c dup undefined error - c

Question

I'm trying to create two child processes:
One child reads its input from a file, which is passed in as an argument, and writes output to the pipe.
The other child reads its output from the pipe and writes its output to a file, also passed in as an argument.
The parent sets up some of the file descriptors for the children and when the children are created they finish manipulating the descriptors to their needs.
However, I'm having an issue with setting up the file descriptors, specifically, when I try to close and dup the input file descriptor to take the place of stdin.
Here's all my code:
#include <sys/types.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
int main(int argc, char *argv[])
{
//Open input file
int fdin;
fdin = open(argv[1], O_RDONLY);
//Check if opening the file was successful
if(fdin > 0)
{
//Open output file
int fdout;
//Create the output file and
//check if the output file was created
if((fdout = creat(argv[2], 0644)) > 0)
{
//Captures whether the dup's were successful
int dup_in;
int dup_out;
//Close stdin so we can replace it with our input file
close(0);
//Attempt to put the input file at position 0
//and check if the dup was successful
if((dup_in = dup(fdin)) > 0)
{
//Close stdout so we can replace it with our output file
close(1);
//Attempt to put the output file at position 1
//and check if the dup was successful
if((dup_out = dup(fdout)) > 0)
{
//Pipe success
int pipecreate;
//Pipe file descriptors
int pipe_fd[2];
//Make the pipe and check
//if it was successful
if((pipecreate = pipe(pipe_fd)) > 0)
{
//close unneeded file descriptors
close(fdin);
close(fdout);
//Process id for first child
int cpid1;
//Create first child process
cpid1 = fork();
//Process creation successful, child block
if(cpid1 == 0)
{
//Read pipe dup success
int rpipe_dup;
//close f_in
close(0);
rpipe_dup = dup(pipe_fd[0]);
//Dup successful
if(rpipe_dup > 0)
{
//close uneeded file descriptors
close(pipe_fd[0]);
close(pipe_fd[1]);
execl("count", "count", (char *) 0);
char readbuf[100] = {0};
read(2, readbuf, 100);
}
//Dup failed
else
{
fprintf(stderr, "Read pipe dup failed.\n");
exit(EXIT_FAILURE);
}
}
//Process creation successful, parent block
else if(cpid1 > 0)
{
//Process id for second child
int cpid2;
//Create second child process
cpid2 = fork();
//Process creation successful, child block
if(cpid2 == 0)
{
//Write pipe dup success
int wpipe_dup;
//close f_out
close(1);
wpipe_dup = dup(pipe_fd[1]);
//Dup successful
if(wpipe_dup > 0)
{
//close uneeded file descriptors
close(pipe_fd[0]);
close(pipe_fd[1]);
execl("convert", "convert", (char *) 0);
char readbuf[100] = {0};
write(1, readbuf, 100);
}
//Dup failed
else
{
fprintf(stderr, "Write pipe dup failed.\n");
exit(EXIT_FAILURE);
}
}
//Process creation successful, parent block
else if(cpid2 > 0)
{
//Close unneeded file descriptors
close(pipe_fd[0]);
close(pipe_fd[1]);
int pid;
int status;
pid = wait(&status);
pid = wait(&status);
}
//Dup failed
else
{
fprintf(stderr, "Error creating child process 2.\n");
exit(EXIT_FAILURE);
}
}
//Process creation unsuccessful
else
{
fprintf(stderr, "Error creating child process 1.\n");
exit(EXIT_FAILURE);
}
}
//Pipe creation failed
else
{
fprintf(stderr, "Error creating pipe.\n");
exit(EXIT_FAILURE);
}
}
//Dup'ing the output file descriptor failed
else
{
fprintf(stderr, "Error dup'ing out file descriptor.\n");
exit(EXIT_FAILURE);
}
}
//Dup'ing the input file descriptor failed
else
{
//fprintf(stderr, "Error dup'ing in file descriptor.\n
perror("\nError dup'ing in file descriptor: ");
exit(EXIT_FAILURE);
}
}
//Creat failed
else
{
fprintf(stderr, "Error creating out file.\n");
exit(EXIT_FAILURE);
}
}
//Opening input file failed
else
{
fprintf(stderr, "Error opening in file.\n");
exit(EXIT_FAILURE);
}
}
I'm compiling and running this on MINIX3.
Here's how I run it, including program output:
cc fork.c
./a.out input.txt output.txt
Error dup'ing in file descriptor: : Undefined error: 0
Any help is greatly appreciated.

Answer 1

I found out why.
dup returns the file descriptor number, which, in this case, is 0. if ((dup_in = dup(fdin)) > 0) should be if ((dup_in >= dup(fdin))).