Question

I have a program that takes two files as an argument. The first file is to be copied into the second. The program forks into 2 children, the first child reads file and throws it thru the pipe to the other, then the other child writes it out to a file. The two files are supposed to be identical in the end.

When I run diff to compare the two files I get the following error:

virtual@ubuntu:~/Documents/OSprojects$ ./parent test.txt test2.txt
virtual@ubuntu:~/Documents/OSprojects$ cat test.txt
123456789112233445566778899
virtual@ubuntu:~/Documents/OSprojects$ cat test2.txt
123456789112233445566778899
virtual@ubuntu:~/Documents/OSprojects$ diff test.txt test2.txt
Binary files test.txt and test2.txt differ
virtual@ubuntu:~/Documents/OSprojects$ 

As you can see they are both the same, but the diff prints out that they are different. Obviously its just something I don’t understand about the diff cmd. Any help would be appreciated.

I believe for some reason the file I create is a binary file whereas the first file is not, but I am unaware as to why it is a binary file. I believe it might have to do with this line of code:

    write(1, buf, BUF_SIZE);            //write to buffer
    memset(buf, '\0', BUF_SIZE);

In one of the children this is writing out to the buffer then I am clearing the buffer. Am I clearing out that buffer wrong?

Here is the result of cat -e:

virtual@ubuntu:~/Documents/OSprojects$ cat -e test2.txt
123456789112233445566778899$
^@^@^@^@virtual@ubuntu:~/Documents/OSprojects$

Here is the result of cmp:

virtual@ubuntu:~/Documents/OSprojects$ cmp test.txt test2.txt
cmp: EOF on test.txt
virtual@ubuntu:~/Documents/OSprojects$ 

I believe that is my problem, how can I clear out that buffer so it doesn’t throw those at the end?

ALL OF MY CODE::

Parent:

#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fcntl.h>

#define BUF_SIZE 16

void exitWithError(char* errorMsg, int exitWith);       //generic error out function
void launch_writer(const char* pathname, char* const argv[], int pfd[]);    
void launch_reader(const char* pathname, char* const argv[], int pfd[]);   

int main(int argc, char* argv[]){

//making the pipe
int pfd[2];
if(pipe(pfd) == -1) //test pipe creation
    exitWithError("PIPE FAILED", 1);

//forking
pid_t reader_child_pid;
pid_t writer_child_pid;

//args for each fork
char *args_1[] = {"reader", argv[1], (char *) 0};
char *args_2[] = {"writer", argv[2], (char *) 0};

if((writer_child_pid = fork()) == -1) {
    exitWithError("WRITER FORK FAILED", 1);
}
else if (writer_child_pid == 0) {       //first child comes here
    launch_writer("./writer", args_2, pfd);
}
else if ((reader_child_pid = fork()) == -1) {
    exitWithError("READER FORK FAILED", 1);
}
else if (reader_child_pid == 0) {       //second child comes here
    launch_reader("./reader", args_1, pfd);
}

//parent picks up here

//close off pipe from parents end
close(pfd[0]);
close(pfd[1]);


//wait for all processes to exit before ending
for(;;) {
    if(wait(NULL) == -1){
        if(errno == ECHILD)
            exit(0);
        else {
            exitWithError("WAIT ERROR", 1);
        }
    }
}
}

void exitWithError(char* errorMsg, int exitWith) {
perror(errorMsg);
exit(exitWith);
}

void launch_writer(const char* pathname, char* const argv[], int pfd[]) {
dup2(pfd[0], 0);
close(pfd[1]);
close(pfd[0]);
execve(pathname, argv, NULL);
perror("execve failed");
}

void launch_reader(const char* pathname, char* const argv[], int pfd[]) {
dup2(pfd[1], 1);
close(pfd[1]);
close(pfd[0]);
execve(pathname, argv, NULL);
perror("execve failed");
}

Child 1:

#include <unistd.h>
#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>

#define BUF_SIZE 16

int main(int argc, char* argv[]){
//Opens file to be read from
int inFile = open(argv[1], O_RDONLY);

//declaring variables
char buf[BUF_SIZE]; //temp hold whats read/written
int read_test;      //check if EOF
for(;;) {
    read_test = read(inFile, buf, BUF_SIZE);    //read from file
    if(read_test == 0) //eof
        break;
    write(1, buf, BUF_SIZE);            //write to buffer
    memset(buf, '\0', BUF_SIZE);
}
close(inFile);
exit(0);
}

Child 2:

#include <unistd.h>
#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>

#define BUF_SIZE 16

int main(int argc, char* argv[]){
//Opens a file for reading/writing, if exists then truncates, otherwise makes new one
//with correct permissions
int wri_inFile = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC , S_IRUSR | S_IWUSR);
if(wri_inFile == -1)
    perror("ERROR OPENING FILE");

//declaring variables
char buf[BUF_SIZE];  //to store what is read in/written out
int read_test;      //test if EOF
for(;;) {
    read_test = read(0, buf, BUF_SIZE); //read from buffer
    if(read_test == 0) //eof
        break;
    write(wri_inFile, buf, BUF_SIZE);   //write to file

}
close(wri_inFile);
exit(0);
}

Answer 1

You don’t check (and use) the read data length. Therefore, your data gets padded with garbage.

There should be actual data bytes read (read_test):

read_test = read(0, buf, BUF_SIZE); //read from buffer
if(read_test == 0) //eof
    break;
write(wri_inFile, buf, BUF_SIZE);   //write to file
-----------------------^^^^^^^^

The same applies to the other child. You should also check the error conditions.