stdio.texi

一个C源代码分析器

@node Formatted Output
@section Formatted Output

@cindex format string, for @code{printf}
@cindex template, for @code{printf}
@cindex formatted output to a stream
@cindex writing to a stream, formatted
The functions described in this section (@code{printf} and related
functions) provide a convenient way to perform formatted output.  You
call @code{printf} with a @dfn{format string} or @dfn{template string}
that specifies how to format the values of the remaining arguments.

Unless your program is a filter that specifically performs line- or
character-oriented processing, using @code{printf} or one of the other
related functions described in this section is usually the easiest and
most concise way to perform output.  These functions are especially
useful for printing error messages, tables of data, and the like.

@menu
* Formatted Output Basics::     Some examples to get you started.
* Output Conversion Syntax::    General syntax of conversion
                                 specifications.
* Table of Output Conversions:: Summary of output conversions and
                                 what they do.
* Integer Conversions::         Details about formatting of integers.
* Floating-Point Conversions::  Details about formatting of
                                 floating-point numbers.
* Other Output Conversions::    Details about formatting of strings,
                                 characters, pointers, and the like.
* Formatted Output Functions::  Descriptions of the actual functions.
* Dynamic Output::		Functions that allocate memory for the output.
* Variable Arguments Output::   @code{vprintf} and friends.
* Parsing a Template String::   What kinds of args does a given template
                                 call for? 
* Example of Parsing::          Sample program using @code{parse_printf_format}.
@end menu

@node Formatted Output Basics
@subsection Formatted Output Basics

The @code{printf} function can be used to print any number of arguments.
The template string argument you supply in a call provides
information not only about the number of additional arguments, but also
about their types and what style should be used for printing them.

Ordinary characters in the template string are simply written to the
output stream as-is, while @dfn{conversion specifications} introduced by
a @samp{%} character in the template cause subsequent arguments to be
formatted and written to the output stream.  For example,
@cindex conversion specifications (@code{printf})

@smallexample
int pct = 37;
char filename[] = "foo.txt";
printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
        filename, pct);
@end smallexample

@noindent
produces output like

@smallexample
Processing of `foo.txt' is 37% finished.
Please be patient.
@end smallexample

This example shows the use of the @samp{%d} conversion to specify that
an @code{int} argument should be printed in decimal notation, the
@samp{%s} conversion to specify printing of a string argument, and
the @samp{%%} conversion to print a literal @samp{%} character.

There are also conversions for printing an integer argument as an
unsigned value in octal, decimal, or hexadecimal radix (@samp{%o},
@samp{%u}, or @samp{%x}, respectively); or as a character value
(@samp{%c}).

Floating-point numbers can be printed in normal, fixed-point notation
using the @samp{%f} conversion or in exponential notation using the
@samp{%e} conversion.  The @samp{%g} conversion uses either @samp{%e}
or @samp{%f} format, depending on what is more appropriate for the
magnitude of the particular number.

You can control formatting more precisely by writing @dfn{modifiers}
between the @samp{%} and the character that indicates which conversion
to apply.  These slightly alter the ordinary behavior of the conversion.
For example, most conversion specifications permit you to specify a
minimum field width and a flag indicating whether you want the result
left- or right-justified within the field.

The specific flags and modifiers that are permitted and their
interpretation vary depending on the particular conversion.  They're all
described in more detail in the following sections.  Don't worry if this
all seems excessively complicated at first; you can almost always get
reasonable free-format output without using any of the modifiers at all.
The modifiers are mostly used to make the output look ``prettier'' in
tables.

@node Output Conversion Syntax
@subsection Output Conversion Syntax

This section provides details about the precise syntax of conversion
specifications that can appear in a @code{printf} template
string.

Characters in the template string that are not part of a
conversion specification are printed as-is to the output stream.
Multibyte character sequences (@pxref{Extended Characters}) are permitted in
a template string.

The conversion specifications in a @code{printf} template string have
the general form:

@example
% @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion}
@end example

For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-}
is a flag, @samp{10} specifies the field width, the precision is
@samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies
the conversion style.  (This particular type specifier says to
print a @code{long int} argument in decimal notation, with a minimum of
8 digits left-justified in a field at least 10 characters wide.)

In more detail, output conversion specifications consist of an
initial @samp{%} character followed in sequence by:

@itemize @bullet
@item 
Zero or more @dfn{flag characters} that modify the normal behavior of
the conversion specification.
@cindex flag character (@code{printf})

@item 
An optional decimal integer specifying the @dfn{minimum field width}.
If the normal conversion produces fewer characters than this, the field
is padded with spaces to the specified width.  This is a @emph{minimum}
value; if the normal conversion produces more characters than this, the
field is @emph{not} truncated.  Normally, the output is right-justified
within the field.
@cindex minimum field width (@code{printf})

You can also specify a field width of @samp{*}.  This means that the
next argument in the argument list (before the actual value to be
printed) is used as the field width.  The value must be an @code{int}.
If the value is negative, this means to set the @samp{-} flag (see
below) and to use the absolute value as the field width.

@item 
An optional @dfn{precision} to specify the number of digits to be
written for the numeric conversions.  If the precision is specified, it
consists of a period (@samp{.}) followed optionally by a decimal integer
(which defaults to zero if omitted).
@cindex precision (@code{printf})

You can also specify a precision of @samp{*}.  This means that the next
argument in the argument list (before the actual value to be printed) is
used as the precision.  The value must be an @code{int}, and is ignored
if it is negative.  If you specify @samp{*} for both the field width and
precision, the field width argument precedes the precision argument.
Other C library versions may not recognize this syntax.

@item
An optional @dfn{type modifier character}, which is used to specify the
data type of the corresponding argument if it differs from the default
type.  (For example, the integer conversions assume a type of @code{int},
but you can specify @samp{h}, @samp{l}, or @samp{L} for other integer
types.)
@cindex type modifier character (@code{printf})

@item
A character that specifies the conversion to be applied.
@end itemize

The exact options that are permitted and how they are interpreted vary 
between the different conversion specifiers.  See the descriptions of the
individual conversions for information about the particular options that
they use.

With the @samp{-Wformat} option, the GNU C compiler checks calls to
@code{printf} and related functions.  It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a @code{printf}-style format string.  
@xref{Function Attributes, , Declaring Attributes of Functions,
gcc.info, Using GNU CC}, for more information.

@node Table of Output Conversions
@subsection Table of Output Conversions
@cindex output conversions, for @code{printf}

Here is a table summarizing what all the different conversions do:

@table @asis
@item @samp{%d}, @samp{%i}
Print an integer as a signed decimal number.  @xref{Integer
Conversions}, for details.  @samp{%d} and @samp{%i} are synonymous for
output, but are different when used with @code{scanf} for input
(@pxref{Table of Input Conversions}).

@item @samp{%o}
Print an integer as an unsigned octal number.  @xref{Integer
Conversions}, for details.

@item @samp{%u}
Print an integer as an unsigned decimal number.  @xref{Integer
Conversions}, for details.

@item @samp{%x}, @samp{%X}
Print an integer as an unsigned hexadecimal number.  @samp{%x} uses
lower-case letters and @samp{%X} uses upper-case.  @xref{Integer
Conversions}, for details.

@item @samp{%f}
Print a floating-point number in normal (fixed-point) notation.
@xref{Floating-Point Conversions}, for details.

@item @samp{%e}, @samp{%E}
Print a floating-point number in exponential notation.  @samp{%e} uses
lower-case letters and @samp{%E} uses upper-case.  @xref{Floating-Point
Conversions}, for details.

@item @samp{%g}, @samp{%G}
Print a floating-point number in either normal or exponential notation,
whichever is more appropriate for its magnitude.  @samp{%g} uses
lower-case letters and @samp{%G} uses upper-case.  @xref{Floating-Point
Conversions}, for details.

@item @samp{%c}
Print a single character.  @xref{Other Output Conversions}.

@item @samp{%s}
Print a string.  @xref{Other Output Conversions}.

@item @samp{%p}
Print the value of a pointer.  @xref{Other Output Conversions}.

@item @samp{%n}
Get the number of characters printed so far.  @xref{Other Output Conversions}.
Note that this conversion specification never produces any output.

@item @samp{%m}
Print the string corresponding to the value of @code{errno}.
(This is a GNU extension.)
@xref{Other Output Conversions}.

@item @samp{%%}
Print a literal @samp{%} character.  @xref{Other Output Conversions}.
@end table

If the syntax of a conversion specification is invalid, unpredictable
things will happen, so don't do this.  If there aren't enough function
arguments provided to supply values for all the conversion
specifications in the template string, or if the arguments are not of
the correct types, the results are unpredictable.  If you supply more
arguments than conversion specifications, the extra argument values are
simply ignored; this is sometimes useful.

@node Integer Conversions
@subsection Integer Conversions

This section describes the options for the @samp{%d}, @samp{%i},
@samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion
specifications.  These conversions print integers in various formats.

The @samp{%d} and @samp{%i} conversion specifications both print an
@code{int} argument as a signed decimal number; while @samp{%o},
@samp{%u}, and @samp{%x} print the argument as an unsigned octal,
decimal, or hexadecimal number (respectively).  The @samp{%X} conversion
specification is just like @samp{%x} except that it uses the characters
@samp{ABCDEF} as digits instead of @samp{abcdef}.

The following flags are meaningful:

@table @asis
@item @samp{-}
Left-justify the result in the field (instead of the normal
right-justification).

@item @samp{+}
For the signed @samp{%d} and @samp{%i} conversions, print a
plus sign if the value is positive.

@item @samp{ }
For the signed @samp{%d} and @samp{%i} conversions, if the result
doesn't start with a plus or minus sign, prefix it with a space
character instead.  Since the @samp{+} flag ensures that the result
includes a sign, this flag is ignored if you supply both of them.

@item @samp{#}
For the @samp{%o} conversion, this forces the leading digit to be
@samp{0}, as if by increasing the precision.  For @samp{%x} or
@samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively)
to the result.  This doesn't do anything useful for the @samp{%d},
@samp{%i}, or @samp{%u} conversions.  Using this flag produces output
which can be parsed by the @code{strtoul} function (@pxref{Parsing of
Integers}) and @code{scanf} with the @samp{%i} conversion
(@pxref{Numeric Input Conversions}).

@item @samp{0}
Pad the field with zeros instead of spaces.  The zeros are placed after
any indication of sign or base.  This flag is ignored if the @samp{-}
flag is also specified, or if a precision is specified.
@end table

If a precision is supplied, it specifies the minimum number of digits to
appear; leading zeros are produced if necessary.  If you don't specify a
precision, the number is printed with as many digits as it needs.  If
you convert a value of zero with an explicit precision of zero, then no
characters at all are produced.

Without a type modifier, the corresponding argument is treated as an
@code{int} (for the signed conversions @samp{%i} and @samp{%d}) or
@code{unsigned int} (for the unsigned conversions @samp{%o}, @samp{%u},
@samp{%x}, and @samp{%X}).  Recall that since @code{printf} and friends
are variadic, any @code{char} and @code{short} arguments are
automatically converted to @code{int} by the default argument
promotions.  For arguments of other integer types, you can use these
modifiers:

@table @samp
@item h
Specifies that the argument is a @code{short int} or @code{unsigned
short int}, as appropriate.  A @code{short} argument is converted to an
@code{int} or @code{unsigned int} by the default argument promotions
anyway, but the @samp{h} modifier says to convert it back to a
@code{short} again.

@item l
Specifies that the argument is a @code{long int} or @code{unsigned long
int}, as appropriate.  Two @samp{l} characters is like the @samp{L}
modifier, below.

@item L
@itemx ll
@itemx q
Specifies that the argument is a @code{long long int}.  (This type is
an extension supported by the GNU C compiler.  On systems that don't
support extra-long integers, this is the same as @code{long int}.)

The @samp{q} modifier is another name for the same thing, which comes
from 4.4 BSD; a @w{@code{long long int}} is sometimes called a ``quad''
@code{int}.

@item Z