STDIO(3C)STDIO(3C)NAME
stdio, stdin, stdout, stderr - standard buffered input/output package
SYNOPSIS
#include <stdio.h>
extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;
DESCRIPTION
The standard I/O functions described in section 3C of this manual con‐
stitute an efficient, user-level I/O buffering scheme. The in-line
macros getc() and putc() handle characters quickly. The macros
getchar(3C) and putchar(3C), and the higher-level routines fgetc(3C),
fgets(3C), fprintf(3C), fputc(3C), fputs(3C), fread(3C), fscanf(3C),
fwrite(3C), gets(3C), getw(3C), printf(3C), puts(3C), putw(3C), and
scanf(3C) all use or act as if they use getc() and putc(); they can be
freely intermixed.
A file with associated buffering is called a stream (see Intro(3)) and
is declared to be a pointer to a defined type FILE. The fopen(3C) func‐
tion 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 and associated with the standard open files:
stdin
standard input file
stdout
standard output file
stderr
standard error file
The following symbolic values in <unistd.h> define the file descriptors
that will be associated with the C-language stdin, stdout and stderr
when the application is started:
STDIN_FILENO Standard input value 0 stdin
STDOUT_FILENO Standard output value 1 stdout
STDERR_FILENO Standard error value 2 stderr
The constant NULL designates a null pointer.
The integer-constant EOF is returned upon end-of-file or error by most
integer functions that deal with streams (see the individual descrip‐
tions for details).
The integer constant BUFSIZ specifies the size of the buffers used by
the particular implementation.
The integer constant FILENAME_MAX specifies the number of bytes needed
to hold the longest pathname of a file allowed by the implementation.
If the system does not impose a maximum limit, this value is the recom‐
mended size for a buffer intended to hold a file's pathname.
The integer constant FOPEN_MAX specifies the minimum number of files
that the implementation guarantees can be open simultaneously. Note
that no more than 255 files may be opened using fopen(), and only file
descriptors 0 through 255 can be used in a stream.
The functions and constants mentioned in the entries of section 3S of
this manual are declared in that header and need no further declara‐
tion. The constants and the following "functions" are implemented as
macros (redeclaration of these names is perilous): getc(), getchar(),
putc(), putchar(), ferror(3C), feof(3C), clearerr(3C), and fileno(3C).
There are also function versions of getc(), getchar(), putc(),
putchar(), ferror(), feof(), clearerr(), and fileno().
Output streams, with the exception of 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(3C)) 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). The set‐
buf() or setvbuf() functions (both described on the setbuf(3C) manual
page) may be used to change the stream's buffering strategy.
Interactions of Other FILE-Type C Functions
A single open file description can be accessed both through streams and
through file descriptors. Either a file descriptor or a stream will be
called a handle on the open file description to which it refers; an
open file description may have several handles.
Handles can be created or destroyed by user action without affecting
the underlying open file description. Some of the ways to create them
include fcntl(2), dup(2), fdopen(3C), fileno(3C) and fork(2) (which
duplicates existing ones into new processes). They can be destroyed by
at least fclose(3C) and close(2), and by the exec functions (see
exec(2)), which close some file descriptors and destroy streams.
A file descriptor that is never used in an operation and could affect
the file offset (for example read(2), write(2), or lseek(2)) is not
considered a handle in this discussion, but could give rise to one (as
a consequence of fdopen(), dup(), or fork(), for example). This
exception does include the file descriptor underlying a stream, whether
created with fopen() or fdopen(), as long as it is not used directly
by the application to affect the file offset. (The read() and write()
functions implicitly affect the file offset; lseek() explicitly
affects it.)
If two or more handles are used, and any one of them is a stream, their
actions shall be coordinated as described below. If this is not done,
the result is undefined.
A handle that is a stream is considered to be closed when either an
fclose() or freopen(3C) is executed on it (the result of freopen() is a
new stream for this discussion, which cannot be a handle on the same
open file description as its previous value) or when the process owning
that stream terminates the exit(2) or abort(3C). A file descriptor is
closed by close(), _exit() (see exit(2)), or by one of the exec func‐
tions when FD_CLOEXEC is set on that file descriptor.
For a handle to become the active handle, the actions below must be
performed between the last other user of the first handle (the current
active handle) and the first other user of the second handle (the
future active handle). The second handle then becomes the active han‐
dle. All activity by the application affecting the file offset on the
first handle shall be suspended until it again becomes the active han‐
dle. (If a stream function has as an underlying function that affects
the file offset, the stream function will be considered to affect the
file offset. The underlying functions are described below.)
The handles need not be in the same process for these rules to apply.
Note that after a fork(), two handles exist where one existed before.
The application shall assure that, if both handles will ever be
accessed, that they will both be in a state where the other could
become the active handle first. The application shall prepare for a
fork() exactly as if it were a change of active handle. (If the only
action performed by one of the processes is one of the exec functions
or _exit(), the handle is never accessed in that process.)
1. For the first handle, the first applicable condition below
shall apply. After the actions required below are taken,
the handle may be closed if it is still open.
a. If it is a file descriptor, no action is required.
b. If the only further action to be performed on any handle
to this open file description is to close it, no action
need be taken.
c. If it is a stream that is unbuffered, no action need be
taken.
d. If it is a stream that is line-buffered and the last
character written to the stream was a newline (that is,
as if a putc('\n') was the most recent operation on that
stream), no action need be taken.
e. If it is a stream that is open for writing or append
(but not also open for reading), either an fflush(3C)
shall occur or the stream shall be closed.
f. If the stream is open for reading and it is at the end
of the file ( feof(3C) is true), no action need be
taken.
g. If the stream is open with a mode that allows reading
and the underlying open file description refers to a
device that is capable of seeking, either an fflush()
shall occur or the stream shall be closed.
h. Otherwise, the result is undefined.
2. For the second handle: if any previous active handle has
called a function that explicitly changed the file offset,
except as required above for the first handle, the applica‐
tion shall perform an lseek() or an fseek(3C) (as appropri‐
ate to the type of the handle) to an appropriate location.
3. If the active handle ceases to be accessible before the
requirements on the first handle above have been met, the
state of the open file description becomes undefined. This
might occur, for example, during a fork() or an _exit().
4. The exec functions shall be considered to make inaccessible
all streams that are open at the time they are called, inde‐
pendent of what streams or file descriptors may be available
to the new process image.
5. Implementation shall assure that an application, even one
consisting of several processes, shall yield correct results
(no data is lost or duplicated when writing, all data is
written in order, except as requested by seeks) when the
rules above are followed, regardless of the sequence of han‐
dles used. If the rules above are not followed, the result
is unspecified. When these rules are followed, it is imple‐
mentation defined whether, and under what conditions, all
input is seen exactly once.
Use of stdio in Multithreaded Applications
All the stdio functions are safe unless they have the _unlocked suffix.
Each FILE pointer has its own lock to guarantee that only one thread
can access it. In the case that output needs to be synchronized, the
lock for the FILE pointer can be acquired before performing a series of
stdio operations. For example:
FILE iop;
flockfile(iop);
fprintf(iop, "hello ");
fprintf(iop, "world);
fputc(iop, 'a');
funlockfile(iop);
will print everything out together, blocking other threads that might
want to write to the same file between calls to fprintf().
An unlocked interface is available in case performace is an issue. For
example:
flockfile(iop);
while (!feof(iop)) {
*c++ = getc_unlocked(iop);
}
funlockfile(iop);
RETURN VALUES
Invalid stream pointers usually cause grave disorder, possibly includ‐
ing program termination. Individual function descriptions describe the
possible error conditions.
SEE ALSOclose(2), lseek(2), open(2), pipe(2), read(2), write(2), ctermid(3C),
cuserid(3C), fclose(3C), ferror(3C), fopen(3C), fread(3C), fseek(3C),
flockfile(3C), getc(3C), gets(3C), popen(3C), printf(3C), putc(3C),
puts(3C), scanf(3C), setbuf(3C), system(3C), tmpfile(3C), tmpnam(3C),
ungetc(3C)
May 18, 2005 STDIO(3C)
