I am doing this homework for some time and it's giving me a headache.
Write a program that writes the integer “i+1” into element “i” of a table of MAXBUF
integers (for every element of the table). MAXBUF should be initially “#define”d as 10 in the source
code of the program. Then, using only one write() operation, the program should write the entire
table of integers in binary format into an initially truncated file, named “filetable.bin”. In the next
step the program should create a child process, and then print the message “The parent process
is terminating.”, and then exit. The child process should separately read, in binary format, from
the file each integer in the same order as the integers are stored in the file, and print each such
integer to the standard output. In the final step of the program, the child process should wait for its
parent process to terminate, and then print to the standard output the message “The child process
is terminating.”, and then terminate. All the operations on the “filetable.bin” file should be
performed using system calls.
#include <fcntl.h>
#include <sys/stat.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#define MAXBUF 10
int decimalToBinary(int n);
int main(int argc, char *argv []) {
int i, fd, n, status, childpid;
char buffer[MAXBUF];
pid_t pid = (long)getpid();
fd = open("filetable.bin", O_CREAT | O_RDWR);
for(i=0; i<MAXBUF; i++) {
n=i+1;
buffer[i] = decimalToBinary(n);
}
write(fd,buffer,sizeof(buffer));
childpid = fork();
if(childpid >0) {
printf("The parent process terminating.\n");
exit(0); // or kill(pid, SIGKILL)
}
if(childpid < 0) {
perror("Failed to fork\n");
}
else {
read(fd,&childpid,sizeof(childpid));
write(STDOUT_FILENO,&childpid,sizeof(childpid));
wait(&status);
printf("The child process is terminating\n");
exit(1);
}
return(0);
}
int decimalToBinary(int n) {
int remainder, binary=0, i=1;
while(n!=0) {
remainder = n%2;
n = n/2;
binary = binary + (remainder*i);
i = i*10;
}
return binary;
}
My problem is how do I read from the childpid and write with it in STDOUT?
EDIT: It appears in the output: "The parent process is terminating\n" "The child process is terminating\n". It's missing the STDOUT
Related
This is a C program where the Parent process tries to write a message to its child process using a simple pipe. The expected output is obtained.
According to the code, the parent calls wait() and waits until the child process exits(returns).
Also, the child process calls read(), which waits for something to be written through the other pipe end?
Thus, shouldn't both processes keep waiting for each other and cause a deadlock? How is it possible that the program works properly?
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#define MSGSIZE 16
char *msg1 = "Hello,Once";
char *msg2 = "Hello,Twice";
char *msg3 = "Hello,Thrice";
int main()
{
char buff[MSGSIZE];
int pi[2],pid,nbytes;
if(pipe(pi) < 0) _exit(1);
if((pid=fork()) > 0)
{
close(pi[0]);
write(pi[1],msg1,MSGSIZE);
write(pi[1],msg2,MSGSIZE);
write(pi[1],msg3,MSGSIZE);
close(pi[1]);
wait(NULL);
}
else
{
close(pi[1]);
while((nbytes = read(pi[0],buff,MSGSIZE)) > 0) printf("%s\n",buff);
printf("Reading Completed\n");
close(pi[0]);
if(nbytes != 0) _exit(2);
}
return 0;
}
I am trying to write a program so that the parent and child process can communicate back and forth between each other. The parent process and the child process ought to print the values from 1-100 where each process prints the value incrementing it by 1 each time. Now the issue I face is that, I know nothing much about pipes. What I gather from reading materials online is that I can use a pipe to read and write values. I have leveraged this to print something in the child process, and send back something to the parent. Now, I am not sure how to get the parent to return to the child after printing for itself? I know my code is probably all wrong, but I am really not sure what I should do.
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int main(int argc, const char * argv[]) {
int fd[2];
if (pipe(fd)== -1){
printf("An error occured while opening the pipe\n");
}
int id = fork();
int i = 0;
if (id == 0){
close(fd[0]);
printf("In child: %d", i);
i ++;
write(fd[1], &i, sizeof(int));
close(fd[1]);
} else {
wait(NULL);
close(fd[1]);
int y;
read(fd[0],&y, sizeof(int));
close(fd[0]);
}
}
To keep it simple, it's up to you to check return values and handle errors. This will only do it between 0 - 9 and you will have to expand the mathematics.
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int main(int argc, char *argv[])
{
int pipefd_1[2];
int pipefd_2[2];
pid_t cpid;
pipe(pipefd_1);
pipe(pipefd_2);
cpid = fork();
if (cpid == 0) { /* Child reads from pipe 1, writes to pipe 2*/
char cval[] = {'0'};
close(pipefd_1[1]); /* Close unused write and read ends */
close(pipefd_2[0]);
while (atoi(cval) != 9) {
read(pipefd_1[0], cval, 1);
printf("Child print %d\n", atoi(cval));
cval[0] += 1;
write(pipefd_2[1], cval, 1);
}
} else {
char cval[] = {'0'}; /* Parent writes buf to pipe 1 */
close(pipefd_1[0]); /* Close unused read end */
close(pipefd_2[1]);
while (atoi(cval) != 9) {
write(pipefd_1[1], cval, 1);
read(pipefd_2[0], cval, 1);
printf("Parent print %d\n", atoi(cval));
cval[0] += 1;
}
}
}
Output
I wrote this code in order to calculate the factorial of a number using processes and pipe(). I wanted to pass on the result from the child process to the child process. For example, to create calculate 5! the main which is the father sends the number 1 in the pipe. Then the first child is created and does 1*2, then it pushes in the pipe the number 2, the second child does 2*3 pushes the result in the pipe etc... Also, I use argv[1][0] thinking that we run the program like this (./ex3 5) where 5 is the number of which the factorial we would like to find. After running the program though, I noticed that a lot of child process was created (I only wanted 4). Why is that?
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
int fd[2];
int w,count=2;
void child_creator(){
pid_t child;
child=fork();
if (child==0) {
close(fd[1]);
read(fd[0],&w,sizeof(w));
close(fd[0]);
w=w*count;
count++;
printf("I am child %d , my father is %d , the prod is %d\n",getpid(),getppid(),w);
sleep(1);
close(fd[0]);
write(fd[1],&w,sizeof(w));
close(fd[1]);
}
}
int main(int argc , char **argv){
int fact=argv[1][0]-'0';
pipe(fd);
w=1;
for (int i=0; i<fact-1; i++){
printf("this is i %d\n", i);
child_creator();
}
return 0;
}
After a suggested answer I tried this code:
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
int fd[1000][2];
int w,count=1,j=0;
void child_creator(){
pid_t child;
j++;
pipe(fd[j]);
child=fork();
if (child==0) {
close(fd[j-1][1]);
read(fd[j-1][0],&w,sizeof(w));
close(fd[j-1][0]);
w=w*count;
printf("I am child %d , my father is %d , the prod is %d\n",getpid(),getppid(),w);
sleep(1);
close(fd[j-1][0]);
write(fd[j][1],&w,sizeof(w));
close(fd[j][1]);
exit(0);
}
}
int main(int argc , char **argv){
int fact=argv[1][0]-'0';
w=1;
for (int i=0; i<fact-1; i++){
count++;
child_creator();
sleep(2);
}
return 0;
}
Both the parent and child are returning to the for loop in main(). Since the child doesn't need to do anything after it writes its result, it should just exit rather than returning.
You also have problems with your handling of the pipe file descriptors. You do close(fd[1]) at the beginning of the child, but later try to write(fd[1],&w,sizeof(w)). You can't write to a closed FD. You don't need to close anything until the child is exiting, and exiting a process automatically closes all its files.
void child_creator(){
pid_t child;
child=fork();
if (child==0) {
read(fd[0],&w,sizeof(w));
w=w*count;
count++;
printf("I am child %d , my father is %d , the prod is %d\n",getpid(),getppid(),w);
sleep(1);
write(fd[1],&w,sizeof(w));
exit(0);
}
}
I am having serious trouble working with pipes in C. I'm supposed to take in arguments from the command line (example: ./myprogram 123 45 67), read the arguments one character at a time into a buffer, send the character to the child process to be counted, and then return the total number of characters read to the parent process. My code is as follows(note: the comments are what I'm supposed to be doing):
// Characters from command line arguments are sent to child process
// from parent process one at a time through pipe.
// Child process counts number of characters sent through pipe.
// Child process returns number of characters counted to parent process.
// Parent process prints number of characters counted by child process.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
static int toChild[2];
static int fromChild[2];
static char buffer;
int main(int argc, char **argv)
{
int status;
int nChars = 0;
pid_t pid;
pipe(toChild);
pipe(fromChild);
if ((pid = fork()) == -1) {
printf("fork error %d\n", pid);
return -1;
}
else if (pid == 0) {
close(toChild[1]);
close(fromChild[0]);
// Receive characters from parent process via pipe
// one at a time, and count them.
int count = 0;
printf("child about to read\n");
while(read(toChild[0], &buffer, 1)){
count++;
}
// Return number of characters counted to parent process.
write(fromChild[1], &count, sizeof(count));
close(toChild[0]);
close(fromChild[1]);
printf("child exits\n");
}
else {
close(toChild[0]);
close(fromChild[1]);
// -- running in parent process --
printf("CS201 - Assignment 3 - Chris Gavette\n");
write(toChild[1], &argv[1], 1);
// Send characters from command line arguments starting with
// argv[1] one at a time through pipe to child process.
read(fromChild[0], &nChars, 1);
// Wait for child process to return. Reap child process.
// Receive number of characters counted via the value
// returned when the child process is reaped.
close(toChild[1]);
close(fromChild[0]);
waitpid(pid, &status, 0);
printf("child counted %d chars\n", nChars);
printf("parent exits\n");
return 0;
}
}
The child process seems to hang even though I've closed both ends of both pipes.
For starters, this is wrong.
write(toChild[1], &count, 1)
It will eventually contribute to your problem. count is a int, not char or unsigned char. You need to send sizeof(count). Also, the read-function upon hitting an error will return EOF, which is non-zero, so your child exit condition is not appropriate. it should look something like this:
while(read(toChild[0], &buffer, 1) == 1)
Finally, your parent process should cycle through each argument in argv[] sending each as a strlen sized buffer.
I'm nearly certain this is what you're trying to do. Note that in order to maintain sanity in knowing which descriptor is used for a specific purpose, I prefer using a #define to note what each process uses for reading and writing. This can be extended to any number of processes, btw, which I'm sure is not too far down the line for your next assignment:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
// P0_READ - parent read source
// P0_WRITE - parent write target
// P1_READ - child read source
// P1_WRITE - child write target
#define P0_READ 0
#define P1_WRITE 1
#define P1_READ 2
#define P0_WRITE 3
#define N_PIPES 4
int main(int argc, char **argv)
{
int fd[N_PIPES], count = 0, i;
pid_t pid;
char c;
if (pipe(fd) || pipe(fd+2))
{
perror("Failed to open pipe(s)");
return EXIT_FAILURE;
}
// fork child process
if ((pid = fork()) == -1)
{
perror("Failed to fork child process");
return EXIT_FAILURE;
}
// child process
if (pid == 0)
{
// close non P1 descriptors
close(fd[P0_READ]);
close(fd[P0_WRITE]);
// get chars from input pipe, counting each one.
while(read(fd[P1_READ], &c, 1) == 1)
count++;
printf("Child: count = %d\n", count);
write(fd[P1_WRITE], &count, sizeof(count));
// close remaining descriptors
close(fd[P1_READ]);
close(fd[P1_WRITE]);
return EXIT_SUCCESS;
}
// parent process. start by closing unused descriptors
close(fd[P1_READ]);
close(fd[P1_WRITE]);
// send each arg
for (i=1; i<argc; ++i)
write(fd[P0_WRITE], argv[i], strlen(argv[i]));
// finished sending args
close(fd[P0_WRITE]);
// Wait for child process to return.
wait(NULL);
// wait for total count
if (read(fd[P0_READ], &count, sizeof(count)) == sizeof(count))
printf("Parent: count = %d\n", count);
// close last descriptor
close(fd[P0_READ]);
return 0;
}
Input
./progname argOne argTwo
Output
Child: count = 12
Parent: count = 12
Edit: Single Pipe with Child Return Status
It seems from the comments of the original question your assignment may call for reaping the return status of the child process as the result count rather than returning it in a pipe. In doing so, you can do this with a single pipe-descriptor pair. I prefer the first method, but this works as well:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
// P0_WRITE - parent write target
// P1_READ - child read source
#define P1_READ 0
#define P0_WRITE 1
#define N_PIPES 2
int main(int argc, char **argv)
{
int fd[N_PIPES], count = 0;
pid_t pid;
char c;
if (pipe(fd))
{
perror("Failed to open pipe(s)");
return EXIT_FAILURE;
}
// fork child process
pid = fork();
if (pid == -1)
{
perror("Failed to fork child process");
return EXIT_FAILURE;
}
if (pid == 0)
{
// close non P1 descriptors
close(fd[P0_WRITE]);
// Return number of characters counted to parent process.
while(read(fd[P1_READ], &c, 1) == 1)
++count;
close(fd[P1_READ]);
printf("Child: count = %d\n", count);
return count;
}
// parent process. start by closing unused descriptors
close(fd[P1_READ]);
// eacn each arg entirely
for (int i=1; i<argc; ++i)
write(fd[P0_WRITE], argv[i], strlen(argv[i]));
// finished sending args
close(fd[P0_WRITE]);
// Wait for child process to return.
if (wait(&count) == -1)
{
perror("Failed to wait for child process");
return EXIT_FAILURE;
}
printf("Parent: count = %d\n", WEXITSTATUS(count));
return 0;
}
The results are the same, but note this is a biach to to debug as most debuggers will signal-trip on your child process and the real exit status is lost. On my Mac, for example, running this under Xcode trips:
Failed to wait for child process: Interrupted system call
while running from the command line gives:
Child: count = 12
Parent: count = 12
One of the many reasons I prefer the two-pipe methodology.
Im creating pipes for multiple children. The code catches a signal fro the keyboard and then the child should sent a message to the father with its result. The handling of the signal works fine but i cant sent the result to the father. I am i doing something wrong here?
The code of the father process is :
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
pid_t *childs; //array for storing childs pids
int number_of_childs;//variable for the number of childs
int exit_count=0;
int *fd;
/*Handler for the SIGINT signal*/
void control_handler(int sig)
{
if (sig==SIGINT){
int j,bytes;
char message[512];
/*Sending SIGUSR1*/
for (j=0;j<number_of_childs;j++)
{
kill(childs[j],SIGUSR1);
}
/*Reading from PIPES*/
for (j=0;j<number_of_childs;j++)
{
close(fd[(2*j)+1]);
bytes = read(fd[(2*j)], message, sizeof(message));
printf("Read from: %s\n",message);
close(fd[2*j]);
}
}
if (sig==SIGUSR2)
{
exit_count++;
}
}
main (int argc,char *argv[]){
int i,child_status;
char cast[512];
char cast2[512];
char cast3[512];
int pid;
number_of_childs=atoi(argv[1]);
signal(SIGINT,control_handler);
signal(SIGUSR2,control_handler);
/*Creating array for children pipes*/
fd=(int*)malloc((2*number_of_childs)*sizeof(int));
/*array that holds the pids for every child used in sending signals*/
childs=malloc(number_of_childs*sizeof (pid_t));
for (i=0;i<number_of_childs;i++){
pid=fork();
/*Create pipes to communicate with all children*/
if(pipe(fd+(2*i))==-1)
{
perror("pipe");exit(1);
}
/*Fathers code goes here*/
if(pid!=0)
{
printf("Parent process: PID= %d,PPID=%d, CPID=%d \n",getpid(),getppid(),pid);
childs[i]=pid; // Keep all your childs in an array
printf("Child:%d\n",childs[i]);
}
/*If you are a child*/
else
{
/*Change the code for the childs and set the time of execution*/
sprintf(cast,"%d",i+1); // make the time char
sprintf(cast2,"%d",(2*i)+1); //make the pipe char
sprintf(cast3,"%d",number_of_childs);
execl("./Child.out","",cast,cast2,cast3,NULL);
}
}
/*Father should never terminate*/
while (exit_count!=number_of_childs);
printf("Father pospastex!!\n");
}
the code for the child children is :
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#include <time.h>
#include <string.h>
#define WRITE 0
#define READ 1
/*Global declerations*/
int alarmflag=0;
double result=0;
int count=0;
int global_pipe;
int *fd;
/*Handler for the alarm and SIGUSR1 signal*/
void signal_handler (int sig)
{
if(sig==SIGALRM)
{
printf("Im child with pid:%d im going to die my value is %lf \n",getpid(),result);
alarmflag=1;
}
if(sig==SIGUSR1)
{
count++;
char message[512];
if(count==1)
{
close(fd[global_pipe-1]);
sprintf(message,"%d,%lf",getpid(),result);
write(fd[global_pipe],message,strlen(message)+1);
close(fd[global_pipe]);
//printf("PID:%d report: %lf\n",getpid(),result);
}
if(count==2)
{
close(fd[global_pipe-1]);
sprintf(message,"%d,%lf",getpid(),result);
write(fd[global_pipe],message,strlen(message)+1);
close(fd[global_pipe]);
//printf("PID:%d report2 : %lf\n",getpid(),result);
//kill(getppid(),SIGUSR2);
//exit(0);
}
}
if(sig==SIGINT)
{
/*Do nothing*/
}
}
double p_calculation ()
{
int i=2;
result=3;
double prosimo=-1;
while(!alarmflag)
{
prosimo=prosimo*(-1);
result=result+(prosimo*(4/((double)i*((double)i+1)*((double)i+2))));
i=i+2;
}
}
main(int argc,char *argv[])
{
int pipe;
int size_fd;
size_fd=(atoi(argv[3]));
pipe=(atoi(argv[2]));
global_pipe=pipe;
fd=(int*)malloc(size_fd*sizeof(int));
/*handling signals*/
signal(SIGALRM,signal_handler);
signal(SIGUSR1,signal_handler);
signal(SIGINT,signal_handler);
/*Notify for execution time*/
printf("PID : %d with PPID : %d executing for %d seconds \n",getpid(),getppid(),atoi(argv[1]));
//printf("pipe:%d\n",pipe);
/*end this after the value passed as argument*/
alarm(atoi(argv[1]));
p_calculation();
/*Notify for finish*/
printf("Done!!!\n");
}
There are a number of problems related to your pipes:
you call pipe AFTER you call fork, so you end up with two independent pipes (one in the child and one in the parent). The parent listens to the one it creates (and noone is writing to), so it never sees anything.
you pass the index in your global fd array to the child rather than the file descriptor of the pipe. The child has its own global fd array that contains random garbage, so you are essentially writing to a random file descriptor rather than the pipe.
you don't close the unneeded pipe ends in the various processes they exist in, so you cna never reliably get EOFs
If you search for pipe+child here on stackoverflow, you'll see a large number of questions with example code trying to do things similar to what you are doing -- you may find it useful to read through those questions and answers.