FILE 
stdin, stdout, stderr;
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#include <stdio.h>FILE
A file with associated buffering is called a stream (see intro(3)) and is declared to be a pointer to a defined type FILE. fopen creates certain descriptive data for a stream and returns a pointer to designate the stream in all further transactions. Normally, there are three open streams with constant pointers declared in the stdio.h header file and associated with the standard open files:
A constant null designates a null pointer.
An integer-constant EOF (-1) is returned
at end-of-file or error by most integer functions that
deal with streams (see the individual descriptions for details).
An integer constant BUFSIZ is a suggested buffer size for use in setvbuf() and the expected size for setbuf().
An integer constant FILENAME_MAX specifies the size needed for an array of char large enough to hold the longest file name string that the implementation guarantees can be opened.
The number of open streams available to a process is limited only by the maximum number of files that the process may open. This limitation is dependent on the operating system.
Any program that uses this package must include the header file of pertinent macro definitions, as follows:
#include <stdio.h>
The functions and constants mentioned in the entries of sub-class 3S of this manual are declared in that header file and need no further declaration. The constants and the following ``functions'' are implemented as macros (redeclaration of these names is perilous): getc, getchar, putc, and putchar. There are also function versions of getc, getchar, putc, and putchar.
Output streams, except for the standard error stream stderr, are by default buffered if the output refers to a file and line-buffered if the output refers to a terminal. The standard error output stream stderr is by default unbuffered, but use of freopen (see fopen(3S)) will cause it to become buffered or line-buffered. When an output stream is unbuffered, information is queued for writing on the destination file or terminal as soon as written; when it is buffered, many characters are saved up and written as a block. When it is line-buffered, each line of output is queued for writing on the destination terminal as soon as the line is completed (that is, as soon as a new-line character is written or terminal input is requested). setbuf or setvbuf (both described in setbuf(3S)) may be used to change the stream's buffering strategy.
Applications should restrict their use of the standard I/O package to the interfaces documented on the Section 3S manual pages. They should not depend on individual members of the internal structures found in stdio.h.