I'm trying to hand pipes with the function below but it results in an infinite loop. My code is based off of this: Implementation of multiple pipes in C.
I'm new to C so I'd appreciate any thoughts or feedback on this solution. Please let me know if any other information would be required. There's no longer an infinite loop if I remove the third if statement, so the issue is somewhere there.
void command_with_pipes(char*** command_table, int count_pipes, bool converted_to_tee) {
int fd[2 * count_pipes];
pid_t pid;
int status;
// Create all necessary pipes at the beginning
for (int i = 0; i < count_pipes; i++) {
if (pipe(fd + i*2) < 0) {
perror("fatal error: pipe");
exit(1);
}
}
// Iterate over every command in the table
for (int commandNum = 0; commandNum <= count_pipes; commandNum++){
pid = fork();
if (pid < 0){
perror("fork error");
exit(-1);
}
else if (pid == 0){
// If it's not the first command, should read to the pipe
if (commandNum != 0) {
if(dup2(fd[(commandNum - 1) * 2], 0) < 0) {
perror("2 can't dup");
exit(1);
}
}
// if not the last command, should write to the pipe
if (commandNum != count_pipes) {
if(dup2(fd[commandNum * 2 + 1], 1) < 0) {
perror("1 can't dup");
exit(1);
}
}
// Last command includes a converted tee, should write to pipe instead of going to standard output
if (commandNum == count_pipes && strcmp(command_table[commandNum][0], "tee") ==0 && converted_to_tee == true){
if(dup2(fd[(commandNum - 1) * 2 + 1], 1) < 0) {
perror("1 can't dup");
exit(1);
}
}
// Close pipes
for (int i = 0; i < 2 * count_pipes; i++){
close(fd[i]);
}
// Execute the command
status = execvp(command_table[commandNum][0], command_table[commandNum]);
if (status < 0) {
perror("exec problem");
exit(1);
}
}
}
// Parent closes everything at the end
for (int i = 0; i < 2 * count_pipes; i++){
close(fd[i]);
}
wait(0);
}
I'm writing a basic shell for course homework that will find a command in the given list of paths, and execute the command. It is also meant to handle pipes.
However, when I fork a child process, I get a "Write error : Broken Pipe" message in gdb, and the program terminates abruptly.
I cannot seem to understand why this is happening, since I've been cautious about opening and closing correct pipes and process forking seems to work as desired. Can someone with more experience in C and unix programming please help me diagnose the problem? Is there something logically incorrect with my fork implementation / pipe implementation?
//commands is of the format {"ls -al", "more", NULL}
//it represents commands connected by pipes, ex. ls -al | more
char **commands = parseArgv(consoleinput, SPECIAL_CHARS[4]);
int numcommands = 0;
while( commands[numcommands]!=NULL )
{
numcommands++;
}
const int numpipes = 2*(numcommands-1);
int pipefds[numpipes];
int i=0;
for(i=0; i<numpipes;i=i+2)
{
pipe(pipefds+i);
}
int pipe_w = 1;
int pipe_r = pipe_w - 3;
int curcommand = 0;
while(curcommand < numcommands)
{
if(pipe_w < numpipes)
{
//open write end
dup2(pipefds[pipe_w], 1);
}
if(pipe_r > 0)
{
//open read end
dup2(pipefds[pipe_r], 0);
}
for(i=0;i<numpipes;i++) //close off all pipes
{
close(pipefds[i]);
}
//Parse current command and Arguments into format needed by execv
char **argv = parseArgv(commands[curcommand], SPECIAL_CHARS[0]);
//findpath() replaces argv[0], i.e. command name by its full path ex. ls by /bin/ls
if(findPath(argv) == 0)
{
int child_pid = fork();
//Program crashes after this point
//Reason: /bin/ls: write error, broken pipe
if(child_pid < 0)
{
perror("fork error:");
}
else if(child_pid == 0) //fork success
{
if(execv(argv[0], argv) == -1)
{
perror("Bad command or filename:");
}
}
else
{
int child_status;
child_pid = waitpid(child_pid, &child_status, 0);
if(child_pid < 0)
{
perror("waitpid error:");
}
}
}
else
{
printf("Bad command or filename");
}
free(argv);
curcommand++;
pipe_w = pipe_w + 2;
pipe_r = pipe_r + 2;
}
//int i=0;
for(i=0;i<numpipes;i++) //close off all pipes
{
close(pipefds[i]);
}
free(commands);
Duplicating the file descriptors after the fork() call, i.e. in the child process, is the correct way.
Also, the waitpid() call makes one child process wait for the other, and the shell hangs. The wait() call should be moved to after the loop, i.e. the parent should wait for all the children.
I'm trying to implement multiple pipes in my shell in C. I found a tutorial on this website and the function I made is based on this example. Here's the function
void executePipes(cmdLine* command, char* userInput) {
int numPipes = 2 * countPipes(userInput);
int status;
int i = 0, j = 0;
int pipefds[numPipes];
for(i = 0; i < (numPipes); i += 2)
pipe(pipefds + i);
while(command != NULL) {
if(fork() == 0){
if(j != 0){
dup2(pipefds[j - 2], 0);
}
if(command->next != NULL){
dup2(pipefds[j + 1], 1);
}
for(i = 0; i < (numPipes); i++){
close(pipefds[i]);
}
if( execvp(*command->arguments, command->arguments) < 0 ){
perror(*command->arguments);
exit(EXIT_FAILURE);
}
}
else{
if(command != NULL)
command = command->next;
j += 2;
for(i = 0; i < (numPipes ); i++){
close(pipefds[i]);
}
while(waitpid(0,0,0) < 0);
}
}
}
After executing it and typing a command like for example ls | grep bin, the shell just hangs there and doesn't output any result. I made sure I closed all pipes. But it just hangs there. I thought that it was the waitpid that's was the problem. I removed the waitpid and after executing I get no results. What did I do wrong? Thanks.
Added code:
void runPipedCommands(cmdLine* command, char* userInput) {
int numPipes = countPipes(userInput);
int status;
int i = 0, j = 0;
pid_t pid;
int pipefds[2*numPipes];
for(i = 0; i < 2*(numPipes); i++){
if(pipe(pipefds + i*2) < 0) {
perror("pipe");
exit(EXIT_FAILURE);
}
}
while(command) {
pid = fork();
if(pid == 0) {
//if not first command
if(j != 0){
if(dup2(pipefds[(j-1) * 2], 0) < 0){
perror(" dup2");///j-2 0 j+1 1
exit(EXIT_FAILURE);
//printf("j != 0 dup(pipefd[%d], 0])\n", j-2);
}
//if not last command
if(command->next){
if(dup2(pipefds[j * 2 + 1], 1) < 0){
perror("dup2");
exit(EXIT_FAILURE);
}
}
for(i = 0; i < 2*numPipes; i++){
close(pipefds[i]);
}
if( execvp(*command->arguments, command->arguments) < 0 ){
perror(*command->arguments);
exit(EXIT_FAILURE);
}
} else if(pid < 0){
perror("error");
exit(EXIT_FAILURE);
}
command = command->next;
j++;
}
for(i = 0; i < 2 * numPipes; i++){
close(pipefds[i]);
puts("closed pipe in parent");
}
while(waitpid(0,0,0) <= 0);
}
}
I believe the issue here is that your waiting and closing inside the same loop that's creating children. On the first iteration, the child will exec (which will destroy the child program, overwriting it with your first command) and then the parent closes all of its file descriptors and waits for the child to finish before it iterates on to creating the next child. At that point, since the parent has closed all of its pipes, any further children will have nothing to write to or read from. Since you are not checking for the success of your dup2 calls, this is going un-noticed.
If you want to keep the same loop structure, you'll need to make sure the parent only closes the file descriptors that have already been used, but leaves those that haven't alone. Then, after all children have been created, your parent can wait.
EDIT: I mixed up the parent/child in my answer, but the reasoning still holds: the process that goes on to fork again closes all of its copies of the pipes, so any process after the first fork will not have valid file descriptors to read to/write from.
pseudo code, using an array of pipes created up-front:
/* parent creates all needed pipes at the start */
for( i = 0; i < num-pipes; i++ ){
if( pipe(pipefds + i*2) < 0 ){
perror and exit
}
}
commandc = 0
while( command ){
pid = fork()
if( pid == 0 ){
/* child gets input from the previous command,
if it's not the first command */
if( not first command ){
if( dup2(pipefds[(commandc-1)*2], 0) < ){
perror and exit
}
}
/* child outputs to next command, if it's not
the last command */
if( not last command ){
if( dup2(pipefds[commandc*2+1], 1) < 0 ){
perror and exit
}
}
close all pipe-fds
execvp
perror and exit
} else if( pid < 0 ){
perror and exit
}
cmd = cmd->next
commandc++
}
/* parent closes all of its copies at the end */
for( i = 0; i < 2 * num-pipes; i++ ){
close( pipefds[i] );
}
In this code, the original parent process creates a child for each command and therefore survives the entire ordeal. The children check to see if they should get their input from the previous command and if they should send their output to the next command. Then they close all of their copies of the pipe file descriptors and then exec. The parent doesn't do anything but fork until it's created a child for each command. It then closes all of its copies of the descriptors and can go on to wait.
Creating all of the pipes you need first, and then managing them in the loop, is tricky and requires some array arithmetic. The goal, though, looks like this:
cmd0 cmd1 cmd2 cmd3 cmd4
pipe0 pipe1 pipe2 pipe3
[0,1] [2,3] [4,5] [6,7]
Realizing that, at any given time, you only need two sets of pipes (the pipe to the previous command and the pipe to the next command) will simplify your code and make it a little more robust. Ephemient gives pseudo-code for this here. His code is cleaner, because the parent and child do not have to do unnecessary looping to close un-needed file descriptors and because the parent can easily close its copies of the file descriptors immediately after the fork.
As a side note: you should always check the return values of pipe, dup2, fork, and exec.
EDIT 2: typo in pseudo code. OP: num-pipes would be the number of pipes. E.g., "ls | grep foo | sort -r" would have 2 pipes.
Here's the correct functioning code
void runPipedCommands(cmdLine* command, char* userInput) {
int numPipes = countPipes(userInput);
int status;
int i = 0;
pid_t pid;
int pipefds[2*numPipes];
for(i = 0; i < (numPipes); i++){
if(pipe(pipefds + i*2) < 0) {
perror("couldn't pipe");
exit(EXIT_FAILURE);
}
}
int j = 0;
while(command) {
pid = fork();
if(pid == 0) {
//if not last command
if(command->next){
if(dup2(pipefds[j + 1], 1) < 0){
perror("dup2");
exit(EXIT_FAILURE);
}
}
//if not first command&& j!= 2*numPipes
if(j != 0 ){
if(dup2(pipefds[j-2], 0) < 0){
perror(" dup2");///j-2 0 j+1 1
exit(EXIT_FAILURE);
}
}
for(i = 0; i < 2*numPipes; i++){
close(pipefds[i]);
}
if( execvp(*command->arguments, command->arguments) < 0 ){
perror(*command->arguments);
exit(EXIT_FAILURE);
}
} else if(pid < 0){
perror("error");
exit(EXIT_FAILURE);
}
command = command->next;
j+=2;
}
/**Parent closes the pipes and wait for children*/
for(i = 0; i < 2 * numPipes; i++){
close(pipefds[i]);
}
for(i = 0; i < numPipes + 1; i++)
wait(&status);
}
The (shortened) relevant code is:
if(fork() == 0){
// do child stuff here
....
}
else{
// do parent stuff here
if(command != NULL)
command = command->next;
j += 2;
for(i = 0; i < (numPipes ); i++){
close(pipefds[i]);
}
while(waitpid(0,0,0) < 0);
}
Which means the parent (controlling) process does this:
fork
close all pipes
wait for child process
next loop / child
But it should be something like this:
fork
fork
fork
close all pipes (everything should have been duped now)
wait for childs
You only need two pipes alternating like below:
typedef int io[2];
extern int I; //piped command current index
extern int pipe_count; //count of '|'
#define CURRENT 0
#define PREVIOUS 1
#define READ 0
#define WRITE 1
#define is_last_command (I == pipe_count)
bool connect(io pipes[2])
{
if (pipe_count)
{
if (is_last_command || I != 0)
dup2(pipes[PREVIOUS][READ], STDIN_FILENO);
if (I == 0 || !is_last_command)
dup2(pipes[CURRENT][WRITE], STDOUT_FILENO);
}
return (true);
}
void close_(io pipes[2])
{
if (pipe_count)
{
if (is_last_command || I != 0)
close(pipes[PREVIOUS][READ]);
if (I == 0 || !is_last_command)
close(pipes[CURRENT][WRITE]);
}
}
void alternate(int **pipes)
{
int *pipe_current;
pipe_current = pipes[CURRENT];
pipes[CURRENT] = pipes[PREVIOUS];
pipes[PREVIOUS] = pipe_current;
}
Example usage:
#define ERROR -1
#define CHILD 0
void execute(char **command)
{
static io pipes[2];
if (pipe_count && pipe(pipes[CURRENT]) == ERROR)
exit_error("pipe");
if (fork()==CHILD && connect(pipes))
{
execvp(command[0], command);
_exit(EXIT_FAILURE);
}
while (wait(NULL) >= 0);
close_(pipes);
alternate((int **)pipes);
}
static void run(char ***commands)
{
for (I = 0; commands[I]; I++)
if (*commands[I])
execute(commands[I]);
}
I'll leave a link to a full working code for someone who needs it.
Building upon the idea of using a maximum of two pipes at a given time mentioned by Christopher Neylan, I put together pseudocode for n-pipes. args is an array of character pointers of size 'args_size' which is a global variable.
// MULTIPLE PIPES
// Test case: char *args[] = {"ls", "-l", "|", "head", "|", "tail", "-4",
0};// "|", "grep", "Txt", 0};
enum fileEnd{READ, WRITE};
void multiple pipes( char** args){
pid_t cpid;
// declare pipes
int pipeA[2]
int pipeB[2]
// I have done getNumberofpipes
int numPipes = getNumberOfPipes;
int command_num = numPipes+1;
// holds sub array of args
// which is a statement to execute
// for example: cmd = {"ls", "-l", NULL}
char** cmd
// iterate over args
for(i = 0; i < args_size; i++){
//
// strip subarray from main array
// cmd 1 | cmd 2 | cmd3 => cmd
// cmd = {"ls", "-l", NULL}
//Open/reopen one pipe
//if i is even open pipeB
if(i % 2) pipe(pipeB);
//if i is odd open pipeA
else pipe(pipeA);
switch(cpid = fork(){
case -1: error forking
case 0: // child process
childprocess(i);
default: // parent process
parentprocess(i, cpid);
}
}
}
// parent pipes must be closed in parent
void parentprocess(int i, pid_t cpid){
// if first command
if(i == 0)
close(pipeB[WRITE]);
// if last command close WRITE
else if (i == numPipes){
// if i is even close pipeB[WRITE]
// if i is odd close pipeA[WRITE]
}
// otherwise if in middle close READ and WRITE
// for appropriate pipes
// if i is even
close(pipeA[READ])
close(pipeB[WRITE])
// if i is odd
close(pipeB[READ])
close(pipeA[WRITE])
}
int returnvalue, status;
waitpid(cpid, returnvalue, status);
}
void childprocess(int i){
// if in first command
if(i == 0)
dup2(pipeB[WRITE], STDOUT_FILENO);
//if in last command change stdin for
// the necessary pipe. Don't touch stdout -
// stdout goes to shell
else if( numPipes == i){
// if i is even
dup2(pipeB[READ], STDIN_FILENO)
//if i is odd
dup2(pipeA[READ], STDIN_FILENO);
}
// otherwise, we are in middle command where
// both pipes are used.
else{
// if i is even
dup2(pipeA[READ], STDIN_FILENO)
dupe(pipeB[WRITE], STDOUT_FILENO)
// if i is odd
dup2(pipeB[READ], STDIN_FILENO)
dup2(pipeA[WRITE], STDOUT_FILENO)
}
// execute command for this iteration
// check for errors!!
// The exec() functions only return if an error has occurred. The return value is -1, and errno is set to indicate the error.
if(exec(cmd, cmd) < 0)
printf("Oh dear, something went wrong with read()! %s\n", strerror(errno));
}
}
Basically what you wanna do is a recursive function where the child executes the first command and the parent executes the second one if no other commands are left or calls the function again.
So I have been trying to implement piping in my own shell program so that I can actually understand what UNIX is doing. I'm very very close at the moment, but for some reason my program is going into an infinite loop when I pipe. I'm pretty sure my problem is stemming from my waitpid arguments not ever letting the last pipe close, because if I make the final loop in the code below be i is less than count-1 instead of i is less than count it will run the command on the left of the pipe. But once I put it back to i is less than count it just loops forever.
if(pipes)
{
for (i=0;i<count-1;i++)
{
if( pipe(fd) ==-1)
{
perror("Pipe failure");
return;
}
read[i+1] = fd[0];
write[i] = fd[1];
}
}
for(i=0;i<count;i++)
{
pid = fork();
if(pid < 0)
{printf("fork failed");}
else if(pid>0)
{pids[i] = pid;}
else
{
if(write[i] != -1)
{
if(dup2(write[i],STDOUT_FILENO) ==-1)
{
perror("dup2 failure");
exit(1);
}
}
if(read[i] !=-1)
{
if (dup2(read[i], STDIN_FILENO)==-1)
{
perror("dup2 failure");
exit(1);
}
}
for (j=0; j<count; j++)
{
close(write[j]);
close(read[j]);
}
execvp(input[i], input);
exit(1);
}//end else
}//end for
for(i=0; i < count; i++){
if(write[i] != -1)
close(write[i]);
if( read[i] != -1)
close (read [i]);
waitpid(pids[i], &status,0);
}
}
return (status);}
I think I'm really really close to the solution but for the time being I'm stuck. I've researched piping a ton, but I guess I'm just not quite getting it. Thanks for any help.
Please format your code. It is unreadable right now.
That being said, a few errors:
You are not setting write[0]. You are setting write[count-1] and read[count].
You are waiting for pids[i] while the parent still has read[i] open. If input[i] wants will not exit until it gets EOF on stdin, it will never happen.
I'm trying to create a simple shell in C for Unix. I've been able to do all the parsing of commands and execution, but I'm having a problem with piping. I think the problem is that I'm not hooking into the correct pipe for the input of the second command.
For example, if I type "ls | wc", it will pause after the "wc" command, which I think is because its waiting for input. I think the problem is when I use dup2(reading[i],0), and its not hooking into the correct pipe.
I know this is a bit of a broad question, but if there are any pointers I could get, I would appreciate it. Here is the code that creates new processes and tries to pipe them.
int fileds[2];
int reading[num_cmds];
int writing[num_cmds];
int p;
for(p=0; p < num_cmds; p++)
{
reading[p] = -1;
writing[p] = -1;
}
int j;
for(j=0; j < num_cmds-1; j++) //Create pipes for commands
{
int fileds[2];
pipe(fileds);
reading[j+1] = fileds[0];
writing[j] = fileds[1];
}
int i = 0;
for(i = 0; i < num_cmds;i++)
{
cmd_args = parse_cmd(cmds[i],output_file,input_file,&run_bg); //Get command and args
pid_t childpid;
int status;
childpid=fork();
if (childpid >= 0)
{
if (childpid == 0)
{
if(writing[i] != -1)
{
dup2(writing[i],1);
close(writing[i]);
}
if(reading[i] != -1)
{
dup2(reading[i],0);
close(reading[i]);
}
int h;
for(h = 0; h < num_cmds; h++)
{
close(writing[h]);
close(reading[h]);
}
if(execvp(cmd_args[0],cmd_args) == -1)
{
perror("Problem with command");
exit(0);
}
}
else
{
wait(&status);
int m;
for(m = 0; m < num_cmds; m++)
{
if( writing[m] != -1) close(writing[m]);
if( reading[m] != -1) close(reading[m]);
}
}
}
else
{
perror("fork");
continue;
}
input_file[0] = 0;
output_file[0] = 0;
run_bg = 0;
}
}
UPDATE: I was able to figure it out, thanks to Richard. It was a combination of closing the file descriptors in the wrong order and not closing some at all. Here's the working code.
int fileds[2];
int reading[num_cmds];
int writing[num_cmds];
int p;
for(p=0; p < num_cmds; p++)
{
reading[p] = -1;
writing[p] = -1;
}
int j;
for(j=0; j < num_cmds-1; j++)
{
int fileds[2];
pipe(fileds);
reading[j+1] = fileds[0];
writing[j] = fileds[1];
}
int i = 0;
for(i = 0; i < num_cmds;i++)
{
cmd_args = parse_cmd(cmds[i],output_file,input_file,&run_bg);
pid_t childpid;
int status;
childpid=fork();
if (childpid >= 0)
{
if (childpid == 0)
{
if(writing[i] != -1)
{
close(1);
dup2(writing[i],1);
}
if(reading[i] != -1)
{
close(0);
dup2(reading[i],0);
}
if(execvp(cmd_args[0],cmd_args) == -1)
{
perror("Problem with command");
exit(0);
}
}
else
{
wait(&status);
close(writing[i]);
if(i > 0)
{
close(reading[i]);
}
}
}
else
{
perror("fork");
}
input_file[0] = 0;
output_file[0] = 0;
run_bg = 0;
}
I think your problem may be that you wait for each process inside the loop and then close all the file descriptors. This makes the file descriptors invalid for the next call to dup2() and results in stdin for the next process staying unchanged.
Just a guess, I haven't run the code.
When I type "ls | wc" wc does as expected and prints the number of words output by the ls command. Remember that when you are piping commands using "|" you don't need to create pipes in your application. The first command needs to output to stdout and the second command needs to read that output from standard in.