Default write behaviour - O_TRUNC or O_APPEND?

Question

What is the default behaviour when you open a file with access mode O_WRONLY or O_RDWR. Is the file opened in append mode or truncate mode? From the man pages:

The argument flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR.
...
In addition, zero or more file creation flags and file status flags can be bitwise-or'd in flags.

This sounds like O_APPEND and O_TRUNC flags are optional. So what does the following do?

void main ( void )
{
    int fd = open( "foo.txt", O_WRONLY );

    write( fd, "hello", 5 );

    close( fd );
}

Answer 1

Neither.

• By default the file is opened with the cursor positioned at the start. Writing overwrites the bytes at the beginning of the file.

• O_TRUNC causes the file to be truncated if it exists.

• O_APPEND causes writes to append to the end of the file instead of overwrite at the start. This flag is persistent. If you move the cursor elsewhere to read data it's always repositioned to the end of the file before each write.

The flags are orthogonal and are not mutually exclusive. You can even combine them if you want to initially truncate the file and ensure all later writes are always appends.