks. In memory, their locations are as follows: + ---------------- + Low-end memory | Program segment | | ---------------- | | Data Segment | | ---------------- | | Stack | + ---------------- + High-end memory A stack is a continuous block in the memory. A register called a stack pointer (SP) points to the top of the stack. The bottom of the stack is a fixed address. A feature of a stack is that the stack goes first, first, and foremost. That is to say, the first data to be put is retrieved. It supports two operations: Push and pop. Push puts data at the top of the stack, and pop pulls the data at the top of the stack. In advanced languages, temporary variables in program function calls and functions are used in stacks. Why? Because when calling a function, we need to protect the current operation, and after the function is executed, the program can find the correct place to continue the execution, therefore, the stack is also used for parameter passing and return values. Generally, local variables are referenced by giving their offset to the SP. There is also a base address pointer (FP, Which is BP in Intel chips), which many compilers actually use to reference local variables and parameters. Generally, the offset of the parameter relative FP is positive, and the local variable is negative. When a function call occurs in the program, the computer first presses the parameter into the stack, and then saves the content in the instruction register (IP) as the return address (RET ); the third place in the stack is the base address register (FP). Then, the current stack pointer (SP) is copied to the FP as the new base address. Finally, some space is reserved for the local variable, subtract the appropriate value from the SP. The variables defined in the function body are usually on the stack. The memory allocated by using malloc, calloc, realloc, and other functions is on the stack. All functions define a global volume in vitro. After the static modifier is added, all functions are stored in the global zone (static zone) no matter where they are located ), static variables defined by all functions in vitro are valid in this file and cannot be used in other files. Static variables defined in the function body are valid only in this function. In addition, strings such as "adgfdf" in the function are stored in the constant area. Comparison: 1. Performance STACK: Stack exists in Ram. Stack is dynamic, and its storage speed is second faster. Stack Heap: the heap is located in Ram and is a general memory pool. All objects are stored in the heap. Heap 2. Application Method STACK: automatically assigned by the system. For example, declare a local variable int B in the function; the system automatically opens up space for B in the stack. Heap: the programmer must apply for it and specify the size. In C, the malloc function is like p1 = (char *) malloc (10 ); In C ++, use the new operator such as P2 = (char *) malloc (10). Note that P1 and P2 are in the stack. 3. system response after application STACK: as long as the remaining space of the stack exceeds the applied space, the system will provide the program with memory. Otherwise, an exception will be reported, prompting stack overflow. Heap: First, you should know that the operating system has a linked list that records idle memory addresses. When the system receives a program application, The linked list is traversed to find the heap node with the first space greater than the requested space. Then, the node is deleted from the idle node linked list and allocated to the program, for most systems, the size of the allocation will be recorded at the first address in the memory space, so that the delete statement in the code can correctly release the memory space. In addition, because the size of the heap node is not necessarily equal to the applied size, the system automatically places the excess part in the idle linked list. 4. Application size limit STACK: in windows, a stack is a data structure extended to a low address and a continuous memory area. This statement indicates that the stack top address and the maximum stack capacity are pre-defined by the system. In Windows, the stack size is 2 MB (OR 1 MB, in short, it is a constant determined during compilation. If the requested space exceeds the remaining space of the stack, overflow will be prompted. Therefore, the space available from the stack is small. Heap: the heap is a data structure extended to the high address and a non-sequential memory area. This is because the system uses the linked list to store the idle memory address, which is naturally discontinuous, And the traversal direction of the linked list is from the low address to the high address. The heap size is limited by the valid virtual memory in the computer system. It can be seen that the space obtained by the heap is flexible and large. 5 Comparison of Application Efficiency The stack is automatically allocated by the system, which is faster. But programmers cannot control it. The heap is the memory allocated by new, which is generally slow and prone to memory fragments. However, it is most convenient to use. In addition, in windows, the best way is to use virtualalloc to allocate memory. Instead of heap or stack, it is to reserve a fast memory in the address space of the process, although it is the most inconvenient to use. However, it is fast and flexible. 6. Storage content in heap and stack STACK: when calling a function, the first entry to the stack is the address of the next instruction in the main function (the next executable statement in the function call statement), and then the parameters of the function, in most C compilers, parameters are written from right to left into the stack, followed by local variables in the function. Note that static variables are not included in the stack. When the function call ends, the local variable first goes out of the stack, then the parameter, and the top pointer of the stack points to the address of the initial storage, that is, the next instruction in the main function, where the program continues to run. Heap: Generally, the heap size is stored in one byte in the heap header. The specific content in the heap is arranged by the programmer. 7 Comparison of access efficiency Char S1 [] = "aaaaaaaaaaaaa "; Char * S2 = "bbbbbbbbbbbbbbbbb "; Aaaaaaaaaaa is assigned a value at the runtime; Bbbbbbbbbbbbb is determined during compilation; However, in future access, the array on the stack is faster than the string pointed to by the pointer (such as the heap. For example: # Include Void main () { Char A = 1; Char C [] = "1234567890 "; Char * P = "1234567890 "; A = C [1]; A = P [1]; Return; } Corresponding assembly code 10: A = C [1]; 00401067 8A 4D F1 mov Cl, byte PTR [ebp-0Fh] 0040106a 88 4D FC mov byte PTR [ebp-4], Cl 11: A = P [1]; 0040106d 8B 55 EC mov edX, dword ptr [ebp-14h] 00401070 8A 42 01 mov Al, byte PTR [edX + 1] 00401073 88 45 FC mov byte PTR [ebp-4], Al The first type reads the elements in the string directly into the CL register, while the second type reads the pointer value into EDX. Reading the characters based on edX is obviously slow. Summary: The difference between stack and stack can be seen in the following metaphor: Using Stacks is like eating at a restaurant, just ordering food (sending an application), paying for it, and eating (using it). If you are full, you can leave, without having to worry about the preparation work, such as cutting and washing dishes, as well as the cleaning and tail work such as washing dishes and flushing pots, his advantage is fast, but his degree of freedom is small. Using heap is like making your favorite dishes. It is troublesome, but it suits your taste and has a high degree of freedom. Global variables, static data, and constants are stored in the global data zone, and the code of all functions is stored in the Code zone, local variables, function parameters, returned data, and return addresses allocated to run functions are stored in the stack. Therefore, in the same process, the global variables and static variables of multiple tasks (threads) should share the same memory (Global Data zone) In different processes, the code is re-loaded. The global variables and static variables between processes certainly do not have the same memory. In PSOs, each task is a different thread, so the global and static variables of each task are in the same memory. In my other program (in sco unix), every running is a new process, so the global and static variables of each process have different memory.