Smart pointers/safe memory management for C

cc99memorypointerssmart-pointers

I, and I think many others, have had great success using smart pointers to wrap up unsafe memory operations in C++, using things like RAII, et cetera. However, wrapping memory management is easier to implement when you have destructors, classes, operator overloading, et cetera.

For someone writing in raw C99, where could you point (no pun intended) to help with safe memory management?

Thanks.

Best Answer

The question is a bit old, but I figured I would take the time to link to my smart pointer library for GNU compilers (GCC, Clang, ICC, MinGW, ...).

This implementation relies on the cleanup variable attribute, a GNU extension, to automatically free the memory when going out of scope, and as such, is not ISO C99, but C99 with GNU extensions.

Example:

simple1.c:

#include <stdio.h>
#include <csptr/smart_ptr.h>

int main(void) {
    smart int *some_int = unique_ptr(int, 1);

    printf("%p = %d\n", some_int, *some_int);

    // some_int is destroyed here
    return 0;
}

Compilation & Valgrind session:

$ gcc -std=gnu99 -o simple1 simple1.c -lcsptr
$ valgrind ./simple1
==3407== Memcheck, a memory error detector
==3407== Copyright (C) 2002-2013, and GNU GPL\'d, by Julian Seward et al.
==3407== Using Valgrind-3.10.0 and LibVEX; rerun with -h for copyright info
==3407== Command: ./simple1 
==3407==
0x53db068 = 1
==3407==
==3407== HEAP SUMMARY:
==3407==     in use at exit: 0 bytes in 0 blocks
==3407==   total heap usage: 1 allocs, 1 frees, 48 bytes allocated
==3407==
==3407== All heap blocks were freed -- no leaks are possible
==3407==
==3407== For counts of detected and suppressed errors, rerun with: -v
==3407== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Windows

Some of the above values are easily available from the appropriate Win32 API, I just list them here for completeness. Others, however, need to be obtained from the Performance Data Helper library (PDH), which is a bit "unintuitive" and takes a lot of painful trial and error to get to work. (At least it took me quite a while, perhaps I've been only a bit stupid...)

Note: for clarity all error checking has been omitted from the following code. Do check the return codes...!

Total Virtual Memory:
```
#include "windows.h"

MEMORYSTATUSEX memInfo;
memInfo.dwLength = sizeof(MEMORYSTATUSEX);
GlobalMemoryStatusEx(&memInfo);
DWORDLONG totalVirtualMem = memInfo.ullTotalPageFile;
```
Note: The name "TotalPageFile" is a bit misleading here. In reality this parameter gives the "Virtual Memory Size", which is size of swap file plus installed RAM.

Virtual Memory currently used:

Same code as in "Total Virtual Memory" and then

 DWORDLONG virtualMemUsed = memInfo.ullTotalPageFile - memInfo.ullAvailPageFile;

Virtual Memory currently used by current process:

#include "windows.h"
#include "psapi.h"

PROCESS_MEMORY_COUNTERS_EX pmc;
GetProcessMemoryInfo(GetCurrentProcess(), (PROCESS_MEMORY_COUNTERS*)&pmc, sizeof(pmc));
SIZE_T virtualMemUsedByMe = pmc.PrivateUsage;

Total Physical Memory (RAM):

Same code as in "Total Virtual Memory" and then
```
DWORDLONG totalPhysMem = memInfo.ullTotalPhys;
```
Physical Memory currently used:

Same code as in "Total Virtual Memory" and then
```
DWORDLONG physMemUsed = memInfo.ullTotalPhys - memInfo.ullAvailPhys;
```
Physical Memory currently used by current process:

Same code as in "Virtual Memory currently used by current process" and then
```
SIZE_T physMemUsedByMe = pmc.WorkingSetSize;
```

CPU currently used:

#include "TCHAR.h"
#include "pdh.h"

static PDH_HQUERY cpuQuery;
static PDH_HCOUNTER cpuTotal;

void init(){
    PdhOpenQuery(NULL, NULL, &cpuQuery);
    // You can also use L"\\Processor(*)\\% Processor Time" and get individual CPU values with PdhGetFormattedCounterArray()
    PdhAddEnglishCounter(cpuQuery, L"\\Processor(_Total)\\% Processor Time", NULL, &cpuTotal);
    PdhCollectQueryData(cpuQuery);
}

double getCurrentValue(){
    PDH_FMT_COUNTERVALUE counterVal;

    PdhCollectQueryData(cpuQuery);
    PdhGetFormattedCounterValue(cpuTotal, PDH_FMT_DOUBLE, NULL, &counterVal);
    return counterVal.doubleValue;
}

CPU currently used by current process:

#include "windows.h"

static ULARGE_INTEGER lastCPU, lastSysCPU, lastUserCPU;
static int numProcessors;
static HANDLE self;

void init(){
    SYSTEM_INFO sysInfo;
    FILETIME ftime, fsys, fuser;

    GetSystemInfo(&sysInfo);
    numProcessors = sysInfo.dwNumberOfProcessors;

    GetSystemTimeAsFileTime(&ftime);
    memcpy(&lastCPU, &ftime, sizeof(FILETIME));

    self = GetCurrentProcess();
    GetProcessTimes(self, &ftime, &ftime, &fsys, &fuser);
    memcpy(&lastSysCPU, &fsys, sizeof(FILETIME));
    memcpy(&lastUserCPU, &fuser, sizeof(FILETIME));
}

double getCurrentValue(){
    FILETIME ftime, fsys, fuser;
    ULARGE_INTEGER now, sys, user;
    double percent;

    GetSystemTimeAsFileTime(&ftime);
    memcpy(&now, &ftime, sizeof(FILETIME));

    GetProcessTimes(self, &ftime, &ftime, &fsys, &fuser);
    memcpy(&sys, &fsys, sizeof(FILETIME));
    memcpy(&user, &fuser, sizeof(FILETIME));
    percent = (sys.QuadPart - lastSysCPU.QuadPart) +
        (user.QuadPart - lastUserCPU.QuadPart);
    percent /= (now.QuadPart - lastCPU.QuadPart);
    percent /= numProcessors;
    lastCPU = now;
    lastUserCPU = user;
    lastSysCPU = sys;

    return percent * 100;
}

Linux

On Linux the choice that seemed obvious at first was to use the POSIX APIs like getrusage() etc. I spent some time trying to get this to work, but never got meaningful values. When I finally checked the kernel sources themselves, I found out that apparently these APIs are not yet completely implemented as of Linux kernel 2.6!?

In the end I got all values via a combination of reading the pseudo-filesystem /proc and kernel calls.

Total Virtual Memory:

#include "sys/types.h"
#include "sys/sysinfo.h"

struct sysinfo memInfo;

sysinfo (&memInfo);
long long totalVirtualMem = memInfo.totalram;
//Add other values in next statement to avoid int overflow on right hand side...
totalVirtualMem += memInfo.totalswap;
totalVirtualMem *= memInfo.mem_unit;

Virtual Memory currently used:

Same code as in "Total Virtual Memory" and then

long long virtualMemUsed = memInfo.totalram - memInfo.freeram;
//Add other values in next statement to avoid int overflow on right hand side...
virtualMemUsed += memInfo.totalswap - memInfo.freeswap;
virtualMemUsed *= memInfo.mem_unit;

Virtual Memory currently used by current process:

#include "stdlib.h"
#include "stdio.h"
#include "string.h"

int parseLine(char* line){
    // This assumes that a digit will be found and the line ends in " Kb".
    int i = strlen(line);
    const char* p = line;
    while (*p <'0' || *p > '9') p++;
    line[i-3] = '\0';
    i = atoi(p);
    return i;
}

int getValue(){ //Note: this value is in KB!
    FILE* file = fopen("/proc/self/status", "r");
    int result = -1;
    char line[128];

    while (fgets(line, 128, file) != NULL){
        if (strncmp(line, "VmSize:", 7) == 0){
            result = parseLine(line);
            break;
        }
    }
    fclose(file);
    return result;
}

Total Physical Memory (RAM):

Same code as in "Total Virtual Memory" and then

long long totalPhysMem = memInfo.totalram;
//Multiply in next statement to avoid int overflow on right hand side...
totalPhysMem *= memInfo.mem_unit;

Physical Memory currently used:

Same code as in "Total Virtual Memory" and then

long long physMemUsed = memInfo.totalram - memInfo.freeram;
//Multiply in next statement to avoid int overflow on right hand side...
physMemUsed *= memInfo.mem_unit;

Physical Memory currently used by current process:

Change getValue() in "Virtual Memory currently used by current process" as follows:

int getValue(){ //Note: this value is in KB!
    FILE* file = fopen("/proc/self/status", "r");
    int result = -1;
    char line[128];

    while (fgets(line, 128, file) != NULL){
        if (strncmp(line, "VmRSS:", 6) == 0){
            result = parseLine(line);
            break;
        }
    }
    fclose(file);
    return result;
}

CPU currently used:

#include "stdlib.h"
#include "stdio.h"
#include "string.h"

static unsigned long long lastTotalUser, lastTotalUserLow, lastTotalSys, lastTotalIdle;

void init(){
    FILE* file = fopen("/proc/stat", "r");
    fscanf(file, "cpu %llu %llu %llu %llu", &lastTotalUser, &lastTotalUserLow,
        &lastTotalSys, &lastTotalIdle);
    fclose(file);
}

double getCurrentValue(){
    double percent;
    FILE* file;
    unsigned long long totalUser, totalUserLow, totalSys, totalIdle, total;

    file = fopen("/proc/stat", "r");
    fscanf(file, "cpu %llu %llu %llu %llu", &totalUser, &totalUserLow,
        &totalSys, &totalIdle);
    fclose(file);

    if (totalUser < lastTotalUser || totalUserLow < lastTotalUserLow ||
        totalSys < lastTotalSys || totalIdle < lastTotalIdle){
        //Overflow detection. Just skip this value.
        percent = -1.0;
    }
    else{
        total = (totalUser - lastTotalUser) + (totalUserLow - lastTotalUserLow) +
            (totalSys - lastTotalSys);
        percent = total;
        total += (totalIdle - lastTotalIdle);
        percent /= total;
        percent *= 100;
    }

    lastTotalUser = totalUser;
    lastTotalUserLow = totalUserLow;
    lastTotalSys = totalSys;
    lastTotalIdle = totalIdle;

    return percent;
}

CPU currently used by current process:

#include "stdlib.h"
#include "stdio.h"
#include "string.h"
#include "sys/times.h"
#include "sys/vtimes.h"

static clock_t lastCPU, lastSysCPU, lastUserCPU;
static int numProcessors;

void init(){
    FILE* file;
    struct tms timeSample;
    char line[128];

    lastCPU = times(&timeSample);
    lastSysCPU = timeSample.tms_stime;
    lastUserCPU = timeSample.tms_utime;

    file = fopen("/proc/cpuinfo", "r");
    numProcessors = 0;
    while(fgets(line, 128, file) != NULL){
        if (strncmp(line, "processor", 9) == 0) numProcessors++;
    }
    fclose(file);
}

double getCurrentValue(){
    struct tms timeSample;
    clock_t now;
    double percent;

    now = times(&timeSample);
    if (now <= lastCPU || timeSample.tms_stime < lastSysCPU ||
        timeSample.tms_utime < lastUserCPU){
        //Overflow detection. Just skip this value.
        percent = -1.0;
    }
    else{
        percent = (timeSample.tms_stime - lastSysCPU) +
            (timeSample.tms_utime - lastUserCPU);
        percent /= (now - lastCPU);
        percent /= numProcessors;
        percent *= 100;
    }
    lastCPU = now;
    lastSysCPU = timeSample.tms_stime;
    lastUserCPU = timeSample.tms_utime;

    return percent;
}

TODO: Other Platforms

I would assume, that some of the Linux code also works for the Unixes, except for the parts that read the /proc pseudo-filesystem. Perhaps on Unix these parts can be replaced by getrusage() and similar functions?

C++ – a smart pointer and when should I use one

UPDATE

This answer is rather old, and so describes what was 'good' at the time, which was smart pointers provided by the Boost library. Since C++11, the standard library has provided sufficient smart pointers types, and so you should favour the use of std::unique_ptr, std::shared_ptr and std::weak_ptr.

There was also std::auto_ptr. It was very much like a scoped pointer, except that it also had the "special" dangerous ability to be copied — which also unexpectedly transfers ownership.
It was deprecated in C++11 and removed in C++17, so you shouldn't use it.

std::auto_ptr<MyObject> p1 (new MyObject());
std::auto_ptr<MyObject> p2 = p1; // Copy and transfer ownership. 
                                 // p1 gets set to empty!
p2->DoSomething(); // Works.
p1->DoSomething(); // Oh oh. Hopefully raises some NULL pointer exception.

OLD ANSWER

A smart pointer is a class that wraps a 'raw' (or 'bare') C++ pointer, to manage the lifetime of the object being pointed to. There is no single smart pointer type, but all of them try to abstract a raw pointer in a practical way.

Smart pointers should be preferred over raw pointers. If you feel you need to use pointers (first consider if you really do), you would normally want to use a smart pointer as this can alleviate many of the problems with raw pointers, mainly forgetting to delete the object and leaking memory.

With raw pointers, the programmer has to explicitly destroy the object when it is no longer useful.

// Need to create the object to achieve some goal
MyObject* ptr = new MyObject(); 
ptr->DoSomething(); // Use the object in some way
delete ptr; // Destroy the object. Done with it.
// Wait, what if DoSomething() raises an exception...?

A smart pointer by comparison defines a policy as to when the object is destroyed. You still have to create the object, but you no longer have to worry about destroying it.

SomeSmartPtr<MyObject> ptr(new MyObject());
ptr->DoSomething(); // Use the object in some way.

// Destruction of the object happens, depending 
// on the policy the smart pointer class uses.

// Destruction would happen even if DoSomething() 
// raises an exception

The simplest policy in use involves the scope of the smart pointer wrapper object, such as implemented by boost::scoped_ptr or std::unique_ptr.

void f()
{
    {
       std::unique_ptr<MyObject> ptr(new MyObject());
       ptr->DoSomethingUseful();
    } // ptr goes out of scope -- 
      // the MyObject is automatically destroyed.

    // ptr->Oops(); // Compile error: "ptr" not defined
                    // since it is no longer in scope.
}

Note that std::unique_ptr instances cannot be copied. This prevents the pointer from being deleted multiple times (incorrectly). You can, however, pass references to it around to other functions you call.

std::unique_ptrs are useful when you want to tie the lifetime of the object to a particular block of code, or if you embedded it as member data inside another object, the lifetime of that other object. The object exists until the containing block of code is exited, or until the containing object is itself destroyed.

A more complex smart pointer policy involves reference counting the pointer. This does allow the pointer to be copied. When the last "reference" to the object is destroyed, the object is deleted. This policy is implemented by boost::shared_ptr and std::shared_ptr.

void f()
{
    typedef std::shared_ptr<MyObject> MyObjectPtr; // nice short alias
    MyObjectPtr p1; // Empty

    {
        MyObjectPtr p2(new MyObject());
        // There is now one "reference" to the created object
        p1 = p2; // Copy the pointer.
        // There are now two references to the object.
    } // p2 is destroyed, leaving one reference to the object.
} // p1 is destroyed, leaving a reference count of zero. 
  // The object is deleted.

Reference counted pointers are very useful when the lifetime of your object is much more complicated, and is not tied directly to a particular section of code or to another object.

There is one drawback to reference counted pointers — the possibility of creating a dangling reference:

// Create the smart pointer on the heap
MyObjectPtr* pp = new MyObjectPtr(new MyObject())
// Hmm, we forgot to destroy the smart pointer,
// because of that, the object is never destroyed!

Another possibility is creating circular references:

struct Owner {
   std::shared_ptr<Owner> other;
};

std::shared_ptr<Owner> p1 (new Owner());
std::shared_ptr<Owner> p2 (new Owner());
p1->other = p2; // p1 references p2
p2->other = p1; // p2 references p1

// Oops, the reference count of of p1 and p2 never goes to zero!
// The objects are never destroyed!

To work around this problem, both Boost and C++11 have defined a weak_ptr to define a weak (uncounted) reference to a shared_ptr.

Best Answer

Related Solutions

C++ – How to determine CPU and memory consumption from inside a process

Windows

Linux

TODO: Other Platforms

C++ – a smart pointer and when should I use one

Related Topic