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<h3 class="section">4.60 <code>sprintf</code>, <code>fprintf</code>, <code>printf</code>, <code>snprintf</code>, <code>asprintf</code>, <code>asnprintf</code>&mdash;format output</h3>
<p><a name="index-fprintf-285"></a><a name="index-g_t_005ffprintf_005fr-286"></a><a name="index-printf-287"></a><a name="index-g_t_005fprintf_005fr-288"></a><a name="index-asprintf-289"></a><a name="index-g_t_005fasprintf_005fr-290"></a><a name="index-sprintf-291"></a><a name="index-g_t_005fsprintf_005fr-292"></a><a name="index-snprintf-293"></a><a name="index-g_t_005fsnprintf_005fr-294"></a><a name="index-asnprintf-295"></a><a name="index-g_t_005fasnprintf_005fr-296"></a><strong>Synopsis</strong>
<pre class="example"> #include &lt;stdio.h&gt;
int printf(const char *<var>format</var>, ...);
int fprintf(FILE *<var>fd</var>, const char *<var>format</var>, ...);
int sprintf(char *<var>str</var>, const char *<var>format</var>, ...);
int snprintf(char *<var>str</var>, size_t <var>size</var>, const char *<var>format</var>,
...);
int asprintf(char **<var>strp</var>, const char *<var>format</var>, ...);
char *asnprintf(char *<var>str</var>, size_t *<var>size</var>, const char *<var>format</var>,
...);
int _printf_r(struct _reent *<var>ptr</var>, const char *<var>format</var>, ...);
int _fprintf_r(struct _reent *<var>ptr</var>, FILE *<var>fd</var>,
const char *<var>format</var>, ...);
int _sprintf_r(struct _reent *<var>ptr</var>, char *<var>str</var>,
const char *<var>format</var>, ...);
int _snprintf_r(struct _reent *<var>ptr</var>, char *<var>str</var>, size_t <var>size</var>,
const char *<var>format</var>, ...);
int _asprintf_r(struct _reent *<var>ptr</var>, char **<var>strp</var>,
const char *<var>format</var>, ...);
char *_asnprintf_r(struct _reent *<var>ptr</var>, char *<var>str</var>,
size_t *<var>size</var>, const char *<var>format</var>, ...);
</pre>
<p><strong>Description</strong><br>
<code>printf</code> accepts a series of arguments, applies to each a
format specifier from <code>*</code><var>format</var>, and writes the
formatted data to <code>stdout</code>, without a terminating NUL
character. The behavior of <code>printf</code> is undefined if there
are not enough arguments for the format. <code>printf</code> returns
when it reaches the end of the format string. If there are
more arguments than the format requires, excess arguments are
ignored.
<p><code>fprintf</code> is like <code>printf</code>, except that output is directed
to the stream <var>fd</var> rather than <code>stdout</code>.
<p><code>sprintf</code> is like <code>printf</code>, except that output is directed
to the buffer <var>str</var>, and a terminating NUL is output.
Behavior is undefined if more output is generated than the
buffer can hold.
<p><code>snprintf</code> is like <code>sprintf</code>, except that output is
limited to at most <var>size</var> bytes, including the terminating
<code>NUL</code>. As a special case, if <var>size</var> is 0, <var>str</var> can be
NULL, and <code>snprintf</code> merely calculates how many bytes would
be printed.
<p><code>asprintf</code> is like <code>sprintf</code>, except that the output is
stored in a dynamically allocated buffer, <var>pstr</var>, which
should be freed later with <code>free</code>.
<p><code>asnprintf</code> is like <code>sprintf</code>, except that the return type
is either the original <var>str</var> if it was large enough, or a
dynamically allocated string if the output exceeds *<var>size</var>;
the length of the result is returned in *<var>size</var>. When
dynamic allocation occurs, the contents of the original
<var>str</var> may have been modified.
<p>For <code>sprintf</code>, <code>snprintf</code>, and <code>asnprintf</code>, the behavior
is undefined if the output <code>*</code><var>str</var> overlaps with one of
the arguments. Behavior is also undefined if the argument for
<code>%n</code> within <code>*</code><var>format</var> overlaps another argument.
<p><var>format</var> is a pointer to a character string containing two
types of objects: ordinary characters (other than <code>%</code>),
which are copied unchanged to the output, and conversion
specifications, each of which is introduced by <code>%</code>. (To
include <code>%</code> in the output, use <code>%%</code> in the format string.)
A conversion specification has the following form:
<pre class="smallexample"> %[<var>pos</var>][<var>flags</var>][<var>width</var>][.<var>prec</var>][<var>size</var>]<var>type</var>
</pre>
<p>The fields of the conversion specification have the following
meanings:
<ul>
<li><var>pos</var>
<p>Conversions normally consume arguments in the order that they
are presented. However, it is possible to consume arguments
out of order, and reuse an argument for more than one
conversion specification (although the behavior is undefined
if the same argument is requested with different types), by
specifying <var>pos</var>, which is a decimal integer followed by
'$'. The integer must be between 1 and &lt;NL_ARGMAX&gt; from
limits.h, and if argument <code>%n$</code> is requested, all earlier
arguments must be requested somewhere within <var>format</var>. If
positional parameters are used, then all conversion
specifications except for <code>%%</code> must specify a position.
This positional parameters method is a POSIX extension to the C
standard definition for the functions.
<li><var>flags</var>
<p><var>flags</var> is an optional sequence of characters which control
output justification, numeric signs, decimal points, trailing
zeros, and octal and hex prefixes. The flag characters are
minus (<code>-</code>), plus (<code>+</code>), space ( ), zero (<code>0</code>), sharp
(<code>#</code>), and quote (<code>'</code>). They can appear in any
combination, although not all flags can be used for all
conversion specification types.
<dl>
<dt><code>'</code><dd>A POSIX extension to the C standard. However, this
implementation presently treats it as a no-op, which
is the default behavior for the C locale, anyway. (If
it did what it is supposed to, when <var>type</var> were <code>i</code>,
<code>d</code>, <code>u</code>, <code>f</code>, <code>F</code>, <code>g</code>, or <code>G</code>, the
integer portion of the conversion would be formatted
with thousands' grouping wide characters.)
<br><dt><code>-</code><dd>The result of the conversion is left
justified, and the right is padded with
blanks. If you do not use this flag, the
result is right justified, and padded on the
left.
<br><dt><code>+</code><dd>The result of a signed conversion (as
determined by <var>type</var> of <code>d</code>, <code>i</code>, <code>a</code>,
<code>A</code>, <code>e</code>, <code>E</code>, <code>f</code>, <code>F</code>, <code>g</code>, or
<code>G</code>) will always begin with a plus or minus
sign. (If you do not use this flag, positive
values do not begin with a plus sign.)
<br><dt><code>" " (space)</code><dd>If the first character of a signed conversion
specification is not a sign, or if a signed
conversion results in no characters, the
result will begin with a space. If the space
( ) flag and the plus (<code>+</code>) flag both
appear, the space flag is ignored.
<br><dt><code>0</code><dd>If the <var>type</var> character is <code>d</code>, <code>i</code>,
<code>o</code>, <code>u</code>, <code>x</code>, <code>X</code>, <code>a</code>, <code>A</code>,
<code>e</code>, <code>E</code>, <code>f</code>, <code>F</code>, <code>g</code>, or <code>G</code>: leading
zeros are used to pad the field width
(following any indication of sign or base); no
spaces are used for padding. If the zero
(<code>0</code>) and minus (<code>-</code>) flags both appear,
the zero (<code>0</code>) flag will be ignored. For
<code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>x</code>, and <code>X</code>
conversions, if a precision <var>prec</var> is
specified, the zero (<code>0</code>) flag is ignored.
<p>Note that <code>0</code> is interpreted as a flag, not
as the beginning of a field width.
<br><dt><code>#</code><dd>The result is to be converted to an
alternative form, according to the <var>type</var>
character:
<dl>
<dt><code>o</code><dd>Increases precision to force the first
digit of the result to be a zero.
<br><dt><code>x</code><dd>A non-zero result will have a <code>0x</code>
prefix.
<br><dt><code>X</code><dd>A non-zero result will have a <code>0X</code>
prefix.
<br><dt><code>a, A, e, E, f, or F</code><dd>The result will always contain a
decimal point even if no digits follow
the point. (Normally, a decimal point
appears only if a digit follows it.)
Trailing zeros are removed.
<br><dt><code>g or G</code><dd>The result will always contain a
decimal point even if no digits follow
the point. Trailing zeros are not
removed.
<br><dt><code>all others</code><dd>Undefined.
</dl>
</dl>
<li><var>width</var>
<p><var>width</var> is an optional minimum field width. You can
either specify it directly as a decimal integer, or
indirectly by using instead an asterisk (<code>*</code>), in
which case an <code>int</code> argument is used as the field
width. If positional arguments are used, then the
width must also be specified positionally as <code>*m$</code>,
with m as a decimal integer. Negative field widths
are treated as specifying the minus (<code>-</code>) flag for
left justfication, along with a positive field width.
The resulting format may be wider than the specified
width.
<li><var>prec</var>
<p><var>prec</var> is an optional field; if present, it is
introduced with `<code>.</code>' (a period). You can specify
the precision either directly as a decimal integer or
indirectly by using an asterisk (<code>*</code>), in which case
an <code>int</code> argument is used as the precision. If
positional arguments are used, then the precision must
also be specified positionally as <code>*m$</code>, with m as a
decimal integer. Supplying a negative precision is
equivalent to omitting the precision. If only a
period is specified the precision is zero. The effect
depends on the conversion <var>type</var>.
<dl>
<dt><code>d, i, o, u, x, or X</code><dd>Minimum number of digits to appear. If no
precision is given, defaults to 1.
<br><dt><code>a or A</code><dd>Number of digits to appear after the decimal
point. If no precision is given, the
precision defaults to the minimum needed for
an exact representation.
<br><dt><code>e, E, f or F</code><dd>Number of digits to appear after the decimal
point. If no precision is given, the
precision defaults to 6.
<br><dt><code>g or G</code><dd>Maximum number of significant digits. A
precision of 0 is treated the same as a
precision of 1. If no precision is given, the
precision defaults to 6.
<br><dt><code>s or S</code><dd>Maximum number of characters to print from the
string. If no precision is given, the entire
string is printed.
<br><dt><code>all others</code><dd>undefined.
</dl>
<li><var>size</var>
<p><var>size</var> is an optional modifier that changes the data
type that the corresponding argument has. Behavior is
unspecified if a size is given that does not match the
<var>type</var>.
<dl>
<dt><code>hh</code><dd>With <code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>x</code>, or
<code>X</code>, specifies that the argument should be
converted to a <code>signed char</code> or <code>unsigned
char</code> before printing.
<p>With <code>n</code>, specifies that the argument is a
pointer to a <code>signed char</code>.
<br><dt><code>h</code><dd>With <code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>x</code>, or
<code>X</code>, specifies that the argument should be
converted to a <code>short</code> or <code>unsigned short</code>
before printing.
<p>With <code>n</code>, specifies that the argument is a
pointer to a <code>short</code>.
<br><dt><code>l</code><dd>With <code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>x</code>, or
<code>X</code>, specifies that the argument is a
<code>long</code> or <code>unsigned long</code>.
<p>With <code>c</code>, specifies that the argument has
type <code>wint_t</code>.
<p>With <code>s</code>, specifies that the argument is a
pointer to <code>wchar_t</code>.
<p>With <code>n</code>, specifies that the argument is a
pointer to a <code>long</code>.
<p>With <code>a</code>, <code>A</code>, <code>e</code>, <code>E</code>, <code>f</code>, <code>F</code>,
<code>g</code>, or <code>G</code>, has no effect (because of
vararg promotion rules, there is no need to
distinguish between <code>float</code> and <code>double</code>).
<br><dt><code>ll</code><dd>With <code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>x</code>, or
<code>X</code>, specifies that the argument is a
<code>long long</code> or <code>unsigned long long</code>.
<p>With <code>n</code>, specifies that the argument is a
pointer to a <code>long long</code>.
<br><dt><code>j</code><dd>With <code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>x</code>, or
<code>X</code>, specifies that the argument is an
<code>intmax_t</code> or <code>uintmax_t</code>.
<p>With <code>n</code>, specifies that the argument is a
pointer to an <code>intmax_t</code>.
<br><dt><code>z</code><dd>With <code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>x</code>, or
<code>X</code>, specifies that the argument is a <code>size_t</code>.
<p>With <code>n</code>, specifies that the argument is a
pointer to a <code>size_t</code>.
<br><dt><code>t</code><dd>With <code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>x</code>, or
<code>X</code>, specifies that the argument is a
<code>ptrdiff_t</code>.
<p>With <code>n</code>, specifies that the argument is a
pointer to a <code>ptrdiff_t</code>.
<br><dt><code>L</code><dd>With <code>a</code>, <code>A</code>, <code>e</code>, <code>E</code>, <code>f</code>, <code>F</code>,
<code>g</code>, or <code>G</code>, specifies that the argument
is a <code>long double</code>.
</dl>
<li><var>type</var>
<p><var>type</var> specifies what kind of conversion <code>printf</code>
performs. Here is a table of these:
<dl>
<dt><code>%</code><dd>Prints the percent character (<code>%</code>).
<br><dt><code>c</code><dd>Prints <var>arg</var> as single character. If the
<code>l</code> size specifier is in effect, a multibyte
character is printed.
<br><dt><code>C</code><dd>Short for <code>%lc</code>. A POSIX extension to the C standard.
<br><dt><code>s</code><dd>Prints the elements of a pointer to <code>char</code>
until the precision or a null character is
reached. If the <code>l</code> size specifier is in
effect, the pointer is to an array of
<code>wchar_t</code>, and the string is converted to
multibyte characters before printing.
<br><dt><code>S</code><dd>Short for <code>%ls</code>. A POSIX extension to the C standard.
<br><dt><code>d or i</code><dd>Prints a signed decimal integer; takes an
<code>int</code>. Leading zeros are inserted as
necessary to reach the precision. A value of 0 with
a precision of 0 produces an empty string.
<br><dt><code>D</code><dd>Newlib extension, short for <code>%ld</code>.
<br><dt><code>o</code><dd>Prints an unsigned octal integer; takes an
<code>unsigned</code>. Leading zeros are inserted as
necessary to reach the precision. A value of 0 with
a precision of 0 produces an empty string.
<br><dt><code>O</code><dd>Newlib extension, short for <code>%lo</code>.
<br><dt><code>u</code><dd>Prints an unsigned decimal integer; takes an
<code>unsigned</code>. Leading zeros are inserted as
necessary to reach the precision. A value of 0 with
a precision of 0 produces an empty string.
<br><dt><code>U</code><dd>Newlib extension, short for <code>%lu</code>.
<br><dt><code>x</code><dd>Prints an unsigned hexadecimal integer (using
<code>abcdef</code> as digits beyond <code>9</code>); takes an
<code>unsigned</code>. Leading zeros are inserted as
necessary to reach the precision. A value of 0 with
a precision of 0 produces an empty string.
<br><dt><code>X</code><dd>Like <code>x</code>, but uses <code>ABCDEF</code> as digits
beyond <code>9</code>.
<br><dt><code>f</code><dd>Prints a signed value of the form
<code>[-]9999.9999</code>, with the precision
determining how many digits follow the decimal
point; takes a <code>double</code> (remember that
<code>float</code> promotes to <code>double</code> as a vararg).
The low order digit is rounded to even. If
the precision results in at most DECIMAL_DIG
digits, the result is rounded correctly; if
more than DECIMAL_DIG digits are printed, the
result is only guaranteed to round back to the
original value.
<p>If the value is infinite, the result is
<code>inf</code>, and no zero padding is performed. If
the value is not a number, the result is
<code>nan</code>, and no zero padding is performed.
<br><dt><code>F</code><dd>Like <code>f</code>, but uses <code>INF</code> and <code>NAN</code> for
non-finite numbers.
<br><dt><code>e</code><dd>Prints a signed value of the form
<code>[-]9.9999e[+|-]999</code>; takes a <code>double</code>.
The digit before the decimal point is non-zero
if the value is non-zero. The precision
determines how many digits appear between
<code>.</code> and <code>e</code>, and the exponent always
contains at least two digits. The value zero
has an exponent of zero. If the value is not
finite, it is printed like <code>f</code>.
<br><dt><code>E</code><dd>Like <code>e</code>, but using <code>E</code> to introduce the
exponent, and like <code>F</code> for non-finite
values.
<br><dt><code>g</code><dd>Prints a signed value in either <code>f</code> or <code>e</code>
form, based on the given value and
precision&mdash;an exponent less than -4 or
greater than the precision selects the <code>e</code>
form. Trailing zeros and the decimal point
are printed only if necessary; takes a
<code>double</code>.
<br><dt><code>G</code><dd>Like <code>g</code>, except use <code>F</code> or <code>E</code> form.
<br><dt><code>a</code><dd>Prints a signed value of the form
<code>[-]0x1.ffffp[+|-]9</code>; takes a <code>double</code>.
The letters <code>abcdef</code> are used for digits
beyond <code>9</code>. The precision determines how
many digits appear after the decimal point.
The exponent contains at least one digit, and
is a decimal value representing the power of
2; a value of 0 has an exponent of 0.
Non-finite values are printed like <code>f</code>.
<br><dt><code>A</code><dd>Like <code>a</code>, except uses <code>X</code>, <code>P</code>, and
<code>ABCDEF</code> instead of lower case.
<br><dt><code>n</code><dd>Takes a pointer to <code>int</code>, and stores a count
of the number of bytes written so far. No
output is created.
<br><dt><code>p</code><dd>Takes a pointer to <code>void</code>, and prints it in
an implementation-defined format. This
implementation is similar to <code>%#tx</code>), except
that <code>0x</code> appears even for the NULL pointer.
<br><dt><code>m</code><dd>Prints the output of <code>strerror(errno)</code>; no
argument is required. A GNU extension.
</dl>
</ul>
<p><code>_printf_r</code>, <code>_fprintf_r</code>, <code>_asprintf_r</code>,
<code>_sprintf_r</code>, <code>_snprintf_r</code>, <code>_asnprintf_r</code> are simply
reentrant versions of the functions above.
<p><br>
<strong>Returns</strong><br>
On success, <code>sprintf</code> and <code>asprintf</code> return the number of bytes in
the output string, except the concluding <code>NUL</code> is not counted.
<code>snprintf</code> returns the number of bytes that would be in the output
string, except the concluding <code>NUL</code> is not counted. <code>printf</code> and
<code>fprintf</code> return the number of characters transmitted.
<code>asnprintf</code> returns the original <var>str</var> if there was enough room,
otherwise it returns an allocated string.
<p>If an error occurs, the result of <code>printf</code>, <code>fprintf</code>,
<code>snprintf</code>, and <code>asprintf</code> is a negative value, and the result of
<code>asnprintf</code> is NULL. No error returns occur for <code>sprintf</code>. For
<code>printf</code> and <code>fprintf</code>, <code>errno</code> may be set according to
<code>fputc</code>. For <code>asprintf</code> and <code>asnprintf</code>, <code>errno</code> may be set
to ENOMEM if allocation fails, and for <code>snprintf</code>, <code>errno</code> may be
set to EOVERFLOW if <var>size</var> or the output length exceeds INT_MAX.
<p><br>
<strong>Bugs</strong><br>
The &ldquo;&rdquo;' (quote) flag does not work when locale's thousands_sep is not empty.
<p><br>
<strong>Portability</strong><br>
ANSI C requires <code>printf</code>, <code>fprintf</code>, <code>sprintf</code>, and
<code>snprintf</code>. <code>asprintf</code> and <code>asnprintf</code> are newlib extensions.
<p>The ANSI C standard specifies that implementations must support at
least formatted output of up to 509 characters. This implementation
has no inherent limit.
<p>Depending on how newlib was configured, not all format specifiers are
supported.
<p>Supporting OS subroutines required: <code>close</code>, <code>fstat</code>, <code>isatty</code>,
<code>lseek</code>, <code>read</code>, <code>sbrk</code>, <code>write</code>.
<p><br>
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