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Writing c code for who | sort | lp

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# 8  
Old 01-29-2003
To redirect as in "2 > log.dat", you close() fd 2 and then open() log.dat prior to the exec().
# 9  
Old 05-23-2007
Hi Perderabo

I have got a similar problem, hope you can help Smilie

I want to execute sort | uniq | wc.

So, on the command line it would be:

sort file1 | uniq -c | wc -l

But if for some reason sort fails (for e.g: if the /tmp fir is full) the above command returns 0 as the answer. I am not able to catch the status of each individual command in the pipe.


So the code I had before is:

Code:
FILE *fptr;

fptr = popen("sort file1 | uniq -c | wc -l" , "r");

if (fptr != NULL) {
     fscanf(fptr, "%lu", &uniqLines);
} else {
    //ERROR
}


What I want is, if sort fails, I should be able to catch it.

Therefore I had to implement pipe, to get the status of each individual calls.

So now, based on your program above, I have the following code:

Code:
#include <unistd.h>

int main()
{
        int pipe1[2]; /* pipe for:  sort ------> uniq */
        int pipe2[2]; /* pipe for:  uniq ------> wc   */

        pipe(pipe1);

        if(!fork()) {
                close(1);
                dup(pipe1[1]);
                close(pipe1[0]);
                close(pipe1[1]);
                execlp("sort","sort","work.fp",NULL);
                /* sort failed */
                printf("\n sort failed");

        } else {
                
                pipe(pipe2);

                if (!fork()) {
                        close(0);
                        dup(pipe1[0]);
                        close(1);
                        dup(pipe2[1]);
                        close(pipe1[0]);
                        close(pipe1[1]);
                        close(pipe2[0]);
                        close(pipe2[1]);
                        execlp("uniq","uniq","-c",NULL);
                } else {
                        close(0);
                        dup(pipe2[0]);
                        close(pipe1[0]);
                        close(pipe1[1]);
                        close(pipe2[0]);
                        close(pipe2[1]);
                        execlp("wc","wc","-l",NULL);
                }
        }

}

This works great !!!

However, what I want is to capture the output of sort | uniq | wc in a variable (like I did before) and not print it on stdout.

Code:
fscanf(fptr, "%lu", &uniqLines);

What to do ???
# 10  
Old 05-23-2007
Quote:
Originally Posted by the_learner
What to do ???
Open a pipe for the last process's stdout and read from it.
# 11  
Old 05-24-2007
Thanks porter..

Does this mean I will need another child-parent processes using fork().. that means another level of if-else ??

Do you mean something like:

Code:
#include <unistd.h>

int main()
{
        int pipe1[2]; /* pipe for:  sort ------> uniq */
        int pipe2[2]; /* pipe for:  uniq ------> wc   */
        int pipe3[2];

        pipe(pipe1);

        if(!fork()) {
                close(1);
                dup(pipe1[1]);
                close(pipe1[0]);
                close(pipe1[1]);
                execlp("sort","sort","work.fp",NULL);
                /* sort failed */
                printf("\n sort failed");

        } else {
                
                pipe(pipe2);

                if (!fork()) {
                        close(0);
                        dup(pipe1[0]);
                        close(1);
                        dup(pipe2[1]);
                        close(pipe1[0]);
                        close(pipe1[1]);
                        close(pipe2[0]);
                        close(pipe2[1]);
                        execlp("uniq","uniq","-c",NULL);
                } else {
                        
                        pipe(pipe3);
                        if (!fork()) {
                                 close(0);
                                 dup(pipe2[0]);
                                 close(1);
                                 dup(pipe3[1]);                                
                                 close(pipe1[0]);
                                 close(pipe1[1]);
                                 close(pipe2[0]);
                                 close(pipe2[1]);
                                 execlp("wc","wc","-l",NULL);
                         } else {
                                 //read from pipe 3
                        }
                }
        }

}

# 12  
Old 06-25-2007
Hi, it seems that a few posts got deleted..

anyway,

I think the following code works to get the status of sort (thats what I wanted)

I call waitpid() to get the status of sort. And then WEXITSTATUS(status) is non-zero if sort had failed.

But As you can see..the code is just too big.. it has lots of if-else conditional statements and too many nestings.. it is getting very complicated. Can someone please help me out here. Porter suggested that I should Use functions to encapsulate. Porter, can you please tell me which piece of code here should I encapsulate. Thanks in advance. !!

Code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main()
{
        int pipe1[2]; /* pipe for:  sort ------> uniq */
        int pipe2[2]; /* pipe for:  uniq ------> wc   */
        int pipe3[2]; /* pipe for:  read result from wc */

        u_long uniqLines;
        pid_t pid;
        int   status;

        pipe(pipe1);

        pid = fork();

        if(pid == 0) {

                /*
                 * Child Process - sort
                 */

                close(1);
                dup(pipe1[1]);
                close(pipe1[0]);
                close(pipe1[1]);
                execlp("sort","sort","work.fp.big",NULL);

        } else {
                /*
                 * Parent Process
                 */

                pipe(pipe2);

                if (!fork()) {

                        /*
                         *     uniq
                         */       
                        close(0);
                        dup(pipe1[0]);
                        close(1);
                        dup(pipe2[1]);
                        close(pipe1[0]);
                        close(pipe1[1]);
                        close(pipe2[0]);
                        close(pipe2[1]);
                        execlp("uniq","uniq","-c",NULL);

                } else {

                        pipe(pipe3);

                        if (!fork()) {

                               /*
                                 *      wc
                                 */

                                close(0);
                                dup(pipe2[0]);
                                close(1);
                                dup(pipe3[1]);
                                close(pipe1[0]);
                                close(pipe1[1]);
                                close(pipe2[0]);
                                close(pipe2[1]);
                                close(pipe3[0]);
                                close(pipe3[1]);
                                execlp("wc","wc","-l",NULL);

                        } else {
                                close(0);
                                dup(pipe3[0]);
                                close(pipe1[0]);
                                close(pipe1[1]);
                                close(pipe2[0]);
                                close(pipe2[1]);
                                close(pipe3[0]);
                                close(pipe3[1]);
                                scanf("%lu",&uniqLines);
                        }

                }

                       /*
                        * Get status of sort
                        */

                        if (waitpid (pid, &status, 0) != pid) {
                                perror("waitpid");
                                fprintf(stderr, "\nstatus not available\n");
                        } else {
                                fprintf(stderr, "\n status = %d\n", status);
                        }

                        if ( WEXITSTATUS(status) == 0 ) {
                                  printf("Sort Success \n");
                        } else {
                                  printf("Sort failed \n");
                        }
                
        }


}

# 13  
Old 06-25-2007
Quote:
Originally Posted by the_learner
Porter, can you please tell me which piece of code here should I encapsulate. Thanks in advance. !!
I will leave that as an exercise for you but consider the following:

General:

1. If you have lot's of lines that all look similar, then try and make those a function.

2. Use structures to hold related information rather than lot's of individual variables.

3. Extract pipe file descriptor pairs from their arrays and put them in structure elements that reflect their purpose.

Specific case:

1. rather than having lot's of "close" in the child fork, you could mark each file descriptor to be closed as "close-on-exec" using either

Code:
fcntl(fd,F_SETFD,FD_CLOEXEC);

or
Code:
int b=1;
ioctl(fd,FIOCLEX,(char *)&b);

# 14  
Old 06-26-2007
Quote:
Originally Posted by porter
1. If you have lot's of lines that all look similar, then try and make those a function.
Do you mean I can put all the close() calls in a function ? Something like this..

Code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int pipe1[2]; /* pipe for:  sort ------> uniq */
int pipe2[2]; /* pipe for:  uniq ------> wc   */
int pipe3[2]; /* pipe for:  read result from wc */

void
close_des()
{
        close(pipe1[0]);
        close(pipe1[1]);
        close(pipe2[0]);
        close(pipe2[1]);
        close(pipe3[0]);
        close(pipe3[1]);
}

int main()
{
        u_long uniqLines;
        pid_t pid;
        int   status;

        pipe(pipe1);

        pid = fork();

        if(pid == 0) {
                /*
                 * Child Process - sort
                 */

                dup2(pipe1[1],1);
                close_des();
                execlp("sort","sort","work.fp",NULL);

        } else {
                /*
                 * Parent Process
                 */

                pipe(pipe2);

                if (!fork()) {

                        /*
                         *     uniq
                         */
                        dup2(pipe1[0],0);
                        dup2(pipe2[1],1);
                        close_des();
                        execlp("uniq","uniq","-c",NULL);

                } else {
                        pipe(pipe3);
                        if (!fork()) {
                                dup2(pipe2[0],0);
                                dup2(pipe3[1],1);
                                close_des();                                
                                 execlp("wc","wc","-l",NULL);
                        } else {
                                dup2(pipe3[0],0);
                                close_des();
                                scanf("%lu",&uniqLines);
                        }
                }


  ..... and so on ....

}


Quote:
Originally Posted by porter
1. rather than having lot's of "close" in the child fork, you could mark each file descriptor to be closed as "close-on-exec" using either

fcntl(fd,F_SETFD,FD_CLOEXEC);
Do you mean something like this...

Code:
int main()
{
        int pipe1[2]; /* pipe for:  sort ------> uniq */
        int pipe2[2]; /* pipe for:  uniq ------> wc   */
        int pipe3[2]; /* pipe for:  read result from wc */

        u_long uniqLines;
        pid_t pid;
        int   status;

        pipe(pipe1);
        fcntl(pipe1[0],F_SETFD,FD_CLOEXEC);
        fcntl(pipe1[1],F_SETFD,FD_CLOEXEC);
        pid = fork();

        if(pid == 0) {
                /*
                 * Child Process - sort
                 */

                dup2(pipe1[1],1);
                execlp("sort","sort","work.fp",NULL);

        } else {
                /*
                 * Parent Process
                 */

                pipe(pipe2);
                fcntl(pipe2[0],F_SETFD,FD_CLOEXEC);
                fcntl(pipe2[1],F_SETFD,FD_CLOEXEC);
                if (!fork()) {

                        /*
                         *     uniq
                         */
                        dup1(pipe1[0],0);
                        close(1);
                        dup(pipe2[1]);
                        execlp("uniq","uniq","-c",NULL);

                } else {

                        pipe(pipe3);
                        fcntl(pipe3[0],F_SETFD,FD_CLOEXEC);
                        fcntl(pipe3[1],F_SETFD,FD_CLOEXEC);
                        if (!fork()) {

                                /*
                                 *      wc
                                 */

                                dup2(pipe2[0],0);
                                dup(pipe3[1],1);
                                execlp("wc","wc","-l",NULL);

                        } else {
                                dup2(pipe3[0],0);
                                fprintf(stderr, "\n reached after wc");
                                scanf("%lu",&uniqLines);
                        }

                }



In both the occasion..the code seems to hang.. I dont know why..

But for some cases, if I leave some of the close statements in there.. it seems to work..

I really dont understand this pipe() and fork() and dup() and fnctl()... I think I need to buy that book which Perderabo suggested.. Advanced Unix Programming..

But I need to get this done urgently Smilie Porter please help...
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