Text and Binary modes

Text and Binary modes

On a UNIX system, when an application reads from a file it gets exactly what's in the file on disk and the converse is true for writing. The situation is different in the DOS/Windows world where a file can be opened in one of two modes, binary or text. In the binary mode the system behaves exactly as in UNIX. However on writing in text mode, a NL (\n, ^J) is transformed into the sequence CR (\r, ^M) NL.

This can wreak havoc with the seek/fseek calls since the number of bytes actually in the file may differ from that seen by the application.

The mode can be specified explicitly as explained in the Programming section below. In an ideal DOS/Windows world, all programs using lines as records (such as bash , make , sed ...) would open files (and change the mode of their standard input and output) as text. All other programs (such as cat , cmp , tr ...) would use binary mode. In practice with Cygwin, programs that deal explicitly with object files specify binary mode (this is the case of od , which is helpful to diagnose CR problems). Most other programs (such as sed , cmp , tr ) use the default mode.