Question

我最近决定了解有关系统编程的更多信息，并认为查看我的代码实际在做什么会有所帮助。

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

我的期望是任何扩展堆的指令（例如使用new关键字或实例化LinkedList对象）都会调用mmap或sbrk / break系统调用。事实并非如此！

事实上，使用-s开关运行dtruss时，我看不到从LinkedList::Add函数内部调用任何系统调用！测试，我正在添加某些元素。

任何人都可以解释为什么我的dtruss输出中没有看到mmap / sbrk的引用？

如果有人可以解释mprotect和madvise的目的，则可获得积分。

我在下面包含了我的LinkedList类，main.cpp和dtruss输出。

谢谢！

dtruss output

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;
    }    
}

的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 OS并没有像大多数UNIX / Linux那样使用sbrk/brk/break()进行内存管理。基本上，它使用Apple从NeXT继承的Mach内核，因此内存调用将是mpadvise(2)和mprotect(2)，它提供比sbrk()更精细的粒度控制。

来自＆＃34; Mac OS X和iOS Internals＆＃34;作者：Jonathan Levin：

因此，为了解释您的内存分配，您需要知道mprotect(2)标头中的sys/mman.h个参数。

#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 */
....

所以你的系统调用意味着：

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User Extensible Heap Manager for Heterogeneous Memory Platforms and Mixed Memory Policies
Mac OS X and iOS Internals
＆＃34; Mac OS X内部：系统方法＆＃34;作者：Amit Singh

Dtruss没有显示mmap / sbrk调用？

1 个答案: