stdio.texi

一个C源代码分析器

Specifies that the argument is a @code{size_t}.  This is a GNU extension.
@end table

Here is an example.  Using the template string:

@smallexample
"|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"
@end smallexample

@noindent
to print numbers using the different options for the @samp{%d}
conversion gives results like:

@smallexample
|    0|0    |   +0|+0   |    0|00000|     |   00|0|
|    1|1    |   +1|+1   |    1|00001|    1|   01|1|
|   -1|-1   |   -1|-1   |   -1|-0001|   -1|  -01|-1|
|100000|100000|+100000| 100000|100000|100000|100000|100000|
@end smallexample

In particular, notice what happens in the last case where the number
is too large to fit in the minimum field width specified.

Here are some more examples showing how unsigned integers print under
various format options, using the template string:

@smallexample
"|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"
@end smallexample

@smallexample
|    0|    0|    0|    0|    0|  0x0|  0X0|0x00000000|
|    1|    1|    1|    1|   01|  0x1|  0X1|0x00000001|
|100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|
@end smallexample


@node Floating-Point Conversions
@subsection Floating-Point Conversions

This section discusses the conversion specifications for floating-point
numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G}
conversions.

The @samp{%f} conversion prints its argument in fixed-point notation,
producing output of the form
@w{[@code{-}]@var{ddd}@code{.}@var{ddd}},
where the number of digits following the decimal point is controlled
by the precision you specify.

The @samp{%e} conversion prints its argument in exponential notation,
producing output of the form
@w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}.
Again, the number of digits following the decimal point is controlled by
the precision.  The exponent always contains at least two digits.  The
@samp{%E} conversion is similar but the exponent is marked with the letter
@samp{E} instead of @samp{e}.

The @samp{%g} and @samp{%G} conversions print the argument in the style
of @samp{%e} or @samp{%E} (respectively) if the exponent would be less
than -4 or greater than or equal to the precision; otherwise they use the
@samp{%f} style.  Trailing zeros are removed from the fractional portion
of the result and a decimal-point character appears only if it is
followed by a digit.

The following flags can be used to modify the behavior:

@comment We use @asis instead of @samp so we can have ` ' as an item.
@table @asis
@item @samp{-}
Left-justify the result in the field.  Normally the result is
right-justified.

@item @samp{+}
Always include a plus or minus sign in the result.

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

@item @samp{#}
Specifies that the result should always include a decimal point, even
if no digits follow it.  For the @samp{%g} and @samp{%G} conversions,
this also forces trailing zeros after the decimal point to be left
in place where they would otherwise be removed.

@item @samp{0}
Pad the field with zeros instead of spaces; the zeros are placed
after any sign.  This flag is ignored if the @samp{-} flag is also
specified.
@end table

The precision specifies how many digits follow the decimal-point
character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions.  For
these conversions, the default precision is @code{6}.  If the precision
is explicitly @code{0}, this suppresses the decimal point character
entirely.  For the @samp{%g} and @samp{%G} conversions, the precision
specifies how many significant digits to print.  Significant digits are
the first digit before the decimal point, and all the digits after it.
If the precision @code{0} or not specified for @samp{%g} or @samp{%G},
it is treated like a value of @code{1}.  If the value being printed
cannot be expressed accurately in the specified number of digits, the
value is rounded to the nearest number that fits.

Without a type modifier, the floating-point conversions use an argument
of type @code{double}.  (By the default argument promotions, any
@code{float} arguments are automatically converted to @code{double}.)
The following type modifier is supported:

@table @samp
@item L
An uppercase @samp{L} specifies that the argument is a @code{long
double}.
@end table

Here are some examples showing how numbers print using the various
floating-point conversions.  All of the numbers were printed using
this template string:

@smallexample
"|%12.4f|%12.4e|%12.4g|\n"
@end smallexample

Here is the output:

@smallexample
|      0.0000|  0.0000e+00|           0|
|      1.0000|  1.0000e+00|           1|
|     -1.0000| -1.0000e+00|          -1|
|    100.0000|  1.0000e+02|         100|
|   1000.0000|  1.0000e+03|        1000|
|  10000.0000|  1.0000e+04|       1e+04|
|  12345.0000|  1.2345e+04|   1.234e+04|
| 100000.0000|  1.0000e+05|       1e+05|
| 123456.0000|  1.2346e+05|   1.234e+05|
@end smallexample

Notice how the @samp{%g} conversion drops trailing zeros.

@node Other Output Conversions
@subsection Other Output Conversions

This section describes miscellaneous conversions for @code{printf}.

The @samp{%c} conversion prints a single character.  The @code{int}
argument is first converted to an @code{unsigned char}.  The @samp{-}
flag can be used to specify left-justification in the field, but no
other flags are defined, and no precision or type modifier can be given.
For example:

@smallexample
printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');
@end smallexample

@noindent
prints @samp{hello}.

The @samp{%s} conversion prints a string.  The corresponding argument
must be of type @code{char *} (or @code{const char *}).  A precision can
be specified to indicate the maximum number of characters to write;
otherwise characters in the string up to but not including the
terminating null character are written to the output stream.  The
@samp{-} flag can be used to specify left-justification in the field,
but no other flags or type modifiers are defined for this conversion.
For example:

@smallexample
printf ("%3s%-6s", "no", "where");
@end smallexample

@noindent
prints @samp{ nowhere }.

If you accidentally pass a null pointer as the argument for a @samp{%s}
conversion, the GNU library prints it as @samp{(null)}.  We think this
is more useful than crashing.  But it's not good practice to pass a null
argument intentionally.

The @samp{%m} conversion prints the string corresponding to the error
code in @code{errno}.  @xref{Error Messages}.  Thus:

@smallexample
fprintf (stderr, "can't open `%s': %m\n", filename);
@end smallexample

@noindent
is equivalent to:

@smallexample
fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));
@end smallexample

@noindent
The @samp{%m} conversion is a GNU C library extension.

The @samp{%p} conversion prints a pointer value.  The corresponding
argument must be of type @code{void *}.  In practice, you can use any
type of pointer.

In the GNU system, non-null pointers are printed as unsigned integers,
as if a @samp{%#x} conversion were used.  Null pointers print as
@samp{(nil)}.  (Pointers might print differently in other systems.)

For example:

@smallexample
printf ("%p", "testing");
@end smallexample

@noindent
prints @samp{0x} followed by a hexadecimal number---the address of the
string constant @code{"testing"}.  It does not print the word
@samp{testing}.

You can supply the @samp{-} flag with the @samp{%p} conversion to
specify left-justification, but no other flags, precision, or type
modifiers are defined.

The @samp{%n} conversion is unlike any of the other output conversions.
It uses an argument which must be a pointer to an @code{int}, but
instead of printing anything it stores the number of characters printed
so far by this call at that location.  The @samp{h} and @samp{l} type
modifiers are permitted to specify that the argument is of type
@code{short int *} or @code{long int *} instead of @code{int *}, but no
flags, field width, or precision are permitted.

For example,

@smallexample
int nchar;
printf ("%d %s%n\n", 3, "bears", &nchar);
@end smallexample

@noindent
prints:

@smallexample
3 bears
@end smallexample

@noindent
and sets @code{nchar} to @code{7}, because @samp{3 bears} is seven 
characters.


The @samp{%%} conversion prints a literal @samp{%} character.  This
conversion doesn't use an argument, and no flags, field width,
precision, or type modifiers are permitted.


@node Formatted Output Functions
@subsection Formatted Output Functions

This section describes how to call @code{printf} and related functions.
Prototypes for these functions are in the header file @file{stdio.h}.
Because these functions take a variable number of arguments, you
@emph{must} declare prototypes for them before using them.  Of course,
the easiest way to make sure you have all the right prototypes is to
just include @file{stdio.h}.
@pindex stdio.h

@comment stdio.h
@comment ANSI
@deftypefun int printf (const char *@var{template}, @dots{})
The @code{printf} function prints the optional arguments under the
control of the template string @var{template} to the stream
@code{stdout}.  It returns the number of characters printed, or a
negative value if there was an output error.
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypefun int fprintf (FILE *@var{stream}, const char *@var{template}, @dots{})
This function is just like @code{printf}, except that the output is
written to the stream @var{stream} instead of @code{stdout}.
@end deftypefun

@comment stdio.h
@comment ANSI
@deftypefun int sprintf (char *@var{s}, const char *@var{template}, @dots{})
This is like @code{printf}, except that the output is stored in the character
array @var{s} instead of written to a stream.  A null character is written
to mark the end of the string.

The @code{sprintf} function returns the number of characters stored in
the array @var{s}, not including the terminating null character.

The behavior of this function is undefined if copying takes place
between objects that overlap---for example, if @var{s} is also given
as an argument to be printed under control of the @samp{%s} conversion.
@xref{Copying and Concatenation}.

@strong{Warning:} The @code{sprintf} function can be @strong{dangerous}
because it can potentially output more characters than can fit in the
allocation size of the string @var{s}.  Remember that the field width
given in a conversion specification is only a @emph{minimum} value.

To avoid this problem, you can use @code{snprintf} or @code{asprintf},
described below.
@end deftypefun

@comment stdio.h
@comment GNU
@deftypefun int snprintf (char *@var{s}, size_t @var{size}, const char *@var{template}, @dots{})
The @code{snprintf} function is similar to @code{sprintf}, except that
the @var{size} argument specifies the maximum number of characters to
produce.  The trailing null character is counted towards this limit, so
you should allocate at least @var{size} characters for the string @var{s}.

The return value is the number of characters stored, not including the
terminating null.  If this value equals @code{@var{size} - 1}, then
there was not enough space in @var{s} for all the output.  You should
try again with a bigger output string.  Here is an example of doing
this:

@smallexample
@group
/* @r{Construct a message describing the value of a variable}
   @r{whose name is @var{name} and whose value is @var{value}.} */
char *
make_message (char *name, char *value)
@{
  /* @r{Guess we need no more than 100 chars of space.} */
  int size = 100;
  char *buffer = (char *) xmalloc (size);
@end group
@group
  while (1)
    @{
      /* @r{Try to print in the allocated space.} */
      int nchars = snprintf (buffer, size,
                             "value of %s is %s",
                             name, value);
      /* @r{If that worked, return the string.} */
      if (nchars < size)
        return buffer;
      /* @r{Else try again with twice as much space.} */
      size *= 2;
      buffer = (char *) xrealloc (size, buffer);
    @}
@}
@end group
@end smallexample