I am trying to execute LP to print a PDF document and wait for it to exit. After
it exists i am trying to delete the file with unlink();
However the wait finishes even before execv execute LP. I am not quite sure
how to handle this and why the wait isn't waiting till execv finishes.
Is there any other way to accomplish this?
if(fork())
{
fprintf(stderr, "Executing command %s %s", "/usr/bin/lp", homedir);
char *const parmList[] = {"/usr/bin/lp", homedir, (char *)0};
execv("/usr/bin/lp", parmList );
}else
{
int pid, status;
fprintf(stderr, "Wait\n");
pid = wait(&status);
fprintf(stderr, "Finished waiting.\n");
unlink(homedir);
}
When executing the above code the ouput would look like this:
Wait
Finished waiting.
Executing command /usr/bin/lp /home/user/Docs/test.pdf
/usr/bin/lp: Error - unable to access "/home/user/Docs/test.pdf" - No such file or directory
fork() returns zero in the child process, and a positive value in the parent process (assuming the fork succeeds), and wait(...) only makes sense in the parent process, so you need to swap the contents of your if and else blocks.
Related
I am working on a tiny shell(tsh) implemented in C(it's an assignment). One part of assignment belongs to PIPING. I have to pipe a command's output to another command. e.g:ls -l | sort
When I run the shell, every command that I execute on it, is processed by a child process that it spawns. After the child finishes the result is returned. For piping I wanted to implement a harcoded example first to check how it works. I wrote a method, that partially works. The problems is when I run the pipe command, after child process finishes, the whole program quits with it! Obviously I am not handling the child process signal properly(Method code below).
My Question:
How does process management with pipe() works? if i run a command ls -l | sort does it create a child process for ls -l and another process for sort ? From the piping examples that I have seen so far, only one process is created(fork()).
When the second command (sort from our example) is processed, how can i get its process ID?
EDIT: Also while running this code I get the result twice. don't know why it runs twice, there is no loop in there.
Here is my code:
pid_t pipeIt(void){
pid_t pid;
int pipefd[2];
if(pipe(pipefd)){
unix_error("pipe");
return -1;
}
if((pid = fork()) <0){
unix_error("fork");
return -1;
}
if(pid == 0){
close(pipefd[0]);
dup2(pipefd[1],1);
close(pipefd[1]);
if(execl("/bin/ls", "ls", (char *)NULL) < 0){
unix_error("/bin/ls");
return -1;
}// End of if command wasn't successful
}// End of pid == 0
else{
close(pipefd[1]);
dup2(pipefd[0],0);
close(pipefd[0]);
if(execl("/usr/bin/tr", "tr", "e", "f", (char *)NULL) < 0){
unix_error("/usr/bin/tr");
return -1;
}
}
return pid;
}// End of pipeIt
Yes, the shell must fork to exec each subprocess. Remember that when you call one of the execve() family of functions, it replaces the current process image with the exec'ed one. Your shell cannot continue to process further commands if it directly execs a subprocess, because thereafter it no longer exists (except as the subprocess).
To fix it, simply fork() again in the pid == 0 branch, and exec the ls command in that child. Remember to wait() for both (all) child processes if you don't mean the pipeline to be executed asynchronously.
Yes, you do need to call fork at least twice, once for each program in the pipeline. Remember that exec replaces the program image of the current process, so your shell stops existing the moment you start running sort or (tr).
I am writing a simple C program to create my own shell. It takes in input as commands and executes them. But when I try to execute a process in background( i.e. I fork a process from parent. The parent won't wait for the child process to finish, it just goes on to take more input commands while the child process runs in the background.) The execvp does execute the command but then gives a segmentation fault immediately.
Can you help me? I'll post my part of the code which I think is relevant. Let me know if you need to know anything more, i'll edit my question accordingly.
while(1){
pid = fork();
if(pid == 0)
executeCommand(info);
else
{
if(info->boolBackground ==1)
{
waitpid(pid , status , WNOHANG);
}
else
wait(NULL);
}
} //Info contains the command to be executed and it's arguments.
Here is me executeCommand function:
void executeCommand(parseInfo * info)
{
FILE *infile, *outfile;
struct commandType * com;
char * cmd;
int i , status;
cmd = (char*)malloc(1024);
strcpy(cmd , info->CommArray[0].command);
if(info->boolOutfile == 1)
{
outfile = fopen(info->outFile, "w");
dup2(fileno(outfile), 1);
}
if(info->boolInfile == 1)
{
infile = fopen(info->inFile, "r");
dup2(fileno(infile), 0);
}
status = execvp(cmd , info->CommArray[0].VarList); //VarList contains the arguments
if(status == -1){
printf("%s\n",strerror(errno));}
exit(0);
}
When I give an input command: ls &
(& means that ls should be executed in background.)
It forks a child process which executes ls and prints the list of files/directories in the directory and then gives segmentation fault. Can you spot the error? I tried running execvp in background with simply ls command. It also lead to a segmentation fault.
Yes, as Mark Plotnick pointed out in a comment, you'll need probably need &status. I'd use &info->status. Also, if you do detached jobs, you need to maintain a list of their info objects and do a waitpid loop on them:
forall (info in detached_detach_job_list) {
pid = waitpid(info->pid,&info->status,WNOHANG);
if (pid > 0) {
report_status(info);
remove_job_from_list(info);
}
}
Hopefully, the code frag you gave for your outer loop does something like this.
Also, I might not do "wait(NULL)" for a foreground. I'd treat it similarly to a detached job. Consider a case where a user does:det1 &
det2 &
...
det9000 &
run_long_30_minute_job
Because your shell is doing a hard wait on the foreground, it can't reap the detached jobs as they finish and you'll end up with zombie processes. Do the list/loop approach, just don't give user a prompt until the foreground completes (e.g. it's in the list, it's just the one with the background flag cleared). In other words, call the list something like child_list to denote all child processes, not just background. Put a sleep in the outer loop. Or, attach to SIGCHLD and do a single long sleep
I am working on a tiny shell(tsh) implemented in C(it's an assignment). One part of assignment belongs to PIPING. I have to pipe a command's output to another command. e.g:ls -l | sort
When I run the shell, every command that I execute on it, is processed by a child process that it spawns. After the child finishes the result is returned. For piping I wanted to implement a harcoded example first to check how it works. I wrote a method, that partially works. The problems is when I run the pipe command, after child process finishes, the whole program quits with it! Obviously I am not handling the child process signal properly(Method code below).
My Question:
How does process management with pipe() works? if i run a command ls -l | sort does it create a child process for ls -l and another process for sort ? From the piping examples that I have seen so far, only one process is created(fork()).
When the second command (sort from our example) is processed, how can i get its process ID?
EDIT: Also while running this code I get the result twice. don't know why it runs twice, there is no loop in there.
Here is my code:
pid_t pipeIt(void){
pid_t pid;
int pipefd[2];
if(pipe(pipefd)){
unix_error("pipe");
return -1;
}
if((pid = fork()) <0){
unix_error("fork");
return -1;
}
if(pid == 0){
close(pipefd[0]);
dup2(pipefd[1],1);
close(pipefd[1]);
if(execl("/bin/ls", "ls", (char *)NULL) < 0){
unix_error("/bin/ls");
return -1;
}// End of if command wasn't successful
}// End of pid == 0
else{
close(pipefd[1]);
dup2(pipefd[0],0);
close(pipefd[0]);
if(execl("/usr/bin/tr", "tr", "e", "f", (char *)NULL) < 0){
unix_error("/usr/bin/tr");
return -1;
}
}
return pid;
}// End of pipeIt
Yes, the shell must fork to exec each subprocess. Remember that when you call one of the execve() family of functions, it replaces the current process image with the exec'ed one. Your shell cannot continue to process further commands if it directly execs a subprocess, because thereafter it no longer exists (except as the subprocess).
To fix it, simply fork() again in the pid == 0 branch, and exec the ls command in that child. Remember to wait() for both (all) child processes if you don't mean the pipeline to be executed asynchronously.
Yes, you do need to call fork at least twice, once for each program in the pipeline. Remember that exec replaces the program image of the current process, so your shell stops existing the moment you start running sort or (tr).
I have to write program that create process using pipe().
My first task is to write a parent process that generates four child processes using the fork() function.
Once the fork() is successful, replace the child process with another process rover1, rover2, rover3, and rover4, though all of them have the same code.
The function of the processes is as follows.
Each child process is initially given its own number. It receives a new number from the parent. Using the following formula it creates its own new number as follows and forwards it to the parent:
mynumber = (3 * mynumber + 4 * numberreceived)/7
This process continues until the parent sends the message that the system is stable. The parent also has its initial number. It receives numbers of all the children and computes its new number as follows:
mynumber = (3 * mynumber + numbers sent by all the children)/7
The parent will send this number to all its children. This process will continue until the parent finds that its number is not changing anymore. At that time it will tell the children the system has become stable.
This is what I did but my professor said I have to use exec() to execute the child and replace child process with another child process. I am not sure how to use exec(). Could you please help me with this.
I am attaching only first child generation.
// I included stdio.h, unistd.h stdlib.h and errno.h
int main(void)
{
// Values returned from the four fork() calls
pid_t rover1, rover2, rover3, rover4;
int parentnumber, mynumber1, mynumber2, mynumber3, mynumber4;
int childownnumber1 = 0, status = 1, childownnumber2 = 0,
childownnumber3 = 0, childownnumber4 = 0, numberreceived = 0;
printf("Enter parent number: ");
printf("%d", parentnumber);
printf("Enter each children number");
printf("%d %d %d %d", mynumber1, mynumber2, mynumber3, mynumber4);
// Create pipes for communication between child and parent
int p1[2], p2[2];
// Attempt to open pipe
if (pipe(p1) == -1) {
perror("pipe call error");
exit(1);
}
// Attempt to open pipe
if (pipe(p2) == -1) {
perror("pipe call error");
exit(1);
}
// Parent process generates 4 child processes
rover1 = fork();
// if fork() returns 0, we're in the child process;
// call exec() for each child to replace itself with another process
if (rover1 == 0) {
for(; numberreceived != 1; ) {
close(p1[1]); // Close write end of pipe
close(p2[0]); // Close read end of second pipe
// Read parent's number from pipe
read(p1[0], &numberreceived, sizeof(int));
if (numberreceived == 1) {
// System stable, end child process
close(p1[0]);
close(p2[1]);
_exit(0); // End child process
}
mynumber1 = (int)((3*mynumber1 + 4*numberreceived)/7.0);
printf("\nrover1 number: ");
printf("%i", mynumber1);
// Write to pipe
write(p2[1], &mynumber1, sizeof(int));
}
}
/* Error:
* If fork() returns a negative number, an error happened;
* output error message
*/
if (rover1 < 0) {
fprintf(stderr,
"can't fork, child process 1 not created, error %d\n",
errno);
exit(EXIT_FAILURE);
}
}
The exec family of functions is used to replace the current process with a new process. Note the use of the word replace. Once exec is called, the current process is gone and the new process starts. If you want to create a separate process, you must first fork, and then exec the new binary within the child process.
Using the exec functions is similar to executing a program from the command line. The program to execute as well as the arguments passed to the program are provided in the call to the exec function.
For example, the following execcommand* is the equivalent to the subsequent shell command:
execl("/bin/ls", "/bin/ls", "-r", "-t", "-l", (char *) 0);
/bin/ls -r -t -l
* Note that "arg0" is the command/file name to execute
Since this is homework, it is important to have a good understanding of this process. You could start by reading documentation on pipe, fork, and exec combined with a few tutorials to gain a better understanding each step.
The following links should help to get you started:
IBM developerWorks: Delve into UNIX process creation
YoLinux Tutorial: Fork, Exec and Process control
Pipe, Fork, Exec and Related Topics
If you are supposed to use exec, then you should split your program into two binaries.
Basically, the code that now gets executed by the child should be in the second binary and should be invoked with exec.
Before calling one of the exec family of functions, you'll also need to redirect the pipe descriptors to the new process' standard input/output using dup2. This way the code in the second binary that gets exec'd won't be aware of the pipe and will just read/write to the standard input/output.
It's also worth noting that some of the data you are using now in the child process is inherited from the parent through the fork. When using exec the child won't share the data nor the code of the parent, so maybe you can consider transmitting the needed data through the pipe as well.
Hi I'm working on a unix shell and I'm running into two problems. I was wondering if any of you could help me out. My first problem is that the shell is not waiting for the child process to terminate. I can actually go type more commands while the child process is running. My second problems is in the following two lines. I'm not getting any display on the shell.
fprintf(stderr, "Process name is: %s\n", commandArgv[0]);
fprintf(stderr, "Child pid = %d\n", pid);
I have the following method to execute a process entered by the user: i.e. firefox, ls -a, etc
void execute(char *command[], char *file, int descriptor){
pid_t pid;
pid = fork();
if(pid == -1){
printf("error in execute has occurred\n");
}
if(pid == 0){
execvp(*command,command);
fprintf(stderr, "Process name is: %s\n", commandArgv[0]);
fprintf(stderr, "Child pid = %d\n", pid);
wait(&status);
exit(EXIT_SUCCESS);
}
else{
printf("ignore for now\n");
}
}
This is where I call the execute command. It works fine and launches a process, but it doesn't wait for it to finish.
execute(commandArgv, "STANDARD",0);
Do you guys have any idea what I might be doing wrong? Thanks I really appreciate any time you take to help me on this.
Once execvp() runs, it will never return. It replaces in-memory the running app with whatever was provided. So your fprintf() and wait() are in the wrong place.
Other than getting the actual logic worked out correctly (Stéphane's suggestions all good) you might also want to fflush(stderr) after fprintf-ing, to ensure your error messages make it out right away instead of being buffered.
You have a little error in how the process works. After execvp is called, there is no turning back. fork() gives you have the parent and an identical child, but execvp overwrite child image to be the command you are calling.
The execvp returns only when a severe errors occur that prevent overwriting the image. So, you need to print things before its call. So you also may want to change the EXIT_SUCCESS to an EXIT_FAILURE there.
Now there is another mistake using wait: you always want the parent waiting for the child, not the other way around. You cannot ask for the child to wait. She has nothing to wait, she will run and terminate. So, you need to move the wait() call to the else part.
void execute(char *command[], char *file, int descriptor)
{
pid_t pid;
pid = fork();
if(pid == -1)
{
printf("fork() error in execute() has occurred\n");
return; /* return here without running the next else statement*/
}
if(pid == 0)
{
fprintf(stderr, "Process name is: %s\n", commandArgv[0]);
fprintf(stderr, "Child pid = %d\n", getpid());
execvp(*command,command);
fprintf(stderr, "Error! Can't overwrite child's image!\n");
exit(EXIT_FAILURE);
}
else
{
printf("Parent waiting for child pid: %d\n", pid);
wait(&status);
printf("Parent running again\n");
}
}
But reading your question, maybe you actually don't want the parent to wait. If that is the case, just don't use the wait() function.
Take care,
Beco
Edited: some minor mistakes. pid of child is getpid()