IO

一 、 unbuffered IO

1. file descriptor

In computer programming, a file descriptor is an abstract indicator for accessing a file. The term is generally used in POSIX operating systems . In Microsoft Windows terminology and in the context of the C standard I/O library , "file handle " is preferred, though the latter case is technically a different object (see below).

In POSIX, a file descriptor is an integer , specifically of the C type int . There are 3 standard POSIX file descriptors which presumably every process (save perhaps a daemon ) should expect to have:

Integer value	Name
0	Standard Input (stdin)
1	Standard Output (stdout)
2	Standard Error (stderr)

Generally, a file descriptor is an index for an entry in a kernel -resident data structure containing the details of all open files. In POSIX this data structure is called a file descriptor table , and each process has its own file descriptor table . The user application passes the abstract key to the kernel through a system call , and the kernel will access the file on behalf of the application, based on the key. The application itself cannot read or write the file descriptor table directly.

In Unix-like systems, file descriptors can refer to files, directories , block or character devices (also called "special files"), sockets , FIFOs (also called named pipes ), or unnamed pipes .

The FILE * file handle in the C standard I/O library routines is technically a pointer to a data structure managed by those library routines; one of those structures usually includes an actual low level file descriptor for the object in question on Unix-like systems. Since file handle refers to this additional layer, it is not interchangeable with file descriptor .