15.3.5.1 Text and Binary Files

Windows supports two different types of files: text files and binary files. On Unix, there is no such distinction. On Windows, any program which uses files must know whether each file is text or binary, and open and use them accordingly.

In a text file on Windows, each line is terminated with a carriage return character followed by a line feed character. When the file is read by a C program in text mode, the C library converts each carriage return/line feed pair into a single line feed character. If the file is read in binary mode, the program will see both the carriage return and the line feed.

You may have seen this distinction when transferring files between Unix and Window systems via FTP. You need to set the FTP program into binary or text mode as appropriate for the file you want to transfer.

When transferring a binary file, the FTP program simply transfers the data unchanged. When transferring a text file, the FTP program must convert each carriage return/line feed pair into a single line feed.

When using the C standard library, a binary file is indicated by adding b after the r, w, or a in the call to fopen. When reading a text file, the program can not simply count characters and use that when computing arguments to fseek.