I am given the task of forking n processes.
For each process, it must start an instance of /bin/xterm
I am done with the part of generating n processes and opening the xterm instance.
I got this output when I tried running the program. (Error in bold)
Number of process to open is 1.
Child (1): 3457
/bin/xterm: Xt error: Can't open display:
/bin/xterm: DISPLAY is not set
My code is below.
I tried googleing for the error but I have no luck so far.
Any solutions?
#include<stdio.h>
#include<stdlib.h>
int main(int argc, char *argv[])
{
int num = atoi(argv[1]);
printf("Number of process to open is %d.\n", num);
int pid;
int i;
for(i = 0; i < num; i++)
{
pid = fork();
if(pid < 0) {
printf("Error");
exit(1);
} else if (pid == 0) {
printf("Child (%d): %d\n", i + 1, getpid());
char * const argv[] = {"/bin/xterm", NULL};
char * const envp[] = {NULL};
int rc = execve ("/bin/xterm", argv, envp);
exit(0);
} else {
wait(NULL);
}
}
return 0;
}
This little changed code works perfectly fine on my system:
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
int main(int argc, char *argv[])
{
int num = atoi(argv[1]);
printf("Number of process to open is %d.\n", num);
int pid;
int i;
for(i = 0; i < num; i++)
{
pid = fork();
if(pid < 0) {
printf("Error");
exit(1);
} else if (pid == 0) {
//printf("Child (%d): %d\n", i + 1, getpid());
//char * const argv[] = {"/bin/xterm", NULL};
//char * const envp[] = {NULL};
execl("/usr/bin/xterm", "/usr/bin/xterm", NULL);
//exit(0);
}else {
wait(NULL);
}
}
return 0;
}
The error was explained in the output you pasted:
/bin/xterm: DISPLAY is not set
You need to set DISPLAY appropriately. Otherwise, it will have no way to know where to put its display.
Also, did you really want to wait for each child after creating it?
Use
char *envp[] = {"TERM=vt100", "PATH=/bin:/usr/bin", "DISPLAY=:0.0",(char *) 0 };
Doing so you set the display on your machine.
Sorry I'm late.
Related
The code I've written finds out the number of words in multiple text files by creating multiple processes with each process being responsible for one file to count its words.
What I want to do is using pipes to find out total number of words in all files.
So the parent should:
creates a pipe between the each child and itself so it can get the number of words from each child
reports the total number of words in all the files by adding the numbers received through pipes
checks the exit status of each child and prints out how that child
exited
also let each child:
sends the number of the words to the parent via the pipe
send 0 as word count through the pipe to the parent if the file does
not exist or any other error happens
returns/exits with 0 if it is successfull in opening the file and
counting the words in that file, returns/exits with 1 if there is an
error (e.g., file does not exist etc.)
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#define MAX_CHAR 100
pid_t getpid(void);
pid_t getppid(void);
char* itoa(int i, char b[]){
char const digit[] = "0123456789";
char* p = b;
if(i<0){
*p++ = '-';
i *= -1;
}
int shifter = i;
do{ //Move to where representation ends
++p;
shifter = shifter/10;
}while(shifter);
*p = '\0';
do{ //Move back, inserting digits as u go
*--p = digit[i%10];
i = i/10;
}while(i);
return b;
}
int countWords(char * fp, int pid) {
FILE * file;
int words = 0;
char word[MAX_CHAR];
//execute this function only if child process of parent, no gradchild is allowed to execute this function!
if (pid == getppid()) {
file = fopen(fp, "r");
if (file == NULL) {
return -1;
}
//find string in the file and count the words.
while (fscanf(file, "%s", word) != EOF) {
words++;
}
return words;
} else {
return -1;
}
return 0;
}
int main(int argc, char * arvg[]) {
//if invalid arguments
if (argc < 2) {
fprintf(stderr, "ERROR: INVALID ARGUMENTS");
exit(-1);
}
int count = 0, pid, ppid, status, totalwords;
int result = -1;
int fd[2];
char string[100];
char readbuffer[80];
int *write_fd = &fd[1];
int *read_fd = &fd[0];
result = pipe(fd);
if(-1 == result){
perror("pipe");
return -1;
}
//creates (argc - 1) child processes using fork()
pid = (int) malloc((argc - 1) * sizeof(int));
//parent pid
ppid = getpid();
//each child process to count the number of words in each file
for (int i = 1; i < argc; i++) {
//child process
pid = fork();
if( pid == -1){
perror("failed to fork");
return -1;
}else if (pid == 0) {
// call a function to count the number of words in file arvg[i]
int words = countWords(arvg[i], ppid);
close(*read_fd);
if (words >= 0) {
printf("Child process pid_%d for %s :number of words is %d\n", i, arvg[i], words);
//I don't know how to write int into the pipe,so below might be wrong
write(*write_fd, words, 1);
return 0;
} else if (words == -1) {
printf("Child process pid_%d for %s :does not exists\n", i, arvg[I]);
//I don't know how to write int into the pipe,so below might be wrong
write(STDOUT_FILENO, words, 1);
exit(1);
}
} else {
close(*write_fd);
//and I have no idea how to read int from pipes
read(*read_fd, &readbuffer, 1);
totalwords += ???
close(*read_fd);
//Wait until all child processes exit/return
if (ppid == getpid()) {
wait( & status);
}
//inspect their exit codes, WEXITSTATUS = return code when child exits
if (WEXITSTATUS(status) == 1) {
count++;
}
}
}
printf("Main process created %d child processes to count words in %d files\n", argc - 1, argc - 1);
printf("Total words is %d", totalwords);
printf("%d files have been counted sucessfully!\n", argc - 1 - count);
printf("%d files did not exist.\n", count);
return 0;
}```
Can someone help me to figure out this? I don't really know how to achieve my goal with pipe.
found some issues with the code. I fixed them for you (however, I would have done the same thing slight differently)
reading and writing int from a pipe is pretty straight forward, just typecast correctly while reading or writing to an int.
malloc to a pid was not necessary. Also malloc returns a pointer and should have been typecasted with (int*)
always add the right includes while using calls. Manual page or reading about the calls while trying to understand the parameters passed and return values is extremely useful.
Enough said, here is your working code
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#define MAX_CHAR 100
pid_t getpid(void);
pid_t getppid(void);
char* itoa(int i, char b[]){
char const digit[] = "0123456789";
char* p = b;
if(i<0){
*p++ = '-';
i *= -1;
}
int shifter = i;
do{ //Move to where representation ends
++p;
shifter = shifter/10;
}while(shifter);
*p = '\0';
do{ //Move back, inserting digits as u go
*--p = digit[i%10];
i = i/10;
}while(i);
return b;
}
int countWords(char * fp, int pid) {
FILE * file;
int words = 0;
char word[MAX_CHAR];
//execute this function only if child process of parent, no gradchild is allowed to execute this function!
if (pid == getppid()) {
file = fopen(fp, "r");
if (file == NULL) {
return -1;
}
//find string in the file and count the words.
while (fscanf(file, "%s", word) != EOF) {
words++;
}
return words;
} else {
return -1;
}
return 0;
}
int main(int argc, char * arvg[]) {
//if invalid arguments
if (argc < 2) {
fprintf(stderr, "ERROR: INVALID ARGUMENTS");
exit(-1);
}
int count = 0, pid, ppid, status, totalwords = 0;
int result = -1;
int fd[2];
char string[100];
char readbuffer[80];
int *write_fd = &fd[1];
int *read_fd = &fd[0];
int recvd = 0;
result = pipe(fd);
if(-1 == result){
perror("pipe");
return -1;
}
//creates (argc - 1) child processes using fork()
//pid = (int) malloc((argc - 1) * sizeof(int));
//parent pid
ppid = getpid();
//each child process to count the number of words in each file
for (int i = 1; i < argc; i++) {
//child process
pid = fork();
if( pid == -1){
perror("failed to fork");
return -1;
}else if (pid == 0) {
printf ("%d child running \n", i);
// call a function to count the number of words in file arvg[i]
int words = countWords(arvg[i], ppid);
close(*read_fd);
if (words >= 0) {
printf("Child process pid_%d for %s :number of words is %d\n", i, arvg[i], words);
//I don't know how to write int into the pipe,so below might be wrong
write(*write_fd, (void *)&words, 1);
return 0;
} else if (words == -1) {
printf("Child process pid_%d for %s :does not exists\n", i, arvg[i]);
//I don't know how to write int into the pipe,so below might be wrong
write(STDOUT_FILENO, (void *)&words, 1);
exit(1);
}
} else {
close(*write_fd);
//and I have no idea how to read int from pipes
read(*read_fd, (void*)&recvd, 1);
totalwords += recvd;
printf("recvd %d \n", totalwords);
close(*read_fd);
//Wait until all child processes exit/return
if (ppid == getpid()) {
wait( & status);
}
//inspect their exit codes, WEXITSTATUS = return code when child exits
if (WEXITSTATUS(status) == 1) {
count++;
}
}
}
printf("Main process created %d child processes to count words in %d files\n", argc - 1, argc - 1);
printf("Total words is %d\n", totalwords);
printf("%d files have been counted sucessfully!\n", argc - 1 - count);
printf("%d files did not exist.\n", count);
return 0;
}
Well, the first time around, I did not focus on the algo. I fixed all of it. The problem is forking in a loop and reading and writing it would lead to wrong results. Moreover, Parent needs to look for EOF to ensure all read has happened. Anyways, Here is the code that should work
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#define MAX_CHAR 100
pid_t getpid(void);
pid_t getppid(void);
char* itoa(int i, char b[]){
char const digit[] = "0123456789";
char* p = b;
if(i<0){
*p++ = '-';
i *= -1;
}
int shifter = i;
do{ //Move to where representation ends
++p;
shifter = shifter/10;
}while(shifter);
*p = '\0';
do{ //Move back, inserting digits as u go
*--p = digit[i%10];
i = i/10;
}while(i);
return b;
}
// count word from file provided
int countWords(char * fp, int pid) {
FILE * file;
int words = 0;
char word[MAX_CHAR];
//execute this function only if child process of parent, no gradchild is allowed to execute this function!
if (pid == getppid()) {
file = fopen(fp, "r");
if (file == NULL) {
return -1;
}
//find string in the file and count the words.
while (fscanf(file, "%s", word) != EOF) {
words++;
}
return words;
} else {
return -1;
}
return 0;
}
//do everything related to child here in this function
void child_process(int write_fd, char *filename, int ppid)
{
// call a function to count the number of words in file argv[i]
printf("counting words of %s\n", filename);
int words = countWords(filename, ppid);
if (words >= 0) {
printf("Child process pid for %s :number of words is %d\n", filename, words);
write(write_fd, (void *)&words, 1);
close(write_fd);
exit(0);
} else if (words == -1) {
printf("Child process pid for %s :does not exist\n", filename);
write(STDOUT_FILENO, (void *)&words, 1);
close(write_fd);
exit(1);
}
return;
}
int main(int argc, char * argv[]) {
//if invalid arguments
if (argc < 2) {
fprintf(stderr, "ERROR: INVALID ARGUMENTS");
exit(-1);
}
int pid = 0;
int ppid = 0;
int totalwords = 0;
int fd[2] = {0};
int write_fd = 0;
int read_fd = 0;
int recvd = 0;
// open a pipe
if(-1 == pipe(fd)){
perror("pipe");
return -1;
}
// assign write_fd and read_fd
write_fd = fd[1];
read_fd = fd[0];
//parent pid
ppid = getpid();
//each child process to count the number of words in each file
pid = fork();
for (int i = 0; i < argc-1; i++)
{
//child process
if (pid == 0) {
close(read_fd);
child_process(write_fd, argv[i+1], ppid);
break;
} else {
pid = fork();
}
}
// don't let child run beyond this point
if (pid == 0) {
exit(0);
}
// parent only code
if (pid > 0)
{
close(write_fd);
while (read(read_fd, (void*)&recvd, 1) > 0)
{
wait(NULL);
totalwords += recvd;
}
close(read_fd);
}
printf("Main process created %d child processes to count words in %d files\n", argc - 1, argc - 1);
printf("Total words is %d\n", totalwords);
printf("%d files have been counted sucessfully!\n", argc - 1);
}
My goal is to send a string from one child process to another. I set up a pipe in the parent process, then fork twice. Both reached statements print, why does the piped message not?
#include<stdlib.h>
#include<stdio.h>
#include<unistd.h>
#include<sys/wait.h>
#include<sys/types.h>
#include<string.h>
int main (char * argv[], int argc)
{
int arr[2];
pipe(arr);
int id = 0;
int pid = fork();
id = 1;
if(pid > 0) pid = fork();
if(pid > 0)
{
close(arr[0]);
close(arr[1]);
wait(NULL);
}
else if (id == 0)
{
close(arr[0]);
char message[] = "HYPERTEXT TRANSFER\n";
write(arr[1],message,strlen(message)+1);
printf("reached\n");
}
else if(id == 1)
{
printf("reached\n");
close(arr[1]);
char * buf = malloc (sizeof(char)* 20);
read(arr[0], buf, 20);
printf("%s", buf);
}
return 0;
}
The program outputs "reached" twice.
id is 1 for all processes. You are not going into the else if (id == 0) to write anything, Hence, you are attempting to read from an empty pipe.
Your problem is that for both your child processes, id = 1. Indeed, both child processes execute the line id = 1 so they are both receivers.
Try:
#include<stdlib.h>
#include<stdio.h>
#include<unistd.h>
#include<sys/wait.h>
#include<sys/types.h>
#include<string.h>
int main (char * argv[], int argc)
{
int arr[2];
pipe(arr);
int id = 0;
int pid = fork();
if(pid > 0) {
pid = fork();
id = 1; // Here only the second child process (and the parent process, but who cares) have id == 1.
}
if(pid > 0)
{
close(arr[0]);
close(arr[1]);
wait(NULL);
}
else if (id == 0)
{
close(arr[0]);
char message[] = "HYPERTEXT TRANSFER\n";
write(arr[1],message,strlen(message)+1);
printf("reached\n");
}
else if(id == 1)
{
printf("reached\n");
close(arr[1]);
char * buf = malloc (sizeof(char)* 20);
read(arr[0], buf, 20);
printf("%s", buf);
}
return 0;
}
I was given this particular task:
Using SIGCONT, SIGSTOP, and SIGCHLD, write a program that accepts
through argv[] a list of commands (none of them has arguments) and
runs the commands in a round-robin style, alternating commands in
1-second intervals.
To achieve it, I tried this:
int dead_children = 0;
int nr_processes;
void inc_dead () {
printf("I, %d, died\n", getpid());
dead_children++;
if (dead_children == nr_processes)
_exit(0);
}
int main (int argc, char *argv[]) {
int pids[argc - 1];
nr_processes = argc - 1;
signal(SIGCHLD, inc_dead);
for (int i = 1; i < argc; i++) {
pid_t pid;
if ( (pid = fork()) == -1 ) {
perror("fork");
return EXIT_FAILURE;
}
if ( !pid ) {
pause();
execlp(argv[i], argv[i], (char *) NULL);
perror("exec");
_exit(1);
}
pids[i - 1] = pid;
}
while (dead_children < nr_processes)
for (int j = 0; j < argc - 1; j++) {
kill(pids[j], SIGCONT);
sleep(1);
kill(pids[j], SIGSTOP);
}
return EXIT_SUCCESS;
}
I tried to run this code with very simple program that would allow me to verify task's correctness:
int main (void) {
int i = 5;
while (i-- > 0) {
printf("This is %d saying HI!\n", getpid());
sleep(1);
}
return EXIT_SUCCESS;
}
However, this does not work. I noticed that after creation, child processes never left pause() instruction, even though parent is sending a SIGCONT. Any thoughts on how to achieve the goal?
I'm trying to execute and schedule my own list of processes read from a file. The files are running in a random order and I'm just curious as to why this is happening. I have simple print statements in the first, second, etc files that tell which is running, and they always print in different (seemingly random) orders. It isn't messing up my functionality thus far, I'm just curious why this is.
main.c below
int main(int argc, char ** argv) {
pid_t pid[50];
pid_t wpid;
int i, j;
int status = 0;
char *newenvp[] = {NULL};
char *newargv[] = {"./files.txt", NULL};
printf("Before forking in the parent\n");
int numProgs = readPrograms();
for (i=0; i<numProgs; i++) {
pid[i] = fork();
if (pid[i] < 0) {
perror("fork error");
exit(EXIT_FAILURE);
}
else if (pid[i] == 0) {
printf("Child process running\n");
execve(programs[i], newargv, newenvp);
perror("execve error");
exit(EXIT_FAILURE);
}
}
for (i=0; i<numProgs; i++) {
wait(&status);
}
return 0;
}
char* programs[50];
int readPrograms();
readPrograms.c below
int readPrograms() {
int i=0;
char line[50];
int numProgs = -1;
FILE *file;
file = fopen("files.txt", "r");
while(fgets(line, sizeof(line), file)!=NULL) {
line[strlen(line)-1] = '\0';
programs[i]=strdup(line);
i++;
numProgs++;
}
fclose(file);
return numProgs;
}
files.txt below
./first
./second
./third
./fourth
When calling fork, your system creates the new process (copy itself,call exec,overlay itself).
Then your fork is ready, both the parent and the child process are marked ready and the running order of the processes is chosen by your system-scheduler.
So depending on your scheduler either your parent or your child is now run.
I am trying to run N concurrent processes in a C program. I've built a simple example that takes commands as arguments, creates a fork for each one, and executes it.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char **argv)
{
int i;
for(i = 1; i < argc; i++)
{
pid_t pid = fork();
if(pid < 0)
{
fprintf(stderr, "forking error\n");
exit(1);
}
else if(pid > 0)
{
int status;
waitpid(pid, &status, 0);
printf("Command %s has completed successfully by PID=%d\n", argv[i], pid);
}
else
{
char cmd[1024];
sprintf(cmd, "%s", argv[i], i);
system(cmd);
_exit(1);
}
}
printf("Finished\n");
return 0;
}
This seems to run the processes correctly, but not concurrently. Any ideas as to what am I doing wrong?
EDIT: I've edited based on suggestions, but this also does not seem to work.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char **argv)
{
int i;
pid_t *pids = malloc( sizeof(pid_t) * (argc) );
int *statuses = malloc( sizeof(int) * (argc) );
for(i = 1; i < argc; i++)
{
pid_t pid = fork();
if(pid < 0)
{
fprintf(stderr, "forking error\n");
exit(1);
}
else if(pid > 0)
{
//int status;
//waitpid(pid, &status, 0);
//printf("Command %s has completed successfully by PID=%d\n", argv[i], pid);
pids[i] = pid;
}
else
{
char cmd[1024];
sprintf(cmd, "%s > out.%d", argv[i], i);
system(cmd);
_exit(1);
}
}
int needtowait = 0;
do
{
needtowait = 0;
for(i = 1; i < argc; i++)
{
if(pids[i] > 0)
{
if(waitpid(pids[i], &statuses[i], 0) != 0)
{
pids[i] = 0;
char *successstr = "successfully";
if(statuses[i])
{
successstr = "unsuccessfully";
}
printf("Command %s has completed %s by PID=%d\n", argv[i], successstr, pids[i]);
}
}
else
{
needtowait = 1;
}
sleep(0);
}
} while(needtowait);
printf("Finished!\n");
return 0;
}
The reason you are not running these processes concurrently is in this line:
waitpid(pid, &status, 0);
The main process that forks out the child process waits for the child process to exit before continuing with the loop, and starting the next process.
Since you want to run your processes concurrently, you can do this: allocate an array of pid_t for process IDs, and fill it in inside the loop. Once you are out of the loop, you can wait for the individual processes to complete by executing waitpid calls in a loop.
pid_t *pids = malloc(argc * sizeof(pid_t));
for (int i = 0 ; i < argc ; i++) { // Start i at 0, not at 1
pid_t pid = fork();
if (pid < 0) {
...
} else if (pid > 0) {
pids[i] = pid;
} else {
char cmd[1024];
sprintf(cmd, "%s", argv[i+1], i+1);
system(cmd);
_exit(1);
}
}
for (int i = 0 ; i < argc ; i++) {
int status;
waitpid(pids[i], &status, 0);
printf("Command %s has completed successfully by PID=%d\n", argv[i+1], pids[i]);
}
Sure. Your parent process is waiting for the child process to finish executing before forking again. You're just running cmd sequentially N times.