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<META NAME="Author" CONTENT="James M. Rogers">
<META NAME="Description" CONTENT="This article is the second in a series designed to explore the Standard C library implementation available for Linux">
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<TITLE>The Standard C Library for Linux Issue 32</TITLE>
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<H4>
"Linux Gazette...<I>making Linux just a little more fun!</I>"
</H4>
<P> <HR> <P>
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<CENTER>
<H1>
<FONT COLOR="#800000">The Standard C Library for Linux</FONT></H1></CENTER>
<CENTER>
<H3>
<FONT COLOR="#000080">Part Three: <stdio.h> formatted input/output</FONT></H3></CENTER>
<CENTER>
<H4>
By <A HREF="mailto:jrogers@u.washington.edu">James M. Rogers</A></H4></CENTER>
<HR>
<P>The last article was on character I/O in the standard input/output library
<stdio.h>. This article is on formatted input and output.
I am assuming a knowledge of c programming on the part of the reader.
There is no guarantee of accuracy in any of this information nor suitability
for any purpose.
<P>As an example of formatted input output we will read in a file containing
a number and a label. We will subtotal the items by label and print
out the subtotal with its label along with a total for all subtotals.
The example is <A HREF="rogers_example3.c">example3.c</A>
and the data file is <A HREF="rogers_example3.dat">example3.dat</A><TT>.</TT>
<P>The code examples given for each function will typically not run unless
the the <angle bracked> items are replaced with real code. Normally
these are items that have to be treated differently depending on what you
are trying to do. As always, if you see an error in my documentation
please tell me and I will correct myself in a later document. See
corrections at end of the document to review corrections to the previous
articles.
<P><B><FONT SIZE=+1>Formatted Output</FONT></B>
<UL><TT>#include <stdio.h></TT>
<P><TT>int printf(const char *format, ...);</TT>
<BR><TT>int fprintf(FILE *stream, const char *format, ...);</TT>
<BR><TT>int sprintf(char *str, const char *format, ...);</TT>
<BR><TT>int snprintf(char *str, size_t size, const char *format,
...);</TT></UL>
<TT>const char *format</TT> is a format that will be used to process the
following arguments.
<BR><TT>...</TT> is a variable number of arguments that must match the
number of arguments used in const char *format.
<BR><TT>FILE *stream</TT> is a previously fopened stream.
<BR><TT>char *str</TT> is a string.
<BR><TT>size_t size</TT> is the maximum size of string that will be produced,
any excess is lost.
<P>p<B>rintf</B> is used to print out a formatted sequence of characters
to standard out.
<P><TT>int x=5;</TT>
<BR><TT>printf("We have %d apples.\n", x);</TT><TT></TT>
<P>This example prints to stdout "We have 5 apples." with a newline following.
<P><B>fprintf</B> is used to print out a formatted sequence of characters
to a file.
<P><TT>fprintf(stderr, "DEBUG\t%s\t%s\t%f", dateString, messageString,
errorNumber);</TT>
<P>Could print to stderr, "DEBUG 199808291055
you are here 1234.4567" followed by a newline.
<P><B>sprintf</B> is used to print out a formatted sequence of characters
to a character array.<TT></TT>
<P><TT>float x=99.1234;</TT>
<BR><TT>sprintf(string, "%d", x)</TT><TT></TT>
<P>This would create a string that contained the characters '9', '9', '.',
'1','2','3','4'. This is the reverse to the atoi function that
we will cover next month.
<P><B>snprint</B> is used to print out a formatted sequence of characters
to a string.
<BR>
<BR><TT>float x=99.1234;</TT>
<BR><TT>returnValue=sprintf(string, 4, "%d", x)</TT>
<P>This will create a string with the characters '9','9','.','1', returnValue
will contain a -1 because the field was truncated.
<P><B><FONT SIZE=+1>Formatted Input</FONT></B>
<UL><TT>#include <stdio.h></TT>
<P><TT>int scanf( const char *format, ...);</TT>
<BR><TT>int fscanf( FILE *stream, const char *format, ...);</TT>
<BR><TT>int sscanf( const char *str, const char *format, ...);</TT></UL>
<TT>const char *format</TT> is a format that will be used to process the
following arguments.
<BR><TT>...</TT> is a variable number of arguments that must match the
number of arguments used in const char *format.
<BR><TT>FILE *stream</TT> is a previously fopened stream.
<BR><TT>char *str</TT> is a string.
<BR><TT>size_t size</TT> is the maximum size of string that will be produced,
any excess is lost.
<P>These functions return an EOF on a read error, or the number of
items that were converted, zero or more.
<P>scanf will read in a format from standard input.
<P><TT>scanf("%f%2d%d", float1, int1, int2)</TT><TT></TT>
<P>with the following on stdin:
<P><TT>12.34 4567</TT>
<P>will set float1=12.34, int1=45 and int2=67.
<P>fscanf will read in a format from the given stream.
<P><TT>fscanf(stdin,"%f%2d%d", float1, int1, int2)</TT>
<P>This example is equivilent to the scanf example.
<P>sscanf will read a format from the given string.
<P><TT>sscanf(string, "%f%2d%d", float1, int1, int2)</TT>
<P>Will scan string for the float and decimal values.
<P><B><FONT SIZE=+1>Format Strings</FONT></B>
<P>The format will look like the above examples. A more general desription
is as follows:
<P>A format string contains zero or more of the following conversion specifications:
<BR>%
A conversion specification is introduced with this required character.
<BR>flags
followed by zero or more flags.
<BR>width
followed by an optional width field.
<BR>precision followed by an optional precision
field.
<BR>argument followed by an optional argument that
differs by specific conversions.
<BR>conversion ending in a required conversion type.
<P><B>Flags</B>
<BR> These are used to change the default behavior of
the conversion.
<P><B>- </B>
left justify the field.
<BR><B>+ </B>
used a sign with a number conversion.
<BR><B>#</B>
use 0 in front of octal conversions, 0x in front of hex conversions and
a decimal point with decimal conversions.
<BR><B>0</B>
pad number conversions with leading zeros, ignore if a - is present.
<BR><B>space </B> if a space follows the % then
a space will be placed before the output, ignore if a - is present.<B></B>
<P><B>Field Width</B>
<BR> The number of characters wide a field is.
Spaces are used to pad the extra characters if a value is not as wide as
the given width. If the value is larger than this number the field
will expand to fit the number.
<P><B>Precision</B>
<BR> The minimum number of digits to appear for integer
types or the number of digits to apear after a floating point number or
the maximum number of characters to print from a string. This takes
the form of a decimal point followed by an optional number. If no
number is given the precision defaults to 0.
<P><B>Argument</B>
<BR> These will be discussed with each relevant conversion.
They are h, l, or L.
<P><B>Conversions</B>
<P><B>d </B> int to signed decimal
-9999 or 99
<BR><B>i</B> int to signed decimal
-9999 or 99
<BR><B>o</B> unsigned int to unsigned
octal 8 becomes 10
<BR><B>u</B> unsigned int to decimal
1 becomes 1
<BR><B>x </B> unsigned int to hexidecimal
13 becomes 1d
<BR><B>X </B> unsigned int to hexidecimal
13 becomes 1D
<BR><B>h</B> is used in front of the above 6 to convert
from the short int and unsigned short int to whatever.
<BR><B>l</B> is used in front of the above 6 to convert
from the long int and unsigned long int to whatever.
<BR><B>f</B> double to decimal
-9999.99 or 99.9
<BR><B>e </B> double to scientific notation
-9.999e+99 or 9.9e-9
<BR><B>E</B> double to scientific notation
-9.999E+99 or 9.9E-9
<BR><B>g</B> double to decimal (f)
or scientific (e) if the # of digits are equal to or greater than the precision.
<BR><B>G</B> double to decimal (f) or scientific
(E) if the # of digits are equal to or greater then the precision.
<BR><B>L</B> is used in front of the above 5 to convert
from long double to whatever.
<BR><B>c</B> int to unsigned char
65 becomes an A in standard ASCII
<BR><B>s</B> pointer to string prints
out the string
<BR><B>n </B> pointer to an int into
which the number of characters already written to stream will be placed.
<BR><B>h</B> is used in front of the above 1 to specify
a short int.
<BR><B>l</B> is used in front of the above 1 to specify
a long int.
<BR><B>%</B> a %% will write out a single %. This
is the way to print a % when needed.
<P>
<HR>
<P>A correction to Part Two:
<UL><TT>Date: Wed, 12 Aug 1998 11:27:08 +0200</TT>
<BR><TT>From: Lars Hesdorf <hesdorf@ibm.net></TT>
<BR><TT>To: jrogers@u.washington.edu</TT>
<BR><TT>Subject: The Standard C Library for Linux, Part Two"</TT>
<P><TT>Hej James M. Rogers</TT>
<P><TT>You wrote somewhere in "The Standard C Library for Linux, Part Two"</TT>
<P><TT>"putchar writes a character to standard out. putchar(x) is
the same as</TT>
<BR><TT>fputc(x, STDIN)"</TT>
<P><TT>You probably meant "...fputc(x, STDOUT)".</TT>
<P><TT>Lars Hesdorf</TT>
<BR><TT>HESDORF@IBM.NET</TT></UL>
Yes I did, thanks a lot. One more problem with all of those examples,
STDOUT and STDIN are suposed to be lower case; stdout and stdin.
Sorry if this caused anyone any frustrations.
<P>
<HR>
<H4>
Bibilography:</H4>
<I>The ANSI C Programming Language, Second Edition</I>, Brian W. Kernighan,
Dennis M. Ritchie, Printice Hall Software Series, 1988
<P><I>The Standard C Library</I>, P. J. Plauger, Printice Hall P T R, 1992
<P><I>The Standard C Library, Parts 1, 2, and 3</I>, Chuck Allison, <I>C/C++
Users Journal</I>, January, February, March 1995
<P>STDIO(3), BSD MANPAGE, <I>Linux Programmer's Manual</I>, 29 November
1993
<P><HR><P>
<center><H4>Previous "The Standard C Library for Linux" Articles</H4></center>
<P><A HREF="http://www.linuxgazette.com/issue24/rogers.html"><I>The Standard
C Library for Linux, Part One</I>, James M. Rogers, January 1998</A>
<P><A HREF="http://www.linuxgazette.com/issue31/rogers1.html"><I>The Standard
C Library for Linux, Part Two</I>, James M. Rogers, January 1998</A>
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<P> <hr> <P>
<center><H5>Copyright © 1998, James M. Robers <BR>
Published in Issue 32 of <i>Linux Gazette</i>, September 1998</H5></center>
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