What does C + + heap and stack mean

I. Preliminary knowledge-memory allocation of a program a program compiled by C + + + + uses memory that is divided into the following sections

1, stack area (stack)-by the compiler automatically assigned to release, store the function of the parameter values, local variables and other values. The operation is similar to the stack in the data structure.

2, heap area (heap)-Generally by the programmer assigned to release, if the programmer does not release, the program at the end may be reclaimed by the OS. Note that it and the data structure of the heap is two different things, the distribution is similar to the list, hehe.

A variable and an uninitialized static variable are in another contiguous area. -System release after the program is finished

4, literal constant area-the constant string is here. Released by system after program is finished

5, program code area-the binary code that holds the function body.

Second, example program this is written by a predecessor, very detailed//main.cpp int a = 0; Global initialization region char *P1; Global uninitialized Zone main () {int b; stack char s[] = "abc"; stack char *p2; stack char *p3 = "123456"; 123456 is in the constant area, p3 on the stack. static int c = 0; Global (static) initialization zone P1 = (char *) malloc (10); P2 = (char *) malloc (20); Areas that are allocated 10 and 20 bytes are in the heap area. strcpy (P1, "123456"); 123456 is placed in the constant area, the compiler may optimize it to a place with the "123456" that P3 points to. }

Theory knowledge of heap and stack

2.1 Application Method Stack: Automatically assigned by the system. For example, declare a local variable int b in a function; The system automatically opens up a space for B in the stack heap: requires the programmer to apply and indicate the size, in C malloc functions such as P1 = (char *) malloc (10); In C + +, use the new operator such as P2 = (char *) malloc (10); But note that P1, p2 itself is in the stack.

2.2 Application after the system response stack: As long as the remaining space of the stack is larger than the application space, the system will provide memory for the program, otherwise it will be reported abnormal tip stack overflow. Heap: You should first know that the operating system has a linked list that records free memory addresses, when the system receives the application, it traverses the list, looks for the first heap node that is larger than the requested space, deletes the node from the Free node list, and assigns the node space to the program, and for most systems, The size of this assignment is recorded at the first address in this memory space, so that the DELETE statement in the code can properly release the memory space. In addition, because the size of the found heap node does not necessarily equal the size of the application, the system automatically puts the extra part back into the free list.

2.3 Application size limit stack: In Windows, the stack is a data structure that extends to a low address, and is a contiguous area of memory. This sentence means that the top of the stack address and the maximum capacity of the stack is the system in advance, in Windows, the size of the stack is 2M (also some said to be 1M, in short is a compile-time constant), if the application of space over the stack of the remaining space, will prompt overflow. Thus, the space available from the stack is small. Heap: The heap is a data structure that is extended to a high address and is a contiguous area of memory. This is because the system is used to store the free memory address of the list, nature is discontinuous, and the link list of the traversal direction is from the low address to the high address. The size of the heap is limited by the virtual memory available in the computer system. This shows that the heap to obtain a more flexible space, but also relatively large.

2.4 Application Efficiency comparison: The stack is automatically allocated by the system, faster. But programmers are out of control. A heap is a memory that is allocated by new, typically slower, and prone to memory fragmentation, but is most convenient to use.

In addition, in Windows, the best way is to allocate memory with VirtualAlloc, he is not in the heap, nor in the stack is directly in the process of the address space to keep a fast memory, although the most inconvenient to use. But fast, and most flexible.

2.5 stacks and stacks of storage content stack: When a function is called, the first stack is the address of the next instruction in the main function (the next executable statement of the function call statement). Then there are the parameters of the function, in most C compilers, the parameters are pushed from right to left, and then the local variables in the function. Note that static variables are not in the stack. When the function call is finished, the local variable first goes out of the stack, then the argument, and the last stack pointer points to the address that was first saved, the next instruction in the main function, where the program continues to run. Heap: The size of the heap is usually stored in a byte at the head of the heap. The specifics of the heap are arranged by the programmer.

2.6 Comparison of the access efficiency char s1[] = "AAAAAAAAAAAAAAA"; Char *s2 = "BBBBBBBBBBBBBBBBB"; The AAAAAAAAAAA is assigned at run time; And the BBBBBBBBBBB is determined at compile time; However, in future accesses, the array on the stack is faster than the string that the pointer points to (for example, a heap). For example: #include void Main () {char a = 1; char c[] = "1234567890"; char *p = "1234567890"; a = c[1]; a = p[1];

Corresponding assembly code 10:A = c[1]; 00401067 8A 4D F1 mov cl,byte ptr [ebp-0fh] 0040106A 4D FC mov byte ptr [ebp-4],cl 11:a = p[1]; 0040106D 8B EC mov edx,dword ptr [ebp-14h] 00401070 8A al,byte ptr [edx+1] 00401073 FC mov byte ptr [eb P-4],al

The first reads the elements in the string directly into the register CL, while the second reads the pointer value into the edx and reads the characters according to EdX, which is obviously slow.

2.7 Summary: Heap and stack differences can be seen in the following analogy: using stacks like we eat in restaurants, just order (apply), pay, and eat (use), go after full, do not have to pay attention to cutting vegetables, vegetables, such as preparation and washing dishes, such as cleaning the pot, his advantage is fast, but the degree of freedom is small. The use of the heap is like a do-it-yourself like to eat dishes, more trouble, but more in line with their own taste, and greater freedom. The main difference between heap and stack: the stack and stack of the operating system, as mentioned above, not much. There is the data structure of the heap and stack, these are different concepts. The heap here actually refers to a data structure of the precedence queue (which satisfies the heap's nature) and the 1th element has the highest precedence; the stack is actually a mathematical or data structure that satisfies the advanced nature of the latter. Although the stack, the stack of the argument is called, but they still have a big difference, connected to the call is only due to historical reasons.