Using pipe between diffrent programs how ? - c

Question

Question asked again and code modified...
I need to create three programs named as program0 program1 and program2 in linux.
Program0:Creates a parent with two child processes and executes program 1 and program 2 with its childs waits them to finish and close.
Program1:Takes a file name from the user and writes text to the file.It finishes writing when CTNL+D pressed and creates a pipe.After that by using cat command it writes file to stdout and uses dup() to create pipe which has file in it.
Program2:It reads filename from the pipe with the help of dup() and then executes wc command.
So far I managed to create all programs and I have no compling errors.Program 0 executes both programs.Program1 is also working and sends file to the pipe but program2 cannot read it from the pipe is prints weird symbols..
When I try to read from the pipe within the program1 it works(see the deactivated code in program1) but same code is not working if I put it inside program2.
So what how can I make program2 to read from the pipe after that I will try to execute wc command in program2 but first I should be able to see that its taking file input from the stdout so how?
I know its kinda long but please help me guys...
Program 0
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#define MAX 999
int main()
{
pid_t pid1, pid2;
pid1 = fork();
if(pid1<0)
{
fprintf(stderr,"Fork basarisiz");
exit(-1);
}
else if (pid1 ==0)/*child prosesleri*/
{
printf("program1\n");
execlp("./program1","program1",NULL);
execlp("./program2","program2",NULL);
}
else /*parent procsesleri */
{
wait(NULL);
pid2 = fork();
if(pid2<0)
{
fprintf(stderr,"Fork basarisiz");
exit(-1);
}
else if (pid2 ==0)/*child prosesleri*/
{
printf("\n");
printf("Program 2\n");
printf("\n");
execlp("./program2","program2",NULL);
//printf("\n");
}
else
{
}
/////////////////////////////////////////////////////////////////////////
wait(NULL);
printf("\n");
printf("Parent:Two child processes have successfully been created\n");
printf("Parent:Two child processes have successfully been terminated\n");
printf("Parent:This process will now terminate\n");
printf("\n");
exit(0);
}
}
Program 1
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#define MAX 999
int main()
{
char c[10000];
char file[10000];
int words;
printf("Child1:A text file will be created\n");
printf("Child1:Enter the name of the file\n");
scanf("%123s",file);
strcat(file,".txt");
FILE * pf;
pf = fopen(file, "w" );
if (!pf)
fprintf( stderr, "I couldn't open the file.\n" );
else
{
printf("Child1: Input a number of text lines ended, each ended by a CR (carriage return).\n");
/////////////////////////////
do
{
if (NULL != fgets(c, sizeof(c), stdin))
{
if (0 == strcmp(c, ".\n"))
{
break;
}
fprintf(pf, "%s", c);
}
else
{
if (0 != ferror(stdin))
{
fprintf(stderr, "An error occured while reading from stdin\n");
}
else
{
printf("Child1: Finish the input by CNTL^D\n");
}
break;
}
} while (1);
/////////////////////////////
}
printf("\nChild1:The file %s is succesfully created and saved in the current dictionary\n",file);
//////////////////////////////////////////////
/////////////////////////pipe///////////////
fclose(pf); // close file
char ch;
int outcount = 0;
int fd[2], nbytes;
pid_t childpid;
int i;
char f2[2];
char readbuffer[80];
pipe(fd);
if((childpid = fork()) == -1)
{
perror("fork");
exit(1);
}
if(childpid == 0)
{ printf("\nChild1:The file written to pipe with cat\n");
close(1) ;
dup(fd[1]);
close(fd[0]);
execlp("/bin/cat", "cat", file,NULL);
}
else
{
wait(NULL);
//close(0) ;
//dup(fd[0]) ;
//close(fd[1]);
//nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
//printf("%s\n",readbuffer);
}
return(0);
}
Program 2
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{
int fd[2],nbytes;
pid_t childpid;
char readbuffer[80];
pipe(fd);
if((childpid = fork()) == -1)
{
perror("fork");
exit(1);
}
if(childpid == 0)
{
}
else
{
close(0) ;
dup(fd[0]) ;
close(fd[1]);
nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
printf("%s\n",readbuffer);
}
return(0);
}

Answer 1

You may want to check the man pages for execve(2) (for starting cat) and dup2(2) (for overriding stdin and stdout as needed) for this one. execve will overwrite the currently executing program by a different one (same PID, same file descriptors), while dup2 will allow you re-define any of the standard file descriptors to point into any file descriptor you provide to it (such as any of the ends of your pipe).