Here I have a program that works fine. This code runs a bash into the parent process and the child process saves stdout from parent process in file called 'toto'. So in the 'toto' file, there are all the outputs of the commands that have been run in the bash from parent process.
#include <stdio.h>
#include <unistd.h>
int main(int argc, char **argv){
int fd1[2];
int pid;
if(pipe(fd1) == -1){
perror("pipe failed\n");
return 1;
}
if((pid = fork()) == -1){
perror("fork failed\n");
return 2;
}
else if(pid == 0){
close(fd1[1]);
dup2(fd1[0], 0);
close(fd1[0]);
argv[0] = "-a";
argv[1] = "toto";
argv[2] = NULL;
execve("/usr/bin/tee", argv,NULL);
return 3;
}
else{
close(fd1[0]);
dup2(fd1[1],1);
close(fd1[1]);
execve("/bin/bash",NULL,NULL);
return 4;
}
return 0;
}
I would also like to save in the 'toto' file the name of the command run into the bash.
Do you think it is possible and if so, how would you do this ?
Thanks ;)
Related
I'm trying to write a c program that is the equivalent of the linux command ps -aux | sort -r -n -k 5 but I'm not getting any output
Here's my code
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
int main(int argc, char ** argv){
int pipes[2];
int r;
r = pipe(pipes);
if (r < 0) {
fprintf(stderr, "pipe failed\n\n"); // stderr is a FILE* variable for the standard error file (terminal)
exit(2);
}
int saved_stdout = dup(1);
int pid = fork();
if(pid > 0){
// Parent
pid = fork();
if(pid > 0){
// Parent
wait(NULL);
}else if (pid == 0){
// Child 1
printf("Child 1\n");
dup2(pipes[1], 1);
close(pipes[0]);
close(pipes[1]);
execlp("/bin/ps", "ps", "-aux", (char*) NULL);
exit(0);
}else{
fprintf(stderr, "FORK FAILED\n\n");
return 1;
}
}else if (pid == 0){
// Child 2
printf("Child 2\n");
dup2(pipes[0], 0);
close(pipes[0]);
close(pipes[1]);
dup2(saved_stdout, 1);
close(saved_stdout);
execlp("/bin/sort", "sort", "-r", "-n", "-k", "5", (char*)NULL);
exit(0);
}else{
fprintf(stderr, "FORK FAILED\n\n");
return 1;
}
wait(NULL);
printf("Exiting parent\n");
}
The output I get is this
Child 1
Child 2
Exiting parent
I doesn't actually print the execlp command, I've tried saving stdout to variable saved_stdout which is the solution I found in another answer, but that doesn't seem to work.
How can I redirect stdout back to the terminal?
Strange my output with your code is:
Child 1
Child 2
and the program don't stop. Or you sure that your output is valid ?
Whatever, your problem is that you don't close your pipe in your parents. Just add:
close(pipes[0]);
close(pipes[1]);
In your both parents (before your two call to wait()).
Plus saved_stdout is useless in your case, because you only change stdout in your child1. saved_stdout and 1 describe the same file in your child2.
I'm writing a program that executes the word count command on the child process. The father process should send a sequence of lines entered by the user trough a pipeline to the child process.
I tried to do this but I ended up with an error.
This is my code:
int main ()
{
int fd[2];
char buff;
int pid;
int pip;
pid = fork();
pip = pipe(fd);
if (pid != 0)
{
pip = pipe(fd);
if (pipe == 0)
{
while (read(fd[0], &buff,1) > 0 )
{
write (fd[1],&buff,1);
}
close(fd[0]);
_exit(0);
}
}
else
{
dup2(fd[1],1);
close(fd[1]);
execlp ("wc","wc",NULL);
_exit(-1);
}
return 0;
}
I've also tried to use dup2 to associate the standard input from the child to the read descriptor of the pipe created by the father process.
But I get this error : wc: standard input: Input/output error.
How can I solve this?
UPDATED (the error is solved but I get an infinite loop)
int main ()
{
int fd[2];
char buff;
int pid;
int pip;
pip = pipe(fd);
if (pip == 0)
{
pid = fork();
if (pid != 0)
{
while (read(fd[0], &buff,1) > 0 )
{
write (fd[1],&buff,1);
}
close(fd[0]);
}
else {
dup2(fd[1],1);
close(fd[1]);
execlp ("wc","wc",NULL);
_exit(-1);
}
}
return 0;
}
#include <unistd.h>
int main ()
{
int fd[2];
char buff;
int pid;
int pip;
int status;
pip = pipe(fd);
if (pip == 0)
{
pid = fork();
if (pid != 0)
{
close(fd[0]);
while (read(0, &buff,1) > 0 )
{
write (fd[1],&buff,1); /* your old loop forwarded internally in the pipe only*/
}
close(fd[1]);
} else {
dup2(fd[0],0); /* you had dup2(fd[1], 1), replacing stdout of wc with the write end from wc */
close(fd[0]);
close(fd[1]);
execlp ("wc","wc",NULL);
_exit(-1);
}
}
wait(&status); /* reap the child process */
return 0;
}
I'm trying to make a program that executes the following commands connecting the output of one to the input of the next using pipes and taking two arguments DIR (directory) and ARG (filetype, example: jpg).
ls DIR -laR | grep ARG | sort
Here's the code:
int main(int argc, char *argv[]) {
if (argc != 3) {
printf("Invalid arguments. <dir> <arg>\n");
exit(1);
}
int pipe_fd1[2];
int pipe_fd2[2];
pid_t ls_pid, grep_pid;
int status;
pipe(pipe_fd1);
pipe(pipe_fd2);
ls_pid = fork();
if (ls_pid == 0) { //first child ls DIR -laR
dup2(pipe_fd1[1], STDOUT_FILENO);
close(pipe_fd1[0]);
execlp("ls", "ls", argv[1], "-laR", NULL);
} else if (ls_pid > 0) {
grep_pid = fork();
if (grep_pid == 0) { //second child grep ARG
dup2(pipe_fd1[0], STDIN_FILENO);
dup2(pipe_fd2[1], STDOUT_FILENO);
close(pipe_fd1[1]);
close(pipe_fd2[0]);
waitpid(ls_pid, &status, 0);
execlp("grep", "grep", argv[2], NULL);
} else if (grep_pid > 0) { //parent sort
dup2(pipe_fd2[0], STDIN_FILENO);
close(pipe_fd2[1]);
waitpid(grep_pid, &status, 0);
execlp("sort", "sort", NULL);
}
}
return 0;
}
It seems to be stuck? Not sure why?
You never close pipe_fd1 on the parent, so grep and sort doen't know when to stop reading input: because the pipe read and write ends are never closed on the parent, the reader blocks waiting for more input that will never arrive. You need to close it.
Also, you don't need waitpid(): the way pipes work ensures that input flows linearly and in order throughout the pipe.
Here's the working version with these issues addressed:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
int main(int argc, char *argv[]) {
if (argc != 3) {
printf("Invalid arguments. <dir> <arg>\n");
exit(1);
}
int pipe_fd1[2];
int pipe_fd2[2];
pid_t ls_pid, grep_pid;
pipe(pipe_fd1);
ls_pid = fork();
if (ls_pid == 0) { //first child ls DIR -laR
dup2(pipe_fd1[1], STDOUT_FILENO);
close(pipe_fd1[0]);
execlp("ls", "ls", argv[1], "-laR", NULL);
} else if (ls_pid > 0) {
dup2(pipe_fd1[0], STDIN_FILENO);
close(pipe_fd1[1]);
pipe(pipe_fd2);
grep_pid = fork();
if (grep_pid == 0) { //second child grep ARG
dup2(pipe_fd2[1], STDOUT_FILENO);
close(pipe_fd2[0]);
execlp("grep", "grep", argv[2], NULL);
} else if (grep_pid > 0) { //parent sort
dup2(pipe_fd2[0], STDIN_FILENO);
close(pipe_fd2[1]);
execlp("sort", "sort", NULL);
}
}
return 0;
}
This is my code.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
int file,parentID,childID;
pid_t pid;
int main(int argc, char *argv[])
{
if( argc != 2 )
{
printf("ERROR ! You have not write an argument\n");
printf("ERROR ! You give more than one argument");
return 1;
}
file = open(argv[1], O_RDONLY); //open file
if(file<0) //test the file
{
printf("Error open file\n");
printf("ERROR : %s\n", strerror(errno));
return 1;
}
// ---------------------------------------------------------------------
pid = fork();
if( pid == -1) //error fork
{
printf("Error fork\n");
return 1;
}
if(pid == 0) // child process
{
childID = getpid();
printf("Child process %d\n",childID);
// if(childID %2 == 1)
// {
// parentID = getppid();
// printf("Process of father of this child= %d\n",parentID);
// }
}
if( pid == 1)
{
parentID = getppid();
printf("ParentProcess %d\n",parentID);
}
}
I have to write a program to create a child process.Depending on the parity of the child process , the parent should transmit to child a message through a file , the message being taken over and showed by the child process( if the child process is a number that is divizible with 2 it will say -"Good morning!" else "Good night!" ).The parent should wait for the final execution of the child to terminate.
I'm trying really hard to do this exercise and i can't find anywere to explain me how or what function/structure object should i use to do this.Above i tried but i failed , and i understand somehow how fork does but... please help me with this code , or suggest me were should i go to read to make this exercise .Sorry for my bad english spelling.
What documentation are you using for the system calls?
There are a number of ways to do this, but what you probably want to do is create a pipe, and then fork the process. Since a fork copies everything, and child processes inherit the environment, each process has a copy of the file descriptors for the pipe. You can then read/write based on the return value of fork().
int main(int argc, char *argv[])
{
int fd[2];
char in[128], out[128];
if( argc != 2 )
{
printf("ERROR ! You have not write an argument\n");
printf("ERROR ! You give more than one argument");
return 1;
}
if (pipe(fd) == -1)
return 1;
// ---------------------------------------------------------------------
pid = fork();
if (!pid)
read(fd[0], in, 128);
else
write(fd[1], out, strlen(out) + 1);
pipe(2)
note, you usually want to close the file descriptor you're not using for one way communication
I think this is the code.
#include <stdio.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
void sighandler (int sig)
{
if (sig == SIGUSR1)
{
FILE *f;
char line[100];
f = fopen("file","r");
fgets(line, 100, f);
printf ("Procesul copil cu pid %d a primit mesajul %s", getpid(), line);
fclose(f);
}
}
int main ()
{
pid_t pid;
FILE * f;
pid = fork();
if (pid < 0)
{
perror ("Eroare la fork()");
return (1);
}
else
if (pid == 0)
{
signal (SIGUSR1, sighandler);
pause();
return 0;
}
else
{
if (pid % 2 == 0)
{
printf ("Notificam procesul fiu cu pid %d", pid);
f = fopen ("file","w");
fprintf (f,"Good morning!");
fclose(f);
kill (pid, SIGUSR1);
}
else
{
printf ("Notificam procesul fiu cu pid %d", pid);
f = fopen ("file","w");
fprintf (f,"Good night!");
fclose(f);
kill (pid, SIGUSR1);
}
}
wait(NULL);
return 0;
}
I have a program which I would like to sort the first column in a file, from a child process, and return the output to the parent process. How can I retrieve the response from the execlp and print it? Here is what I have so far:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#define WRITE 1
#define READ 0
int main(int argc, char **argv)
{
int i, k;
int p1[2], p2[2];
int p1[2], p2[2];
pid_t childID;
if (pipe(p1) < 0 || pipe(p2) < 0) {
perror("pipe");
exit(0);
}
childID = fork();
if (childID < 0) {
perror("fork");
exit(0);
}
else if (childID == 0){
close(p1[WRITE]);
close(p2[READ]);
dup2(p1[READ], STDIN_FILENO);
close(p1[READ]);
dup2(p2[WRITE], STDOUT_FILENO);
close(p2[WRITE]);
execlp("sort", "-k1", "-n", "temp.txt", (char *)NULL);
perror("exec");
exit(0);
}
else {
//parent process
//Not sure how to get response from exec
}
}
After call execlp(), the memory image of current process will be replaced by the called progame, so you cannot get what you want through return value. What you can do is let the child process write its result to somehere, such as a temporal file or a pipe, and the parent process read the result from this place.
After proper setup a pipe to communite between parent and child processes, you can write the result of child process in its stdout, and read the result in parent processes from its stdin.
Something like this:
else if (childID == 0){
close(p1[READ]);
dup2(p1[WRITE], STDOUT_FILENO);
close(p1[WRITE]);
execlp("sort", "-k1", "-n", "temp.txt", (char *)NULL);
perror("exec");
exit(0);
}
else {
close(p1[WRITE]);
dup2(p1[READ], STDIN_FILENO);
close(p1[READ]);
while (scanf("%ms ", &l) != EOF) {
printf("%s\n", l);
free(l);
}
}
Here is full code:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#define WRITE 1
#define READ 0
int main(int argc, char **argv)
{
int p1[2];
char *l;
pid_t childID;
if (pipe(p1) < 0) {
perror("pipe");
exit(0);
}
childID = fork();
if (childID < 0) {
perror("fork");
exit(0);
}
else if (childID == 0){
close(p1[READ]);
dup2(p1[WRITE], STDOUT_FILENO);
close(p1[WRITE]);
execlp("sort", "-k1", "-n", "temp.txt", (char *)NULL);
perror("exec");
exit(0);
}
else {
close(p1[WRITE]);
dup2(p1[READ], STDIN_FILENO);
close(p1[READ]);
while (scanf("%ms ", &l) != EOF) {
printf("%s\n", l);
free(l);
}
}
return 0;
}
And test file temp.txt:
$ cat temp.txt
a
e
b
d
f
c
Result of a test run:
$ ./a.out
a
b
c
d
e
f