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How to use dup2() and pipe() to implement “ls | wc” in my shell? [duplicate]

Classic C. Using pipes in execvp function, stdin and stdout redirectionHow do you set, clear, and toggle a single bit?pipe not being read by subprocess?Pipe with printf commanddup2 - Creating a piping programexecvp() never finishes on pipeUnix - Pipe, forks, execlp, dup2, c programPipes and Redirection, small program to test understandingC - dup2() not executingdup2 for Input File Redirection Not WorkingPiping/dup2() not working properly (Implementing Unix Shell in C)

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-2

This question already has an answer here:

• 
  Classic C. Using pipes in execvp function, stdin and stdout redirection
  
  2 answers
  
  

How would I use the dup2() and pipe() functions to commit a redirect of the output of ls into the input of wc. Therefore printing the word count.

here is my attempt, however this is not working.

 if(!strcmp(*ptr,"|")) //argv[] contains the input partitioned by spaces
 //ignore the ptr for this case
char arr[2];
pipe(arr); // 0 is read 1 is write
dup2(stdout,arr[1]); // my redirected output

int a = fork();
if(a == 0)
 close(arr[0]);
 execvp(argv[0],argv);

 wait(NULL);

 dup2(0,arr[0]); // my redirected input 

int b = fork();
 if(b == 0)
 close(arr[1]);
 execvp(argv[2],argv);
 
 wait(NULL);

 close(arr[0]);
 close(arr[1]);

return;

c

share|improve this question

edited Mar 21 at 18:19

Joshua

24.3k550104

asked Mar 9 at 0:58

pgh78pgh78

1

marked as duplicate by Charles Duffy shell
Users with the  shell badge can single-handedly close shell questions as duplicates and reopen them as needed.

StackExchange.ready(function()
if (StackExchange.options.isMobile) return;

$('.dupe-hammer-message-hover:not(.hover-bound)').each(function()
var $hover = $(this).addClass('hover-bound'),
$msg = $hover.siblings('.dupe-hammer-message');

$hover.hover(
function()
$hover.showInfoMessage('',
messageElement: $msg.clone().show(),
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dismissable: false,
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StackExchange.helpers.removeMessages();

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Mar 9 at 21:19

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  We really should build a canonical instance for this -- someone asks a new version of this question every week at least.
  
  – Charles Duffy
  Mar 9 at 1:25
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  stdout is a FILE *. The first argument to dup2 should be an int.
  
  – William Pursell
  Mar 9 at 1:25
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @CharlesDuffy: homework + each question unique issues each time
  
  – Joshua
  Mar 9 at 1:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @CharlesDuffy I would definitely support a canonical instance for "how do I pipe two programs together in C"
  
  – that other guy
  Mar 9 at 1:33
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Joshua, ...not that unique. I'm pretty sure we've seen 80% of the ways people can screw it up by now. Even if we can tell people to fix the mistakes covered in the canonical instance and come back only with new/different bugs, that's a big improvement from the status quo.
  
  – Charles Duffy
  Mar 9 at 3:07
  
  

  
  
  
  

 | 
show 1 more comment

-2

This question already has an answer here:

• 
  Classic C. Using pipes in execvp function, stdin and stdout redirection
  
  2 answers
  
  

How would I use the dup2() and pipe() functions to commit a redirect of the output of ls into the input of wc. Therefore printing the word count.

here is my attempt, however this is not working.

 if(!strcmp(*ptr,"|")) //argv[] contains the input partitioned by spaces
 //ignore the ptr for this case
char arr[2];
pipe(arr); // 0 is read 1 is write
dup2(stdout,arr[1]); // my redirected output

int a = fork();
if(a == 0)
 close(arr[0]);
 execvp(argv[0],argv);

 wait(NULL);

 dup2(0,arr[0]); // my redirected input 

int b = fork();
 if(b == 0)
 close(arr[1]);
 execvp(argv[2],argv);
 
 wait(NULL);

 close(arr[0]);
 close(arr[1]);

return;

c

share|improve this question

edited Mar 21 at 18:19

Joshua

24.3k550104

asked Mar 9 at 0:58

pgh78pgh78

1

marked as duplicate by Charles Duffy shell
Users with the  shell badge can single-handedly close shell questions as duplicates and reopen them as needed.

StackExchange.ready(function()
if (StackExchange.options.isMobile) return;

$('.dupe-hammer-message-hover:not(.hover-bound)').each(function()
var $hover = $(this).addClass('hover-bound'),
$msg = $hover.siblings('.dupe-hammer-message');

$hover.hover(
function()
$hover.showInfoMessage('',
messageElement: $msg.clone().show(),
transient: false,
position: my: 'bottom left', at: 'top center', offsetTop: -7 ,
dismissable: false,
relativeToBody: true
);
,
function()
StackExchange.helpers.removeMessages();

);
);
);
Mar 9 at 21:19

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  We really should build a canonical instance for this -- someone asks a new version of this question every week at least.
  
  – Charles Duffy
  Mar 9 at 1:25
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  stdout is a FILE *. The first argument to dup2 should be an int.
  
  – William Pursell
  Mar 9 at 1:25
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @CharlesDuffy: homework + each question unique issues each time
  
  – Joshua
  Mar 9 at 1:27
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @CharlesDuffy I would definitely support a canonical instance for "how do I pipe two programs together in C"
  
  – that other guy
  Mar 9 at 1:33
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Joshua, ...not that unique. I'm pretty sure we've seen 80% of the ways people can screw it up by now. Even if we can tell people to fix the mistakes covered in the canonical instance and come back only with new/different bugs, that's a big improvement from the status quo.
  
  – Charles Duffy
  Mar 9 at 3:07
  
  

  
  
  
  

 | 
show 1 more comment

This question already has an answer here:

• 
  Classic C. Using pipes in execvp function, stdin and stdout redirection
  
  2 answers
  
  

How would I use the dup2() and pipe() functions to commit a redirect of the output of ls into the input of wc. Therefore printing the word count.

here is my attempt, however this is not working.

 if(!strcmp(*ptr,"|")) //argv[] contains the input partitioned by spaces
 //ignore the ptr for this case
char arr[2];
pipe(arr); // 0 is read 1 is write
dup2(stdout,arr[1]); // my redirected output

int a = fork();
if(a == 0)
 close(arr[0]);
 execvp(argv[0],argv);

 wait(NULL);

 dup2(0,arr[0]); // my redirected input 

int b = fork();
 if(b == 0)
 close(arr[1]);
 execvp(argv[2],argv);
 
 wait(NULL);

 close(arr[0]);
 close(arr[1]);

return;

c

share|improve this question

edited Mar 21 at 18:19

Joshua

24.3k550104

asked Mar 9 at 0:58

pgh78pgh78

1

 if(!strcmp(*ptr,"|")) //argv[] contains the input partitioned by spaces
 //ignore the ptr for this case
char arr[2];
pipe(arr); // 0 is read 1 is write
dup2(stdout,arr[1]); // my redirected output

int a = fork();
if(a == 0)
 close(arr[0]);
 execvp(argv[0],argv);

 wait(NULL);

 dup2(0,arr[0]); // my redirected input 

int b = fork();
 if(b == 0)
 close(arr[1]);
 execvp(argv[2],argv);
 
 wait(NULL);

 close(arr[0]);
 close(arr[1]);

return;

share|improve this question

edited Mar 21 at 18:19

Joshua

24.3k550104

asked Mar 9 at 0:58

pgh78pgh78

1

share|improve this question

edited Mar 21 at 18:19

Joshua

24.3k550104

asked Mar 9 at 0:58

pgh78pgh78

1

edited Mar 21 at 18:19

Joshua

24.3k550104

edited Mar 21 at 18:19

Joshua

24.3k550104

asked Mar 9 at 0:58

pgh78pgh78

1

asked Mar 9 at 0:58

pgh78pgh78

1

1 Answer
1

active

oldest

votes

0

So you have 6obvious mistakes.

1. Indent your code.


2. Only call dup2 in the child processes.


3. 1 not stdout, 0 not stdin.


4. If execvp fails, call _exit.


5. wait(NULL) is before the second fork not after


6. arguments to dup2 are swapped

Because this is homework I'm not going to just give you the answer. Get this into shape and ping me and I'll see what more I can find.

share|improve this answer

edited Mar 9 at 1:32

answered Mar 9 at 1:26

JoshuaJoshua

24.3k550104

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, the dup2 arguments are switched
  
  – that other guy
  Mar 9 at 1:29
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also his arr is the wrong type.
  
  – Shawn
  Mar 9 at 1:38
  

  
  
  
  

add a comment | 

0

So you have 6obvious mistakes.

1. Indent your code.


2. Only call dup2 in the child processes.


3. 1 not stdout, 0 not stdin.


4. If execvp fails, call _exit.


5. wait(NULL) is before the second fork not after


6. arguments to dup2 are swapped

Because this is homework I'm not going to just give you the answer. Get this into shape and ping me and I'll see what more I can find.

share|improve this answer

edited Mar 9 at 1:32

answered Mar 9 at 1:26

JoshuaJoshua

24.3k550104

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, the dup2 arguments are switched
  
  – that other guy
  Mar 9 at 1:29
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also his arr is the wrong type.
  
  – Shawn
  Mar 9 at 1:38
  

  
  
  
  

add a comment | 

0

So you have 6obvious mistakes.

1. Indent your code.


2. Only call dup2 in the child processes.


3. 1 not stdout, 0 not stdin.


4. If execvp fails, call _exit.


5. wait(NULL) is before the second fork not after


6. arguments to dup2 are swapped

Because this is homework I'm not going to just give you the answer. Get this into shape and ping me and I'll see what more I can find.

share|improve this answer

edited Mar 9 at 1:32

answered Mar 9 at 1:26

JoshuaJoshua

24.3k550104

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, the dup2 arguments are switched
  
  – that other guy
  Mar 9 at 1:29
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also his arr is the wrong type.
  
  – Shawn
  Mar 9 at 1:38
  

  
  
  
  

add a comment | 

So you have 6obvious mistakes.

1. Indent your code.


2. Only call dup2 in the child processes.


3. 1 not stdout, 0 not stdin.


4. If execvp fails, call _exit.


5. wait(NULL) is before the second fork not after


6. arguments to dup2 are swapped

Because this is homework I'm not going to just give you the answer. Get this into shape and ping me and I'll see what more I can find.

share|improve this answer

edited Mar 9 at 1:32

answered Mar 9 at 1:26

JoshuaJoshua

24.3k550104

share|improve this answer

edited Mar 9 at 1:32

answered Mar 9 at 1:26

JoshuaJoshua

24.3k550104

answered Mar 9 at 1:26

JoshuaJoshua

24.3k550104

answered Mar 9 at 1:26

JoshuaJoshua

24.3k550104