I have something like this:
pipe
close(pipe[0]);
parent writes something to pipe
close(pipe[1]);
fork();
if(child)
{
close(pipe[1]);
child reads from pipe
close(pipe[0]);
child does some operations
child writes to pipe
close(pipe[1]);
}
else
{
back to parent
close(pipe[0]);
wait(&code);
parent tries to read what the terminated child just wrote but fails to do so
}
I'm not really sure what can i do to make the parent read from the terminated child. Do i need to make use of dup? I'm not so very sure in what situations dup or dup2 is useful.
writing and reading is done using the write() and read() functions.
I have to use pipes and not fifo's or other means to communicate between processes.
A sample from this article says:
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
main()
{
int fd[2];
pid_t childpid;
pipe(fd);
if((childpid = fork()) == -1)
{
perror("fork");
exit(1);
}
if(childpid == 0)
{
/* Child process closes up input side of pipe */
close(fd[0]);
}
else
{
/* Parent process closes up output side of pipe */
close(fd[1]);
}
.
.
}
IIRC that's the way doing it. The crucial thing is to close the unused fd's in parent and child process.
I think fifo suites your need and I don't think you need to use a dup either. Here is a working code:
#include <fcntl.h>
int main()
{
int e=open("fif",O_RDONLY|O_NONBLOCK);
if(fork()==0)
{
int d=open("fif",O_WRONLY);
write(d,"hi there\n",9);
close(d);
//sleep(5);
exit(0);
}
wait();
char buf[15];
int n=read(e,buf,15);
buf[n]=0;
printf("%s", buf);
//wait();
return 0;
}
Related
Here is my example code
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#include <signal.h>
#include <errno.h>
id_t pid;
void handle_sigterm(int sig)
{
printf("handle me \n");
}
void forkexample()
{
// child process because return value zero
pid = fork();
int status = 0;
if (pid == 0)
{
printf("Hello from Child!\n");
char *newargv[] = { "test2", NULL };
char *newenviron[] = { NULL };
newargv[0] = "test2";
execve("test2", newargv, newenviron);
printf("error -> %d", errno);
fflush(stdout);
}
// parent process because return value non-zero.
else
{
struct sigaction psa;
psa.sa_handler = handle_sigterm;
sigaction(SIGTERM, &psa, NULL);
printf("Hello from Parent!\n");
fflush(stdout);
int result = waitpid(pid, &status, 0);
printf("result -> %d\n", result);
fflush(stdout);
}
}
int main()
{
printf("pid -> %d\n", getpid());
forkexample();
return 0;
}
test2 is just a while(true). Lets say both parent and child process receive SIGTERM at the time, how can I make parent process wait until child process terminates and then exit? I've read from documentation that:
The wait() function shall cause the calling thread to become blocked
until status information
generated by child process termination is made available to the thread, or until delivery of a
signal whose action is either to execute a signal-catching function or to terminate the process
So it means that when SIGTERM is received in parent, it exits the wait() and the process is killed. But I want it to wait until child terminates and then exit. How can I achieve that?
You can use waitpid() to wait for the child inside signal handler of parent also. This does make sure that parent will wait for child even though it receives signal. Some advices are as below.
Why do you think it is a C++ program?
signal handler name you set for sa_handler is wrong. handle_sigint()
is not defined.
I fork a child process which in turn spawns a grand-child process and returns. If there are any errors in spawning that grand-child, the child writes to stderr using fprintf statements and exits.
I am trying to read stderr from the child process in the parent. But, the parent gets hung in the while loop reading from child process. When I look at ps -ef the child is <defunct>.
Following is the code I wrote to accomplish this. I am new to this and the search on web does not give enough information about why the parent is hung when the child is
int pipe_out[2];
// Create the pipe
if (pipe(pipe_out) < 0) {
perror("Failed pipe");
exit(1);
}
// Create the child process
int status;
pid_t pid = fork();
switch(pid) {
case -1:
perror("Failed fork");
exit(1);
case 0: { // Child
close(pipe_out[0]); // close read end
// Make stderr go to pipe write end
dup2(pipe_out[1], STDERR_FILENO);
close(pipe_out[1]);
// start my process
execvp(inp_args[0], inp_args);
_exit(EXIT_FAILURE);
break;
}
default: { // Parent
close(pipe_out[1]); // close write end
// read from child
while( read(pipe_out[0], buffer, sizeof(buffer)) )
log(stdout, "%s\n",buffer);
}
// wait for end then close other end of pipe
waitpid(pid, &status, 0);
if (WIFSIGNALED(status))
log(stdout, "killed by signal %d\n", WTERMSIG(status));
close(pipe_out[0]);
}
}
I'm trying to make a program that forks once and while the parent waits for the child terminates, this child forks again and then executes two execs. There is a Pipe on the program and I've checked the return values of every dup2() and pipe() on the program -just omitted them here to make it looks more concise-. The problem is that I only get the result of ls -a | sort -r AFTER the program finishes.
The code is:
#include <cstdio>
#include <cstring>
#include <sys/wait.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
int main(int argc, char *argv[]) {
printf("Shell> \n"); fflush(stdout);
pid_t pid1;
pid_t pid2;
int status = 0;
int fd[2];
if(pipe(fd) < 0) {
printf("FATAL ERROR.\n");
}
pid1 = fork();
if(pid1 > 0) { // Parent
waitpid(pid1, &status, 0);
printf("\t\t------PID1 Complete-------\n\n");
}
else { // Child
if(pid1 == 0) {
printf("ON CHILD\n");
pid2 = fork();
if(pid2 > 0) { // Child -> Parent
printf("ON CHILD-Parent\n");
close(fd[1]);
dup2(fd[0], STDIN_FILENO);
waitpid(pid2, &status, 0);
printf("ON CHILD-Parent after wait\n");
execlp("sort", "sort", "-r", NULL);
perror("Problem with execlp\n");
exit(1);
}
else { // Child -> Child
printf("ON CHILD->Child\n");
close(fd[0]);
dup2(fd[1], STDOUT_FILENO);
execlp("ls", "ls", "-a", NULL);
perror("Problem with execvp\n");
exit(1);
}
} // End of if(pid1 == 0)
} // End of Child
printf("\nEnd of program.\n");
return 0;
}
My current output is:
Shell>
ON CHILD
ON CHILD-Parent
ON CHILD->Child
ON CHILD-Parent after wait
I think the problem is on the waits, but I just can't figure out how to make this work. Any ideas? Thanks!
The problem is that you call pipe in the grandparent process. After the grandchild process (ls -a) exits, the parent process (sort -r) blocks indefinitely waiting to read more input from the pipe since some process - the grandparent - holds an open descriptor to the write end of the pipe.
If you close the pipe descriptors in the grandparent process, or better yet move the pipe call into the first forked process, then the sort process will terminate when the last process with an open descriptor for the write end of the pipe exits (DEMO):
int main() {
// Turn off buffering of stdout, to help with debugging
setvbuf(stdout, NULL, _IONBF, 0);
printf("Shell> \n");
pid_t pid1 = fork();
if(pid1 < 0) {
perror("fork failed");
}
if(pid1 > 0) { // Parent
int status;
waitpid(pid1, &status, 0);
printf("\t\t------PID1 Complete (%d) -------\n\n", status);
} else { // Child
printf("ON CHILD\n");
int fd[2];
if(pipe(fd) < 0) {
perror("pipe failed");
return 1;
}
pid_t pid2 = fork();
if(pid2 < 0) {
perror("fork failed");
}
if(pid2 > 0) { // Child -> Parent
printf("ON CHILD-Parent\n");
close(fd[1]);
dup2(fd[0], STDIN_FILENO);
execlp("sort", "sort", "-r", NULL);
perror("Problem with execlp");
return 1;
} else { // Child -> Child
printf("ON CHILD->Child\n");
close(fd[0]);
dup2(fd[1], STDOUT_FILENO);
execlp("ls", "ls", "-a", NULL);
perror("Problem with execvp");
return 1;
}
}
printf("\nEnd of program.\n");
}
The other problem with the program is the one #nategoose commented on: the call to waitpid could lead to a deadlock if the output of "ls -a" is too large to fit in the pipe's buffer. There's no reason to wait, so it should simply be eliminated.
This isn't a real answer, but I have some into that I'd like to share.
To make sure that your output comes out in the order that it should, I'm flushing a lot more than you were. Remember that when you are calling functions like fork(), clone(), vfork(), dup(), dup2(), close(), or any of the exec() family of functions you are doing stuff that is BELOW the C runtime environment, which includes stdio. If you do:
printf("cat");
fork();
fflush(stdout);
You are very likely to get:
catcat
as your output because you've duplicated the stdout structure, including all buffered data, so unless stdio decided that it was time to flush anyway before the end of the printf function, then "cat" is in each process's stdout buffer.
There's also the fact that since data can stay buffered when you run a function in the exec family your data may not be flushed before your program is replaced with the new program. When your program is replaced by ls or sort then any data pending in stdout gets lost forever.
Also, when you use dup you have the another issue since you are swapping the file descriptor out from under stdio so it may not have flushed yet and the data may end up getting flushed to the new file after the dup.
Because of these things you should have a lot more calls to fflush, but I don't think that's your problem here.
I want to use ptrace to check what system calls a program spawned by my program makes. I started out from this tutorial as it was explained in an answer to my previous question. I modified the code by adapting it to the platform I'm using (SLES 11 64 bit), and put together the following test code that prints out every system call the spawned process makes:
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/reg.h>
#include <sys/syscall.h> /* For SYS_write etc */
pid_t child;
void run()
{
long orig_eax;
int status;
while(1) {
int pid = wait(&status);
if (pid == -1) {
perror("wait");
kill(child, SIGKILL);
return;
}
printf("Got event from %d.\n", pid);
if(WIFEXITED(status))
break;
orig_eax = ptrace(PTRACE_PEEKUSER,
pid, 8 * ORIG_RAX, NULL);
if (orig_eax == -1) {
perror("ptrace");
kill(child, SIGKILL);
return;
} else {
printf("Syscall %ld called.\n", orig_eax);
}
ptrace(PTRACE_SYSCALL,
pid, NULL, NULL);
}
}
int main(int /*argc*/, char* argv[])
{
child = fork();
if(child == 0) {
ptrace(PTRACE_TRACEME, 0, NULL, NULL);
execl(argv[1], argv[1], NULL);
}
else {
printf("Child process id = %d.\n", child);
run();
}
return 0;
}
It works pretty well: it prints the id of the system calls made by the program (actually it prints each one twice, once at entry and once for exit, but that doesn't matter now). However, my program needs to do other things to do other than checking the system calls, so I decided to move the checking to a separate thread (I'm more comfortable with C++ than C, so I did it the C++ way, but I don't think that matters). Of course in this thest program, I only start the thread and then join it.
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/reg.h>
#include <sys/syscall.h> /* For SYS_write etc */
#include <boost/thread.hpp>
pid_t child;
void run()
{
long orig_eax;
int status;
while(1) {
int pid = wait(&status);
if (pid == -1) {
perror("wait");
kill(child, SIGKILL);
return;
}
printf("Got event from %d.\n", pid);
if(WIFEXITED(status))
break;
orig_eax = ptrace(PTRACE_PEEKUSER,
pid, 8 * ORIG_RAX, NULL);
if (orig_eax == -1) {
perror("ptrace");
kill(child, SIGKILL);
return;
} else {
printf("Syscall %ld called.\n", orig_eax);
}
ptrace(PTRACE_SYSCALL,
pid, NULL, NULL);
}
}
int main(int /*argc*/, char* argv[])
{
child = fork();
if(child == 0) {
ptrace(PTRACE_TRACEME, 0, NULL, NULL);
execl(argv[1], argv[1], NULL);
}
else {
printf("Child process id = %d.\n", child);
boost::thread t(run);
t.join();
}
return 0;
}
This time I get an error message:
Child process id = 24682.
Got event from 24682.
ptrace: No such process
Why is this? I tried searching for an answer but found nothing like this. I found that ptrace won't trace threads started by the child process, but that's another thing needs to be dealed with later. Is that even possible to check the child process from a different therad?
The other strange thing is that in my real application I do basically the same thing (but from a much more complicated context: classes, mutexes etc.), and I get a different kind of error. Instead of ptrace returning with an error, wait doesn't even return for system calls on the child process (and the child process doesn't even stop). On the other hand, wait works as expected when the child process exits.
As far as I can tell, ptrace allows just one tracer per process. This means that if you try to attach, which you can try and force it with PTRACE_ATTACH, you will receive an error, telling that ptrace was not able to attach to the specified process.
Thus, your error appears because your thread is not attached to the child process, and this way, when you try to ptrace it, it fails, sending a -ESRCH code.
Furthermore, you can have a look at this post here, it might answer some other questions you might have apart from this one.
I am really new to C++ and I am trying to get the output from:
execv("./rdesktop",NULL);
I am programming in C++ and on RHEL 6.
Like a FTP client, I would like to get all the status updates from my external running program. Can someone please tell me how I could do this?
execv replaces the current process, so immediately after executing it what's executing will be whatever executable you specified.
Normally you do a fork, and then execv only in the child process. The parent process receives the PID of the new child, which it can use to monitor the child's execution.
You can examine the exit status of a child process by calling wait, waitpid, wait3 or wait4.
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main () {
pid_t pid = fork();
switch(pid) {
case 0:
// We are the child process
execl("/bin/ls", "ls", NULL);
// If we get here, something is wrong.
perror("/bin/ls");
exit(255);
default:
// We are the parent process
{
int status;
if( waitpid(pid, &status, 0) < 0 ) {
perror("wait");
exit(254);
}
if(WIFEXITED(status)) {
printf("Process %d returned %d\n", pid, WEXITSTATUS(status));
exit(WEXITSTATUS(status));
}
if(WIFSIGNALED(status)) {
printf("Process %d killed: signal %d%s\n",
pid, WTERMSIG(status),
WCOREDUMP(status) ? " - core dumped" : "");
exit(1);
}
}
case -1:
// fork failed
perror("fork");
exit(1);
}
}