Question

#include <unistd.h> 
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>

int main(int argc, char *argv[]){ 
 int fd1, fd2;
 char buffer[100];  long int n1;

 if(((fd1 = open(argv[1], O_RDONLY)) == -1) ||  ((fd2 = open(argv[2],  
       O_CREAT|O_WRONLY|O_TRUNC,0700)) == -1)){
     perror("file problem ");  
     exit(1);
 }

 while((n1=read(fd1, buffer, 512) > 0))  
      if(write(fd2, buffer, n1) != n1){
             perror("writing problem ");  exit(3);
       }

// Case of an error exit from the loop  
 if(n1 == -1){
    perror("Reading problem ");  
    exit(2);
  }
  close(fd2);  
  exit(0);
}


There is an issue with this code when I run gdb. fd1 and fd2 dont give same values. I set break at 

while((n1=read(fd1, buffer, 512) > 0)) to get the values of fd1 and fd2 and then "n" and thentry it again. the values are different. Can you point the mistake and tell me how to fix it

Answer 1

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char* argv[])
{
int fd1, fd2;
char buffer[100];
long int n1;
if((fd1 = open(argv[1], O_RDONLY)) == -1)
{

printf("problem to open file for reading ");
exit(1);
}
if((fd2 = open(argv[2], O_CREAT|O_WRONLY|O_TRUNC,0700)) == -1)
{
printf("problem to open file for writing ");
exit(2);
}
while((n1=read(fd1, buffer, 512)) > 0) //read n1 bytes each time until EOF
if(write(fd2, buffer, n1) != n1) // if the bytes read and written has mismatch
{
printf("writing problem ");
exit(3);
}
if(n1 == -1) // if unable to read bytes
{
printf("Reading problem ");
exit(4);
}
close(fd2);
exit(0);
}

It can be solved using the above code