path: root/lib/libc/stdio/stdio.3

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.Dd $Mdocdate: April 12 2012 $
.Dt STDIO 3
.Os
.Sh NAME
.Nm stdio
.Nd standard input/output library functions
.Sh SYNOPSIS
.Fd #include <stdio.h>
.Pp
.Fd FILE *stdin;
.Fd FILE *stdout;
.Fd FILE *stderr;
.Sh DESCRIPTION
The standard I/O library
provides a simple and efficient buffered stream I/O interface.
Input and output is mapped into logical data streams and the physical I/O
characteristics are concealed.
The functions and macros are listed below;
more information is available from the individual man pages.
.Pp
A stream is associated with an external file (which may be a physical
device) by
.Dq opening
a file, which may involve creating a new file.
Creating an existing file causes its former contents to be discarded.
If a file can support positioning requests (such as a disk file, as opposed
to a terminal) then a
.Dq file position indicator
associated with the stream is positioned at the start of the file (byte
zero), unless the file is opened with append mode.
If append mode
is used, the position indicator will be placed at the end-of-file.
The position indicator is maintained by subsequent reads, writes,
and positioning requests.
All input occurs as if the characters
were read by successive calls to the
.Xr fgetc 3
function; all output takes place as if all characters were
written by successive calls to the
.Xr fputc 3
function.
.Pp
A file is disassociated from a stream by
.Dq closing
it.
Output streams are flushed (any unwritten buffer contents are transferred
to the host environment) before the stream is disassociated from the file.
The value of a pointer to a
.Dv FILE
object is indeterminate (garbage) after a file is closed.
.Pp
A file may be subsequently reopened, by the same or another program
execution, and its contents reclaimed or modified (if it can be repositioned
at the start).
If the main function returns to its original caller, or the
.Xr exit 3
function is called, all open files are closed (hence all output
streams are flushed) before program termination.
Other methods of program termination may not close files properly and hence
buffered output may be lost.
In particular,
.Xr _exit 2
does not flush
.Nm
files.
Neither does an exit due to a signal.
Buffers are flushed by
.Xr abort 3 ,
as required by POSIX, although in previous implementations they were not.
.Pp
This implementation needs and makes
no distinction between
.Dq text
and
.Dq binary
streams.
In effect, all streams are binary.
No translation is performed and no extra padding appears on any stream.
.Pp
At program startup, three streams are predefined and need not be
opened explicitly:
.Pp
.Bl -bullet -compact -offset indent
.It
.Em standard input
(for reading conventional input),
.It
.Em standard output
(for writing conventional output), and
.It
.Em standard error
(for writing diagnostic output).
.El
.Pp
These streams are abbreviated
.Em stdin ,
.Em stdout ,
and
.Em stderr .
Initially, the standard error stream
is unbuffered; the standard input and output streams are
fully buffered if and only if the streams do not refer to
an interactive or
.Dq terminal
device, as determined by the
.Xr isatty 3
function.
In fact,
.Em all
freshly opened streams that refer to terminal devices
default to line buffering, and
pending output to such streams is written automatically
whenever such an input stream is read.
Note that this applies only to
.Dq "true reads" ;
if the read request can be satisfied by existing buffered data,
no automatic flush will occur.
In these cases,
or when a large amount of computation is done after printing
part of a line on an output terminal, it is necessary to
.Xr fflush 3
the standard output before going off and computing so that the output
will appear.
Alternatively, these defaults may be modified via the
.Xr setvbuf 3
function.
.Pp
The
.Nm stdio
library is a part of the library libc
and routines are automatically loaded as needed by the compiler.
The
.Sx SYNOPSIS
sections of the following manual pages indicate which include files
are to be used, what the compiler declaration for the function
looks like, and which external variables are of interest.
.Pp
The following are defined as macros;
these names may not be re-used
without first removing their current definitions with
.Dv #undef :
.Dv BUFSIZ ,
.Dv EOF ,
.Dv FILENAME_MAX ,
.Dv FOPEN_MAX ,
.Dv L_cuserid ,
.Dv L_ctermid ,
.Dv L_tmpnam ,
.Dv NULL ,
.Dv SEEK_END ,
.Dv SEEK_SET ,
.Dv SEEK_CUR ,
.Dv TMP_MAX ,
.Dv clearerr ,
.Dv feof ,
.Dv ferror ,
.Dv fileno ,
.Dv freopen ,
.Dv fwopen ,
.Dv getc ,
.Dv getchar ,
.Dv putc ,
.Dv putchar ,
.Dv stderr ,
.Dv stdin ,
.Dv stdout .
Function versions of the macro functions
.Xr feof 3 ,
.Xr ferror 3 ,
.Xr clearerr 3 ,
.Xr fileno 3 ,
.Xr getc 3 ,
.Xr getchar 3 ,
.Xr putc 3 ,
and
.Xr putchar 3
exist and will be used if the macro
definitions are explicitly removed.
.Sh LIST OF FUNCTIONS
.Bl -column "sys_errlist" "Description"
.It Sy Function Ta Sy Description
.It asprintf Ta "formatted output conversion with allocation"
.It clearerr Ta "check and reset stream status"
.It fclose Ta "close a stream"
.It fdopen Ta "stream open functions"
.It feof Ta "check and reset stream status"
.It ferror Ta "check and reset stream status"
.It fflush Ta "flush a stream"
.It fgetc Ta "get next character or word from input stream"
.It fgetln Ta "get a line from a stream"
.It fgetpos Ta "reposition a stream"
.It fgets Ta "get a line from a stream"
.It fileno Ta "get a stream's underlying file descriptor"
.It fopen Ta "stream open functions"
.It fprintf Ta "formatted output conversion"
.It fpurge Ta "flush a stream"
.It fputc Ta "output a character or word to a stream"
.It fputs Ta "output a line to a stream"
.It fread Ta "binary stream input/output"
.It freopen Ta "stream open functions"
.It fropen Ta "open a stream"
.It fscanf Ta "input format conversion"
.It fseek Ta "reposition a stream"
.It fsetpos Ta "reposition a stream"
.It ftell Ta "reposition a stream"
.It funopen Ta "open a stream"
.It fwopen Ta "open a stream"
.It fwrite Ta "binary stream input/output"
.It getc Ta "get next character or word from input stream"
.It getchar Ta "get next character or word from input stream"
.It getdelim Ta "read a delimited record from a stream"
.It getline Ta "read a delimited record from a stream"
.It gets Ta "get a line from a stream"
.It getw Ta "get next character or word from input stream"
.It mkstemp Ta "create unique temporary file"
.It mktemp Ta "create unique temporary file"
.It perror Ta "system error messages"
.It printf Ta "formatted output conversion"
.It putc Ta "output a character or word to a stream"
.It putchar Ta "output a character or word to a stream"
.It puts Ta "output a line to a stream"
.It putw Ta "output a character or word to a stream"
.It remove Ta "remove directory entry"
.It rewind Ta "reposition a stream"
.It scanf Ta "input format conversion"
.It setbuf Ta "stream buffering operations"
.It setbuffer Ta "stream buffering operations"
.It setlinebuf Ta "stream buffering operations"
.It setvbuf Ta "stream buffering operations"
.It snprintf Ta "formatted output conversion"
.It sprintf Ta "formatted output conversion"
.It sscanf Ta "input format conversion"
.It strerror Ta "system error messages"
.It sys_errlist Ta "system error messages"
.It sys_nerr Ta "system error messages"
.It tempnam Ta "temporary file routines"
.It tmpfile Ta "temporary file routines"
.It tmpnam Ta "temporary file routines"
.It ungetc Ta "un-get character from input stream"
.It vasprintf Ta "formatted output conversion with allocation"
.It vfprintf Ta "formatted output conversion"
.It vfscanf Ta "input format conversion"
.It vprintf Ta "formatted output conversion"
.It vscanf Ta "input format conversion"
.It vsnprintf Ta "formatted output conversion"
.It vsprintf Ta "formatted output conversion"
.It vsscanf Ta "input format conversion"
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr open 2 ,
.Xr read 2 ,
.Xr write 2
.Sh STANDARDS
The
.Nm stdio
library conforms to
.St -ansiC .
.Sh BUGS
The standard buffered functions do not interact well with certain other
library and system functions, especially
.Xr vfork 2
and
.Xr abort 3 .