C + + memory allocation

A program is generally divided into 3 segments: Text segment, data segment, BSS segmenttext paragraph: is the program code, compile-time determination, read-only,data segment: stored in the compile phase (not the runtime) can be determined, readable writableis usually referred to as static storage, the global variables assigned the initial value and static variables are stored in this area, and constants are stored in this areaBSS segment: A global variable and a static variable that are defined without an initial value assigned to it, placed in this areaThe memory used by a program compiled by C + + is divided into the following sections 1, stack (stack)-Automatically allocated by the compiler to release, store the function's parameter value, local variable value, etc. It operates in a manner similar to a stack in a data structure. 2, heap area (heap)-generally by the programmer to allocate the release, if the programmer does not release, the end of the program may be recycled by the OS. Note that it is not the same as the heap in the data structure, the distribution is similar to the list, hehe. 3, Global (static)-the storage of global variables and static variables is placed in a block, initialized global variables and static variables in an area, uninitialized global variables and uninitialized static variables in another adjacent area. -System release after the end of the program 4, literal constant area-the constant string is placed here. After the end of the program, the system releases 5, program code area-the binary code that holds the function body. Second, the example program int a = 0; Global initialization Area (code area) char *P1; The global (static) uninitialized area main () {int b; stack char s[] = "abc"; stack char *p2; stack char *p3 = "123456"; 123456\0 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\0 is placed in a constant area, the compiler may optimize it to a place with the "123456" that P3 points to. } Ii. theoretical knowledge of heaps and Stacks 2.1 application method stack: Automatically assigned by the system. For example, declare a local variable int b in the function; The system automatically opens a space heap for B in the stack: requires the programmer to apply himself, and indicates the size, in C, the malloc function such as P1 = (char *) malloc (10); In C + + with the new operator such as P2 = (char *) malloc (10); But note that P1, p2 itself is in the stack. 2.2 Post-Application system response stack: As long as the remaining space on the stack is larger than the requested space, the system will provide memory for the program, or will report the exception prompt stack overflow. Heap: First of all should know that the operating system has a record of the free memory address of the list, when the system receives the application of the program, it will traverse the list, look for the first space is larger than the requested space of the heap node, and then delete the node from the list of idle nodes, and the node's space allocated to the program, in addition,The size of this allocation is recorded at the first address in the block memory space, so that the DELETE statement in the code correctly frees the memory space. Also, because the size of the found heap node does not necessarily equal the size of the request, the system automatically re-places the extra portion into the idle list. 2.3 Application Size limit stack: Under Windows, the stack is the data structure to the low address extension, which is a contiguous area of memory. This sentence means that the top of the stack of the address and the maximum capacity of the stack is the system pre-defined, in Windows, the size of the stack is 2M (also said 1M, in short, is a compile-time determination of the constant), if the request for more space than the stack's remaining space, will prompt overflow. Therefore, the space available from the stack is small. Heap: A heap is a data structure that extends to a high address, and is a discontinuous area of memory. This is because the system is stored with a linked list of free memory address, is naturally discontinuous, and the chain of the list of traversal direction is from the low address to high address. The size of the heap is limited by the valid virtual memory in the computer system. Thus, the space of the heap is more flexible and relatively large. 2.4 Application Efficiency comparison: Stacks are automatically assigned by the system, faster. But programmers can't control it. Heap is the memory allocated by new, the general speed is relatively slow, and prone to memory fragmentation, but the most convenient to use. In addition, under Windows, the best way is to use VirtualAlloc to allocate memory, he is not in the heap, nor in the stack is directly in the process's address space to keep a fast memory, although the most inconvenient to use. But the speed is fast, also the most flexible. 2.5 stacks and stacks of storage content stack: When the 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), and then the parameters of the function, in most C compilers, the arguments are left-to-right 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 is first out of the stack, then the parameter, and the last stack pointer points to the first saved address, which is the next instruction in the main function, and the program continues to run from that point. Heap: The size of a heap is typically stored in a heap at the head of a pile. The concrete contents of the heap are arranged by programmers. 2.6 Comparison of the access efficiency char s1[] = "AA"; Char *s2 = "BBB"; AA is assigned at run time, while BB is determined at compile time, but in subsequent accesses, the array on the stack is faster than the string that the pointer points to (for example, the heap). For example: #include void Main () {char a = 1; char c[] = "1234567890"; char *p = "1234567890"; a = c[1]; a = p[1]; return;} correspondingassembly 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 mov al,byte ptr [edx+1] 00401073 FC mov byte ptr [eb  P-4],al first reads the elements in the string directly into the register CL, while the second one reads the pointer values into EDX, which is obviously slow to read the characters according to EdX.

C + + memory allocation