Linux – Process started from system command in C inherits parent fd’s

clinuxnetstatpppdsystem

I have a sample application of a SIP server listening on both tcp and udp ports 5060.
At some point in the code, I do a system("pppd file /etc/ppp/myoptions &");

After this if I do a netstat -apn, It shows me that ports 5060 are also opened for pppd!
Is there any method to avoid this? Is this standard behaviour of the system function in Linux?

Thanks,
Elison

Best Answer

Yes, by default whenever you fork a process (which system does), the child inherits all the parent's file descriptors. If the child doesn't need those descriptors, it SHOULD close them. The way to do this with system (or any other method that does a fork+exec) is to set the FD_CLOEXEC flag on all file descriptors that shouldn't be used by the children of you process. This will cause them to be closed automatically whenever any child execs some other program.

In general, ANY TIME your program opens ANY KIND of file descriptor that will live for an extended period of time (such as a listen socket in your example), and which should not be shared with children, you should do

fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) | FD_CLOEXEC);

on the file descriptor.

As of the 2016? revision of POSIX.1, you can use the SOCK_CLOEXEC flag or'd into the type of the socket to get this behavior automatically when you create the socket:

listenfd = socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC, 0);
bind(listenfd, ...
listen(listemfd, ...

which guarentees it will be closed properly even if some other simultaneously running thread does a system or fork+exec call. Fortunately, this flag has been supported for awhile on Linux and BSD unixes (but not OSX, unfortunately).

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?

Linux – How to obtain the number of CPUs/cores in Linux from the command line

grep -c ^processor /proc/cpuinfo

will count the number of lines starting with "processor" in /proc/cpuinfo

For systems with hyper-threading, you can use

grep ^cpu\\scores /proc/cpuinfo | uniq |  awk '{print $4}'

which should return (for example) 8 (whereas the command above would return 16)

Best Answer

Related Solutions

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

Windows

Linux

TODO: Other Platforms

Linux – How to obtain the number of CPUs/cores in Linux from the command line

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