Replacing getpid with my own implementation

Question

I have an application where I need to write a new getpid function to replace the original one of the OS. The implementation would be similar to:

pid_t getpid(void)
{
    if (gi_PID != -1)
    {
     return gi_PID;
    }
    else
    {
     // OS level getpid() function
    }
}

How can I call the original getpid() implementation of the OS through this function?

EDIT: I tried:

pid_t getpid(void)
{
    if (gi_PID != -1)
    {
     return gi_PID;
    }
    else
    {
     return _getpid();
    }
}

as Jonathan has suggested. This gave me the following errors when compiling with g++:

In function pid_t getpid()': SerendibPlugin.cpp:882: error: _getpid' undeclared (first use this function) SerendibPlugin.cpp:882: error: (Each undeclared identifier is reported only once for each function it appears in.)

EDIT 2: I've managed to get this to work by using a function pointer and setting it to the next second symbol with the id "getpid", using dlsym(RTLD_NEXT, "getpid").

Here's my sample code:

vi xx.c
"xx.c" 23 lines, 425 characters 
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <iostream>
#include <dlfcn.h>

using namespace std;
pid_t(*___getpid)();

pid_t getpid(void)
{
    cout << "My getpid" << endl;
    cout << "PID :" << (*___getpid)() << endl;
    return (*___getpid)();
}

int main(void)
{
    ___getpid = (pid_t(*)())dlsym(RTLD_NEXT, "getpid");
    pid_t p1 = getpid();
    printf("%d \n", (int)p1);
    return(0);
}

g++ xx.c -o xout

My getpid
PID :7802
7802

Answer 1

On many systems, you will find that getpid() is a 'weak symbol' for _getpid(), which can be called in lieu of getpid().

---

The first version of the answer mentioned __getpid(); the mention was removed swiftly since it was erroneous.

This code works for me on Solaris 10 (SPARC) - with a C++ compiler:

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

extern "C" pid_t _getpid();

pid_t getpid(void)
{
    return(-1);
}

int main(void)
{
    pid_t p1 = getpid();
    pid_t p2 = _getpid();
    printf("%d vs %d\n", (int)p1, (int)p2);
    return(0);
}

This code works for me on Solaris 10 (SPARC) - with a C compiler:

Black JL: cat xx.c
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

pid_t getpid(void)
{
    return(-1);
}

int main(void)
{
    pid_t p1 = getpid();
    pid_t p2 = _getpid();
    printf("%d vs %d\n", (int)p1, (int)p2);
    return(0);
}
Black JL: make xx && ./xx
cc     xx.c   -o xx
"xx.c", line 13: warning: implicit function declaration: _getpid
-1 vs 29808
Black JL:

Answer 2

You're using the terminology a bit incorrectly. It's not possible to override getpid() because it's not a virtual function. All you can do is attempt to replace getpid with a different function by various evil means.

But I must ask, why are you doing this? Replacing getpid means that any component which was depending on the return of getpid will now be receiving you're presumably modified result. This change has a very high risk of changing some other component.

What you're offering is a new functionality and hence should be a different function.

That being said if you truly want to take this approach the best way is to dynamic loading of the function. The original DLL will still contain the getpid function and you can access that via a combination of LoadLibrary/GetProcAddress on Windows or dlopen/dlsym on Linux. If you're using a different OS please specify.

EDIT Responding to comments that getpid needs to be testable

If testing is the concern then why not instead have a custom getpid method for you're application. For example, applicationGetPid(). For normal execution this could be forwarded off to the system getpid function. But during Unit Testing it could be used to produce more predictable values.

pid_t applicationGetPid() { 
#if UNIT_TEST
return SomeCodeForUnitTests;
#else
return getpid();
#endif
}

Answer 3

You can use a macro:

in a .h, included in every file where you want to replace the getpid function

#define getpid mygetpid

Then, put your own implementation in a .cpp Be aware to not include the previous .h!

pid_t mygetpid() {
    // do what you want
    return getpid();
}