How does read(2) interact with carriage returns?

Question

I am writing a simple program to flip all the bits in a file, but right now it only does the first 1000 bytes until I get that much working. Why does my call to read() ignore \r characters? When I run this code on a file that only contains \r\n\r\n, the read call returns 2 and the buffer contains \n\n. The \r characters are completely ignored. I'm running this on Windows (this wouldn't even be an issue on Linux machines)

Why does read(2) skip over the \r character when it finds it? Or is that what is happening?

EDIT: Conclusion is that windows defaults to opening files in "text" mode as opposed to "binary" mode. For this reason, when calling open, we must specify O_BINARY as the mode.

Thanks, code below.

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

void invertBytes(size_t amount, char* buffer);

int main(int argv, char** argc)
{
   int fileCount = 1;
   char* fileName;
   int fd = 0;
   size_t bufSize = 1000;
   size_t amountRead = 0;
   char* text;
   int offset = 0;

   if(argv <= 1)
   {
      printf("Usages: encode [filenames...]\n");
      return 0;
   }

   text = (char *)malloc(sizeof(char) * bufSize);

   for(fileCount = 1; fileCount < argv; fileCount++)
   {
      fileName = argc[fileCount];
      fd = open(fileName, O_RDWR);
      printf("fd: %d\n", fd);
      amountRead = read(fd, (void *)text, bufSize);
      printf("Amount read: %d\n", amountRead);
      invertBytes(amountRead, text);
      offset = (int)lseek(fd, 0, SEEK_SET);
      printf("Lseek to %d\n", offset);
      offset = write(fd, text, amountRead);
      printf("write returned %d\n", offset);
      close(fd);
   }

   return 0;
}

void invertBytes(size_t amount, char* buffer)
{
   int byteCount = 0;
   printf("amount: %d\n", amount);
   for(byteCount = 0; byteCount < amount; byteCount++)
   {
      printf("%x, ", buffer[byteCount]);
      buffer[byteCount] = ~buffer[byteCount];
      printf("%x\r\n", buffer[byteCount]);
   }
   printf("byteCount: %d\n", byteCount);
}

What environment are you using? `open` and `read` are not native to windows. — CB Bailey, Jan 06 '12 at 20:33
This is all done on windows xp right now. Im compiling it from the command line. — Akron, Jan 06 '12 at 20:35
Surely you are using a compiler and one or more runtime libraries? — CB Bailey, Jan 06 '12 at 20:44
You might read here http://stackoverflow.com/q/229924/694576 for more details on this issue. — alk, Jan 06 '12 at 20:45

score 4 · Accepted Answer · edited May 23 '17 at 12:21

4

fd = open(fileName, O_RDWR);

should be

fd = open(fileName, O_RDWR | O_BINARY);

See read() only reads a few bytes from file for details.

edited May 23 '17 at 12:21

Community

1
1

answered Jan 06 '12 at 20:30

Eugen Rieck

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Wow, you are right. I changed that one line and it is now correctly processing the \r character. If I compiled this on a Linux machine, would I need to specify the O_BINARY mode? I know unix files do not typically contain \r. – Akron Jan 06 '12 at 20:33
1

I'd always use O_BINARY for binary files, it's just the polite thing to do and promotes portability for cross compiling with cygwin. – Joachim Isaksson Jan 06 '12 at 20:36
1

No, on linux there's no difference between binary mode and text(?) mode. But of course for portability, it's better to use O_BINARY anyway. – Daniel Fischer Jan 06 '12 at 20:36
@Akron this doesn't have to do with \r at all: Windows uses different modes to read a file as Text or Binary, AFAIK this concept doesn't exist in Linux. So: Yes, you are right, that you don't need O_BINARY on Linux, but no, that is not because of Linux text files using \n as a line delimiter, but because of Linux not using this (IMHO braindead) way to handle a file. – Eugen Rieck Jan 06 '12 at 20:38

score 2 · Answer 2 · answered Jan 06 '12 at 20:32

2

Try opening with O_BINARY to use binary mode, text mode may be default and may ignore \r.

open(fileName, O_RDWR|O_BINARY);

answered Jan 06 '12 at 20:32

Joachim Isaksson

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How does read(2) interact with carriage returns?

2 Answers2