没有显示mmap/sbrk调用？

Question

最近，我决定学习更多关于系统编程的知识，并且觉得看看我的代码在幕后到底在做什么是有帮助的。

为此，我用C++编写了一个简短的C++类，并决定使用dtruss (read: dtrace)跟踪它。

我的期望是，任何扩展堆的指令(例如使用new关键字，或者实例化LinkedList对象)都会调用mmap或sbrk/break系统调用。不是这样的！

实际上，在使用dtruss开关运行-s时，我没有看到从-s函数中调用任何系统调用！测试，我确定元素被添加了。

有谁能解释一下为什么我在mmap**/**sbrk输出中没有看到对的引用？

如果有人能解释mprotect和madvise的目的，就会得到额外的分数。

我在下面包含了我的LinkedList类、main.cpp和d桁架输出。

谢谢!

d构架输出

SYSCALL(args)        = return
Created new LinkedList
Created new LinkedList
Destroyed a LinkedList
open("/dev/dtracehelper\0", 0x2, 0xFFFFFFFFE3236D70)         = 3 0
ioctl(0x3, 0x80086804, 0x7FFEE3236CD0)       = 0 0
close(0x3)       = 0 0
access("/AppleInternal/XBS/.isChrooted\0", 0x0, 0x0)         = -1 Err#2
thread_selfid(0x0, 0x0, 0x0)         = 198178 0
bsdthread_register(0x7FFF5BAB5C50, 0x7FFF5BAB5C40, 0x2000)       = 1073742047 0
issetugid(0x0, 0x0, 0x0)         = 0 0
mprotect(0x10C9D0000, 0x1000, 0x0)       = 0 0
mprotect(0x10C9D5000, 0x1000, 0x0)       = 0 0
mprotect(0x10C9D6000, 0x1000, 0x0)       = 0 0
mprotect(0x10C9DB000, 0x1000, 0x0)       = 0 0
mprotect(0x10C9CE000, 0x88, 0x1)         = 0 0
mprotect(0x10C9DC000, 0x1000, 0x1)       = 0 0
mprotect(0x10C9CE000, 0x88, 0x3)         = 0 0
mprotect(0x10C9CE000, 0x88, 0x1)         = 0 0
getpid(0x0, 0x0, 0x0)        = 1698 0
stat64("/AppleInternal/XBS/.isChrooted\0", 0x7FFEE32362E8, 0x0)      = -1 Err#2
stat64("/AppleInternal\0", 0x7FFEE3236380, 0x0)      = -1 Err#2
csops(0x6A2, 0x7, 0x7FFEE3235E20)        = -1 Err#22
sysctl([CTL_KERN, 14, 1, 1698, 0, 0] (4), 0x7FFEE3235F68, 0x7FFEE3235F60, 0x0, 0x0)      = 0 0
csops(0x6A2, 0x7, 0x7FFEE3235710)        = -1 Err#22
getrlimit(0x1008, 0x7FFEE32374F0, 0x0)       = 0 0
fstat64(0x1, 0x7FFEE3237508, 0x0)        = 0 0
ioctl(0x1, 0x4004667A, 0x7FFEE3237554)       = 0 0
write_nocancel(0x1, "Created new LinkedList\n\0", 0x17)      = 23 0
write_nocancel(0x1, "Created new LinkedList\n\0", 0x17)      = 23 0
write_nocancel(0x1, "Destroyed a LinkedList\n\0", 0x17)      = 23 0

LinkedList.cpp

#include <iostream>
#include "LinkedList.h"

using namespace std;

LinkedList::LinkedList() {
    this->length = 0;
    this->head = NULL;
    this->tail = NULL;
    cout << "Created new LinkedList" << endl;
}

LinkedList::~LinkedList() {
    Node* curr;
    Node* temp;
    curr = this->head;

    while ( curr ) {
        temp = curr;
        curr = curr->next;
        delete temp;
    }

    cout << "Destroyed a LinkedList" << endl;
}

void LinkedList::Add(int v) {

    Node* n = new Node();
    n->val = v;
    n->next = NULL;

    if (!this->head) {
        this->head = n;
        this->tail = n;
    } else {
        this->tail->next = n;
        this->tail = n;
    }    
}

main.cpp

#include <iostream>
#include "LinkedList.h"

using namespace std;

int main() {

    LinkedList l; // You should require a heap increase, right?

    LinkedList* ll = new LinkedList(); // Surely you require more heap!

    for (int i=0; i<1000; i++) 
        l.Add(i);

    return 0;

}

Answer 1

我发现Mac不像大多数UNIX/Linuxes那样使用sbrk/brk/break()进行内存管理。基本上，它使用的是苹果从NeXT继承的Mach内核，因此内存调用将是mpadvise(2)和mprotect(2)，它们提供了比sbrk()更好的粒度控制。

来自Jonathan的"Mac和iOS内部“：

​

​

因此，要解释内存分配，需要了解来自mprotect(2)头的sys/mman.h参数。

#define PROT_NONE       0x00    /* [MC2] no permissions */
#define PROT_READ       0x01    /* [MC2] pages can be read */
#define PROT_WRITE      0x02    /* [MC2] pages can be written */
#define PROT_EXEC       0x04    /* [MC2] pages can be executed */
....

所以你的系统呼叫意味着：

mprotect(0x10C9D0000, 0x1000, 0x0)       = 0 0
mprotect(0x10C9D5000, 0x1000, 0x0)       = 0 0
mprotect(0x10C9D6000, 0x1000, 0x0)       = 0 0
mprotect(0x10C9DB000, 0x1000, 0x0)       = 0 0
mprotect(0x10C9CE000, 0x88, 0x1)         = 0 0  <-- Allow reads starting at 0x10C9CE000 for 136 bytes
mprotect(0x10C9DC000, 0x1000, 0x1)       = 0 0
mprotect(0x10C9CE000, 0x88, 0x3)         = 0 0  <-- Allow reads and writes starting at 0x10C9CE000 for 136 bytes
mprotect(0x10C9CE000, 0x88, 0x1)         = 0 0

关于mmap(2)，在Linux系统上，它用于映射共享库的目标代码，但不用于malloc/free或new/delete。

参考文献：

• 白皮书用于异构内存平台和混合内存策略的用户可扩展堆管理器
• Mac和iOS内部设备
• Amit Singh的"Mac内部:一种系统方法“