Why am I not getting an output when reading from a file descriptor?

Question

I am trying to read from a file and print it for the user to read. My task is similar to the UNIX 'cat' command but my code does not print anything.

#include<unistd.h>
#include<fcntl.h>
#include<stdio.h> //for printf

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

  if(argc==1)
    fd=open(argv[1],O_RDONLY);
  else
    if (fd=open(argv[1],O_RDONLY) ==0){
    printf("Error opening");
    return(0);
  }

  while((read(fd,&buffer,1)) != -1){
     read(fd,&buffer,1);
     write(STDOUT_FILENO,buffer, 1);

  }
    return(0);
}

Answer 1

You're reading from the wrong FD due to a precedence issue. if (fd=open(argv[1],O_RDONLY) ==0) is parsed as if (fd=(open(argv[1],O_RDONLY) ==0)) . You wanted it to be parsed as if ((fd=open(argv[1],O_RDONLY)) ==0) instead, so write that.

Other problems:

• 0 is a legitimate FD. You should check against -1 instead to see if open failed.
• If argc is 1, then argv[1] is a null pointer, which you shouldn't try to open .
• Calling read twice in a row instead of just once means you're going to throw away every other character.
• write expects a pointer, so pass it &buffer too like you do with read . Remember it's not printf .
• read can return 0 when you get to EOF, and right now that will send your program into an infinite loop printing the final character.