.. _fopen:


fopen modes
===========

    "r" -- text file for read.

    "w" -- text file for write.

    "a" -- text file for append.

    "r+" -- text file for read and write.

    "a+" -- text file for append and read.

    "rb" -- binary file for read.

    "wb" -- binary file for write.

    "ab" -- binary file for append and write.

    "rb+" -- binary file for read and write.

    "ab+" -- binary file for append and read.


In read-write mode, reading may not directly follow output unless
EOF is reached or a call to one of the file positioning functions
`fseek()`, `fsetpos()`, `rewind()` has occurred.

Similarly, writing may not directly follow reading without first
making a call to one of `fflush()`, `fseek()`, `fsetpos()`, `rewind()`.

Safe fopen
==========

Always test the result of `fopen()`.

    ::

        FILE *fp;
        
        if ((fp = fopen("filename", mode)) == NULL) {
           fprintf(stderr, "cannot open filename" );
           return 1;             // in Unix: exit(1);
        }