reads from a file
If buf is empty then no more data is available.
Otherwise, if buf is not an error-object, then buf contains as many bytes as could be read without further blocking.
If an error occurs then an appropriate error-object is returned.
This function needs I/O-access rights.
| std::read | a low level read function |
| std::write | a low level write function |
| std::open | opens a file |
| std::close | closes the specified file without returning success status |
| std::fsync | flushes all buffers associated with the file descriptor |