C++ Fork child, ask child for process list, kill a process in Linux

Question

I'm trying to create a child process, send the child process a command "LISTALL". The child process should then issue the system the command ps and return that list to the parent process. The parent process should then choose a process and kill it. This is what I have so far but I'm having trouble just getting it to run.

#include <stdio.h>
#include <unistd.h>
#include <cstring>
#include <stdlib.h>
#include <iostream>
#include <sys/wait.h>

char* getlistOfProcesses(const char* cmd) 
{
    FILE* pipe = popen(cmd, "r");
    if (!pipe) return (char*)"ERROR";
    char buffer[128];
    char *result = new char[1024];
    while(!feof(pipe)) {
        if(fgets(buffer, 128, pipe) != NULL)
            strcat(result, buffer);
    }
    pclose(pipe);
    return result;
}

int spawnGEdit()
{
    pid_t gPid = fork();
    if(gPid == 0)
    {
        execl("gedit", "gedit", NULL);
        exit(-1);
    }
    else
    {
    }
    return 0;
}

int main(int argc, char **argv)
{   
    int P2C[2];
    int C2P[2];
    pipe(P2C);
    pipe(C2P);
    pid_t cPid = fork();
    char cmd[50];
    char* listOfProcesses = new char[1024];
    spawnGEdit();

    if (cPid == 0)
    {
        close(P2C[1]); 
        close(C2P[0]); 
        read(P2C[0], cmd, 10);
        if(strcmp(cmd,"LISTALL") == 0)
        {
            write(C2P[1], getlistOfProcesses("ps"), 1024);
            close(P2C[0]);
            close(C2P[1]);
        }
    }
    else if (cPid > 0)
    {
        close(C2P[1]); 
        close(P2C[0]); 
        write(P2C[1], "LISTALL", 10);
        wait(NULL);
        read(C2P[0], listOfProcesses,1024);
        printf("%s",listOfProcesses); 
        //TODO
        //get user input of a PID
        //kill the PID
        close(C2P[0]);
        close(P2C[1]);
    }
    else
    {
        // fork failed
        printf("Forking failed!\n");
        exit(1);
    }
    return 0;
}

These are the errors I'm getting when I try to compile it:

/tmp/cciTPIOZ.o: In function `getlistOfProcesses(char const*)':
test.cpp:(.text+0x53): undefined reference to `operator new[](unsigned long)'
/tmp/cciTPIOZ.o: In function `main':
test.cpp:(.text+0x166): undefined reference to `operator new[](unsigned long)'
/tmp/cciTPIOZ.o: In function `__static_initialization_and_destruction_0(int, int)':
test.cpp:(.text+0x2c0): undefined reference to `std::ios_base::Init::Init()'
test.cpp:(.text+0x2cf): undefined reference to `std::ios_base::Init::~Init()'
collect2: error: ld returned 1 exit status

I'm compiling with with:

cc test.cpp -o test

Answer 1

Compilation Errors occurred on line number:9,53,64 can be solved by using these:

line 9: FILE* pipe = popen(cmd.data(), "r");

line 53: write(C2P[1], getlistOfProcesses("ps").data(), 1024);

line 64: printf("%s",listOfProcesses.data());

Reason: These popen,write,printf requires char* as their arguments but you are passing them std::string. You have to use std::string.data() function instead as it returns pointer to the character array represented by std::string object.

And for your error on line 63, refer this.

PS:- For your edits in question:

line 10: if (!pipe) return (char*)"ERROR";

line 12: char *result = new char[1024];

line 53: (change in line 7) char* getlistOfProcesses(const char* cmd)

A bit of advice: use wait(NULL); in parent process before reading listOfProcesses and exit(0); at the end of child process.

Working code:

#include <stdio.h>
#include <unistd.h>
#include <cstring>
#include <stdlib.h>
#include <iostream>
#include <sys/wait.h>

char* getlistOfProcesses(const char* cmd) 
{
    FILE* pipe = popen(cmd, "r");
    if (!pipe) return (char*)"ERROR";
    char buffer[128];
    char *result = new char[1024];
    while(!feof(pipe)) {
        if(fgets(buffer, 128, pipe) != NULL)
            strcat(result, buffer);
    }
    pclose(pipe);
    return result;
}

int spawnGEdit()
{
    pid_t gPid = fork();
    if(gPid == 0)
    {
        execl("gedit", "gedit", NULL);
        exit(-1);
    }
    else
    {
    }
    return 0;
}

int main(int argc, char **argv)
{   
    int P2C[2];
    int C2P[2];
    pipe(P2C);
    pipe(C2P);
    pid_t cPid = fork();
    char cmd[50];
    char* listOfProcesses = new char[1024];
    spawnGEdit();

    if (cPid == 0)
    {
        close(P2C[1]); 
        close(C2P[0]); 
        read(P2C[0], cmd, 10);
        if(strcmp(cmd,"LISTALL") == 0)
        {
            write(C2P[1], getlistOfProcesses("ps"), 1024);
            close(P2C[0]);
            close(C2P[1]);
        }
        exit(0);
    }
    else if (cPid > 0)
    {
        close(C2P[1]); 
        close(P2C[0]); 
        write(P2C[1], "LISTALL", 10);
        wait(NULL);
        read(C2P[0], listOfProcesses,1024);
        printf("%s",listOfProcesses); 
        //TODO
        //get user input of a PID
        //kill the PID
        close(C2P[0]);
        close(P2C[1]);
    }
    else
    {
        // fork failed
        printf("Forking failed!\n");
        exit(1);
    }
    return 0;
}