I am trying to link the output of a child process into the input of a parent process; the parent process is to execute a system call or command using the child's output.
I have looked to the following threads for answer; however, I didn't quite get the answer I am looking for.
Pipe - C++ piping issue
Linux Pipes as Input and Output
The problem I am having is that the command in the parent process is not being printed to the terminal.
Why isn't the output being printed to the terminal? I have closed the ends of my pipe in both the parent and child processes. Furthermore, the parent's std_out isn't being modified.
Here is my code.
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <iostream>
using namespace std;
int main(int argc, const char * argv[])
{
enum {RD, WR};
int fd[2];
pid_t pid;
if (pipe(fd) < 0)
perror("pipe error");
else if ((pid = fork()) < 0)
perror("fork error");
else if (pid == 0) { //In child process
close(fd[RD]);
dup2(fd[WR], STDOUT_FILENO);
close(fd[WR]);
execlp("/bin/ps", "ps", "-A", NULL);
}
else { //In parent process
close(fd[WR]);
dup2(fd[RD], STDIN_FILENO);
close(fd[RD]);
wait(NULL);
execlp("/bin/wc", "wc", "-l", NULL);
}
return 0;
}
You don't check for the failure of execlp.
Are you sure all of your programs are where you think they are? Your program works for me if I just change "/bin/wc" to "/usr/bin/wc".
Related
I know wait(NULL) waits for all child process to terminate. But while working with fork(),execv()/execvp() and wait(), it seems wait(NULL) is not waiting for all the child processed to terminate.
I was trying to understand the unexpected behaviour of the wait(NULL) system call in my code.
I tried to recreate the same situation with a different but smaller code, it is shown below.
#include <unistd.h>
#include <filesystem>
#include <sys/wait.h>
#include <bits/stdc++.h>
#include <fcntl.h>
using namespace std;
int main ()
{
int pid;
int fd[2];
char command[] = "square";
//constructing a pipe
if(pipe(fd)==-1)
cout<<"Pipe1 Failed";
//first fork
pid = fork();
if(pid<0)
{
cout<<"fork failed";
}
else if(pid == 0)
{
cout<<"inside child 1\n";
close(0);
//opening input file at fd = 0
if(open("input",O_RDONLY) != 0)
cerr<<"Failed input open\n";
//connecting write end of pipe to first child process
dup2(fd[1],1);
close(fd[1]);
close(fd[0]);
char *args[] = {command,NULL};
//calling exec system call
execv(command,args);
cerr<<"execv1 failed\n";
exit(0);
}
//second fork
pid = fork();
if(pid<0)
{
cout<<"fork failed";
}
else if(pid == 0)
{
cout<<"inside child 2\n";
//connecting read end of the pipe to second child process
dup2(fd[0],0);
close(fd[0]);
close(fd[1]);
char *args[] = {command,NULL};
//calling exec system call
execv(command,args);
cerr<<"execv2 failed\n";
exit(0);
}
wait(NULL);
cout<<"After wait\n";
return 0;
}
What this code effectively tries to do is something like ./square < input | ./square in bash. input is a file that contains a number (it was 5 when I ran the code), square is a function that takes an integer input and prints its square (n*n).
The code of square is
#include<stdio.h>
int main()
{
int n;
scanf("%d",&n);
printf("%d",n*n);
return 0;
}
The final output I was expecting is (with input file containing number 5)
inside child 1
inside child 2
625
After wait
But the final output I am getting is
inside child 1
inside child 2
After wait
625
Output image
Can somebody help me figure out what is happening here or where my understanding is wrong.
NOTE: I used g++ compiler to compile this C++ codes.
Thanks in advance.
wait(NULL) doesn't wait for all children to exit. wait(&wstatus) waits for one child, and puts its exit status into the int wstatus. wait(NULL) waits for one child, and ignores its exit status.
Citation: man 2 wait
Does using the C function daemon() have any security or stability disadvantages for a linux daemon compared to using explicit functions like fork(), setsid(), umask(), etc. (beside being unable to set all daemon parameters)?
I was wondering why I should write
#include <sys/types.h>
#include <sys/stat.h>
#include <cstdio>
#include <cstdlib>
#include <fcntl.h>
#include <cerrno>
#include <unistd.h>
#include <syslog.h>
#include <string>
int main()
{
//Set our Logging Mask and open the Log
setlogmask(LOG_UPTO(LOG_NOTICE));
openlog(DAEMON_NAME, LOG_CONS | LOG_NDELAY | LOG_PERROR | LOG_PID, LOG_USER);
syslog(LOG_INFO, "Entering Daemon");
pid_t pid, sid;
//Fork the Parent Process
pid = fork();
if (pid < 0)
exit(EXIT_FAILURE);
//We got a good pid, Close the Parent Process
if (pid > 0)
exit(EXIT_SUCCESS);
//Change File Mask
umask(0);
//Create a new Signature Id for our child
sid = setsid();
if (sid < 0)
exit(EXIT_FAILURE);
//Change Directory
//If we cant find the directory we exit with failure.
if ((chdir("/")) < 0)
exit(EXIT_FAILURE);
//Close Standard File Descriptors
close(STDIN_FILENO);
close(STDOUT_FILENO);
close(STDERR_FILENO);
while (true)
{
sleep(5);
//Do something
}
closelog ();
}
instead of
#include <unistd.h>
int main()
{
daemon(0, 0);
while (true)
{
//Do something
sleep(5);
}
}
Per the manpage, it's not in POSIX so you're always taking a risk regarding its existence.
Otherwise, no.
I'm writing a fuzzer in C++ on linux. It spawns multiple threads and has a timeout function if the thread hangs for whatever reason. I cannot figure out the correct way to kill the thread off after it's timer runs out. What I am doing now is something along the lines of:
`
#include <signal.h>
#include <string.h>
#include <sys/wait.h>
#include <errno.h>
#include <iostream>
#include <fstream>
#include <cstdlib>
#include <chrono>
#include <thread>
#include <stdlib.h>
#include <sys/wait.h>
#include <stdio.h>
#include <vector>
#include <iostream>
#include <sys/time.h>
#include <random>
#include <cstdlib>
#include <climits>
#include <sstream>
#include <iomanip>
#include <unistd.h>
#include <cstring>
#define READ 0
#define WRITE 1
void reaper (int c_pid, int t_timeout) {
std::this_thread::sleep_for(std::chrono::milliseconds(t_timeout));
kill (c_pid, 9);
}
FILE * popen2 (std::string command, std::string type, int & pid, std::string low_lvl_user) {
pid_t child_pid;
int fd[2];
pipe(fd);
if((child_pid = fork()) == -1) {
perror("fork");
exit(1);
}
if (child_pid == 0) { // child begins
if (type == "r") {
close(fd[READ]); //Close the READ
dup2(fd[WRITE], 1); //Redirect stdout to pipe
}
else {
close(fd[WRITE]); //Close the WRITE
dup2(fd[READ], 0); //Redirect stdin to pipe
}
if (getuid() == 0) {
execl("/bin/su", "su", "-c", "/bin/sh", "-c", command.c_str(), low_lvl_user.c_str(), NULL); // fixes not being able to reap suid 0 processes
}
else {
execl("/bin/sh", "/bin/sh", "-c", command.c_str(), NULL); // runs it all
}
exit(0);
}
else {
if (type == "r") {
close(fd[WRITE]); //Close the WRITE
}
else {
close(fd[READ]); //Close the READ
}
}
pid = child_pid;
if (type == "r") {
return fdopen(fd[READ], "r");
}
return fdopen(fd[WRITE], "w");
}
int pclose2(FILE * fp, pid_t pid) // close it so we don't fuck outselves
{
int stat;
fclose(fp);
while (waitpid(pid, &stat, 0) == -1) {
if (errno != EINTR) {
stat = -1;
break;
}
}
return stat;
}
int spawn_ch (std::string out_str) {
std::string low_lvl_user = "nobody";
int t_timeout = 500;
int pid; // initializes child
FILE * fp = popen2(out_str, "r", pid, low_lvl_user); // opens child process fork
char command_out[4096] = {0};
std::stringstream output;
std::thread reaper_thread(reaper, pid, t_timeout); // takes care of killing it off if it takes too long
reaper_thread.join();
while (read(fileno(fp), command_out, sizeof(command_out)-1) != 0) {
output << std::string(command_out);
memset(&command_out, 0, sizeof(command_out));
}
pclose2(fp, pid);
std::string token;
}
int main () {
std::string command = "HOME=AAAAAAAAA MAIL=AA /usr/sbin/exim4 -Ac AAAAAA -G -MCP,9,-Mar -Mf -Mset b -S 999999 -X,,-bF 999 -bdf -bpc -bpr -bpru,,-bt -exim4,AAA, -f,AAAAAAAAA,-oA -oMa,5Mu^i, -oMaa,, -oMas,,-oMs -oX isotanr -odb -oee -oem,999, -oo,99999999 -r 999999999 -t -ti 999999";
std::vector<std::thread> threads;
int num_threads = 2;
for (int cur_thread=1; cur_thread <= num_threads; ++cur_thread) threads.push_back(std::thread(spawn_ch, command)); // Thrift Shop
for (auto& all_thread : threads) all_thread.join(); // is that your grandma's coat?
exit(0);
}
But as the processes are spawned as suid 101 in this example (or 0, or whatever else), the kill function can be run as root to reap the processes it spawned... which would work, except exim4 apparently tries to spawn multiple processes, and when one dies the others don't. Is there a way to let the program know what processes were spawned to kill them, or preferably, a way to just terminate the entire thread that spawned them (which I think should work, as if you ctrl+c my program it will kill off what it spawned)?
The whole codebase is on github.
Thanks in advance.
The std::thread class provides no means to arbitrary terminate an active execution thread. This functionality is not implemented in the current C++ standard.
The sample code you posted is pretty much the only thing that can be done using purely the functionality in the C++ and C libraries.
The POSIX thread API is an alternative option. It does provide the means to terminate an active thread; however that comes with many important caveats, and it's very difficult to avoid undefined behavior, when terminating an execution thread using pthread_cancel(), since this will not properly unwind the terminated thread's stack, and invoke all the needed destructors; furthermore the executing thread must reach a cancellation point, in order for pthread_cancel() to take effect.
Additionally, if the executing thread execs another process, the new process is going to replace the thread's entire process, not just the executing thread's context. If this was your intent all along, trying to cancel the thread won't do much good anyway, and you pretty much have to do what you are already doing.
I'm trying to check that the cmd variable is set to "LISTALL" but it isn't when I try printing it out.
#include <stdio.h>
#include <unistd.h>
#include <cstring>
#include <stdlib.h>
#include <iostream>
#include <sys/wait.h>
int main(int argc, char **argv)
{
pid_t cPid = fork();
int P2C[2];
int C2P[2];
pipe(P2C);
pipe(C2P);
char cmd[50];
char* listOfProcesses = new char[1024];
if (cPid == 0)
{
...
read(P2C[0], cmd, 50);
printf("%s\n", cmd);
if(strcmp(cmd,"LISTALL") == 0)
{
//printf("Executing the command: %s", cmd);
write(C2P[1], getlistOfProcesses("ps -ax -o pid,cmd"), 1024);
...
}
}
else if (cPid > 0)
{
...
write(P2C[1], "LISTALL", 50);
wait(NULL);
read(C2P[0], listOfProcesses,1024);
...
}
else
{
// fork failed
printf("Forking failed!\n");
exit(1);
}
return 0;
}
What I get from that is a mini box symbol with 00 at the top and 01 or 02 at the bottom. I tried pasting the symbol here but it doesn't show.
You create 4 pipes: two in the parent process and two in the child process.
Create the pipes before forking! Then fork, then check whether you are in the parent process or in the child process.
That way you have only two pipes, both processes know about these pipes and can communicate by reading or writing to the appropriate file descriptors of the pipes.
I am working on a project and I got it mostly figured out except for one minor(big) problem. I can't seem to figure out how to create pipes between any number of children.
for example I am taking in command line arguments to determine how many children will be produced. The first child doesn't have input but has output and the last child outputs to STD output. I need to pass values into the first child and into each child after that in order. Here is what i got:
#include <errno.h>
#include <cstdio>
#include <iostream>
#include <sys/wait.h>
using namespace std;
int main(int argc, char *argv[]) {
pid_t childpid;
int x2ypipe[2];
pipe(x2ypipe);
if(x2ypipe==0) {
cout<<"ERROR:"<<errno<<endl;
}
int y2zpipe[2];
pipe(y2zpipe);
if(y2zpipe==0) {
cout<<"ERROR:"<<errno<<endl;
}
pid_t xchild =fork();
if(xchild==0) {
dup2(x2ypipe[1],STDOUT_FILENO);
close(x2ypipe[0]);
close(x2ypipe[1]);
int a=execl(argv[1],argv[1], (char*)NULL);
if(a==-1) {
perror("The following error occurred at A");
}
}
for(int i=2; i<(argc-1); i++) {
childpid =fork();
if(childpid==0) {
dup2(x2ypipe[0],STDIN_FILENO);
close(x2ypipe[0]);
close(x2ypipe[1]);
//direct y2z pipe to standard output and replace the child with the program part2
dup2(x2ypipe[1],y2zpipe[1]);
dup2(y2zpipe[1],STDOUT_FILENO);
close(y2zpipe[0]);
close(y2zpipe[1]);
int b=execl(argv[i],argv[i],(char *)NULL);
if(b==-1) {
perror("The following error occurred at B");
}
}
}
pid_t zchild =fork();
if(zchild==0) {
dup2(y2zpipe[0],STDIN_FILENO);
close(y2zpipe[0]);
close(y2zpipe[1]);
int c=execl(argv[argc-1],argv[argc-1],(char *)NULL);
if(c==-1) {
perror("The following error occurred at C");
}
}
close(x2ypipe[0]);
close(x2ypipe[1]);
wait(NULL);
wait(NULL);
wait(NULL);
}
now right now I am only passing in three programs in to the argv[] and it works fine. I will have to add a if statement in my for loop to check for the last/highest possible value of i to connect the y2z pipe to the zchild. What I am having trouble doing it connecting the children to each other within the for loop. How would I go about creating a new pipe for each child from the last child?
Maybe this will help. Notice how I call pipe() inside my for loop, so I don't have to think of new "x2y", "y2z", "z2omega", etc, etc names for the pipe pairs.
Also notice how I used a variable prevfd from outside the for loop to carry the previous iterations's pipe file descriptor into the next iteration. And how it points to "/dev/null" to start with.
Finally, notice how I call wait() precisely as many times as I need to, in a loop, rather than writing it 3 (or 4 or 5 or ... 1,397) times.
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <cstdlib>
#include <cstdio>
#include <sys/wait.h>
int main(int argc, char *argv[]) {
int prevfd;
prevfd = open("/dev/null", O_RDONLY);
if(prevfd < 0) {
perror("/dev/null");
exit(1);
}
for(int i = 1; i < argc; ++i) {
int pipefd[2];
int kid;
if(i != argc-1 && pipe(pipefd)) {
perror("pipe");
break;
}
if(!fork()) {
dup2(prevfd, 0);
close(prevfd);
if(i != argc-1) {
dup2(pipefd[1], 1);
close(pipefd[0]);
close(pipefd[1]);
}
execl(argv[i], argv[i], (char*)0);
perror(argv[i]);
exit(1);
}
close(prevfd);
prevfd = pipefd[0];
close(pipefd[1]);
}
while(wait((int*)0) != -1)
;
return 0;
}
You need a separate pipe between each pair of connected processes.