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Next: <a href="stdio_005fext.html#stdio_005fext" accesskey="n" rel="next">stdio_ext</a>, Previous: <a href="sprintf.html#sprintf" accesskey="p" rel="prev">sprintf</a>, Up: <a href="Stdio.html#Stdio" accesskey="u" rel="up">Stdio</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="sscanf_002c-fscanf_002c-scanf_002d_002d_002dscan-and-format-input"></a>
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<h3 class="section">4.62 <code>sscanf</code>, <code>fscanf</code>, <code>scanf</code>—scan and format input</h3>
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<a name="index-scanf"></a>
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<a name="index-_005fscanf_005fr"></a>
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<a name="index-fscanf"></a>
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<a name="index-_005ffscanf_005fr"></a>
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<a name="index-sscanf"></a>
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<a name="index-_005fsscanf_005fr"></a>
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<p><strong>Synopsis</strong>
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</p><div class="example">
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<pre class="example">#include <stdio.h>
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int scanf(const char *restrict <var>format</var>, ...);
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int fscanf(FILE *restrict <var>fd</var>, const char *restrict <var>format</var>, ...);
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int sscanf(const char *restrict <var>str</var>, const char *restrict <var>format</var>, ...);
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int _scanf_r(struct _reent *<var>ptr</var>, const char *restrict <var>format</var>, ...);
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int _fscanf_r(struct _reent *<var>ptr</var>, FILE *restrict <var>fd</var>,
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const char *restrict <var>format</var>, ...);
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int _sscanf_r(struct _reent *<var>ptr</var>, const char *restrict <var>str</var>,
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const char *restrict <var>format</var>, ...);
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</pre></div>
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<p><strong>Description</strong><br>
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<code>scanf</code> scans a series of input fields from standard input,
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one character at a time. Each field is interpreted according to
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a format specifier passed to <code>scanf</code> in the format string at
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<code>*<var>format</var></code>. <code>scanf</code> stores the interpreted input from
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each field at the address passed to it as the corresponding argument
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following <var>format</var>. You must supply the same number of
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format specifiers and address arguments as there are input fields.
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</p>
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<p>There must be sufficient address arguments for the given format
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specifiers; if not the results are unpredictable and likely
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disasterous. Excess address arguments are merely ignored.
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</p>
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<p><code>scanf</code> often produces unexpected results if the input diverges from
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an expected pattern. Since the combination of <code>gets</code> or <code>fgets</code>
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followed by <code>sscanf</code> is safe and easy, that is the preferred way
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to be certain that a program is synchronized with input at the end
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of a line.
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</p>
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<p><code>fscanf</code> and <code>sscanf</code> are identical to <code>scanf</code>, other than the
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source of input: <code>fscanf</code> reads from a file, and <code>sscanf</code>
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from a string.
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</p>
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<p>The routines <code>_scanf_r</code>, <code>_fscanf_r</code>, and <code>_sscanf_r</code> are reentrant
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versions of <code>scanf</code>, <code>fscanf</code>, and <code>sscanf</code> that take an additional
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first argument pointing to a reentrancy structure.
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</p>
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<p>The string at <code>*<var>format</var></code> is a character sequence composed
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of zero or more directives. Directives are composed of
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one or more whitespace characters, non-whitespace characters,
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and format specifications.
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</p>
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<p>Whitespace characters are blank (<code> </code>), tab (<code>\t</code>), or
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newline (<code>\n</code>).
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When <code>scanf</code> encounters a whitespace character in the format string
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it will read (but not store) all consecutive whitespace characters
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up to the next non-whitespace character in the input.
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</p>
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<p>Non-whitespace characters are all other ASCII characters except the
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percent sign (<code>%</code>). When <code>scanf</code> encounters a non-whitespace
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character in the format string it will read, but not store
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a matching non-whitespace character.
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</p>
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<p>Format specifications tell <code>scanf</code> to read and convert characters
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from the input field into specific types of values, and store then
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in the locations specified by the address arguments.
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</p>
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<p>Trailing whitespace is left unread unless explicitly
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matched in the format string.
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</p>
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<p>The format specifiers must begin with a percent sign (<code>%</code>)
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and have the following form:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> %[*][<var>width</var>][<var>size</var>]<var>type</var>
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</pre></div>
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<p>Each format specification begins with the percent character (<code>%</code>).
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The other fields are:
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</p><ul>
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<li> *
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<p>an optional marker; if present, it suppresses interpretation and
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assignment of this input field.
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</p>
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</li><li> <var>width</var>
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<p>an optional maximum field width: a decimal integer,
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which controls the maximum number of characters that
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will be read before converting the current input field. If the
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input field has fewer than <var>width</var> characters, <code>scanf</code>
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reads all the characters in the field, and then
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proceeds with the next field and its format specification.
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</p>
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<p>If a whitespace or a non-convertable character occurs
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before <var>width</var> character are read, the characters up
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to that character are read, converted, and stored.
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Then <code>scanf</code> proceeds to the next format specification.
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</p>
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</li><li> <var>size</var>
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<p><code>h</code>, <code>j</code>, <code>l</code>, <code>L</code>, <code>t</code>, and <code>z</code> are optional size
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characters which override the default way that <code>scanf</code>
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interprets the data type of the corresponding argument.
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</p>
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<table>
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<thead><tr><th width="18%">Modifier</th><th width="30%">Type(s)</th><th width="52%"></th></tr></thead>
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<tr><td width="18%">hh</td><td width="30%">d, i, o, u, x, n</td><td width="52%">convert input to char, store in char object</td></tr>
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<tr><td width="18%">h</td><td width="30%">d, i, o, u, x, n</td><td width="52%">convert input to short, store in short object</td></tr>
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<tr><td width="18%">h</td><td width="30%">D, I, O, U, X, e, f, c, s, p</td><td width="52%">no effect</td></tr>
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<tr><td width="18%">j</td><td width="30%">d, i, o, u, x, n</td><td width="52%">convert input to intmax_t, store in intmax_t object</td></tr>
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<tr><td width="18%">j</td><td width="30%">all others</td><td width="52%">no effect</td></tr>
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<tr><td width="18%">l</td><td width="30%">d, i, o, u, x, n</td><td width="52%">convert input to long, store in long object</td></tr>
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<tr><td width="18%">l</td><td width="30%">e, f, g</td><td width="52%">convert input to double, store in a double object</td></tr>
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<tr><td width="18%">l</td><td width="30%">D, I, O, U, X, c, s, p</td><td width="52%">no effect</td></tr>
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<tr><td width="18%">ll</td><td width="30%">d, i, o, u, x, n</td><td width="52%">convert to long long, store in long long object</td></tr>
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<tr><td width="18%">L</td><td width="30%">d, i, o, u, x, n</td><td width="52%">convert to long long, store in long long object</td></tr>
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<tr><td width="18%">L</td><td width="30%">e, f, g, E, G</td><td width="52%">convert to long double, store in long double object</td></tr>
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<tr><td width="18%">L</td><td width="30%">all others</td><td width="52%">no effect</td></tr>
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<tr><td width="18%">t</td><td width="30%">d, i, o, u, x, n</td><td width="52%">convert input to ptrdiff_t, store in ptrdiff_t object</td></tr>
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<tr><td width="18%">t</td><td width="30%">all others</td><td width="52%">no effect</td></tr>
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<tr><td width="18%">z</td><td width="30%">d, i, o, u, x, n</td><td width="52%">convert input to size_t, store in size_t object</td></tr>
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<tr><td width="18%">z</td><td width="30%">all others</td><td width="52%">no effect</td></tr>
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</table>
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</li><li> <var>type</var>
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<p>A character to specify what kind of conversion
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<code>scanf</code> performs. Here is a table of the conversion
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characters:
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</p>
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<dl compact="compact">
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<dt><code>%</code></dt>
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<dd><p>No conversion is done; the percent character (<code>%</code>) is stored.
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</p>
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</dd>
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<dt><code>c</code></dt>
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<dd><p>Scans one character. Corresponding <var>arg</var>: <code>(char *arg)</code>.
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</p>
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</dd>
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<dt><code>s</code></dt>
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<dd><p>Reads a character string into the array supplied.
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Corresponding <var>arg</var>: <code>(char arg[])</code>.
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</p>
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</dd>
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<dt><code>[<var>pattern</var>]</code></dt>
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<dd><p>Reads a non-empty character string into memory
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starting at <var>arg</var>. This area must be large
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enough to accept the sequence and a
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terminating null character which will be added
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automatically. (<var>pattern</var> is discussed in the paragraph following
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this table). Corresponding <var>arg</var>: <code>(char *arg)</code>.
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</p>
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</dd>
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<dt><code>d</code></dt>
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<dd><p>Reads a decimal integer into the corresponding <var>arg</var>: <code>(int *arg)</code>.
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</p>
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</dd>
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<dt><code>D</code></dt>
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<dd><p>Reads a decimal integer into the corresponding
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<var>arg</var>: <code>(long *arg)</code>.
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</p>
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</dd>
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<dt><code>o</code></dt>
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<dd><p>Reads an octal integer into the corresponding <var>arg</var>: <code>(int *arg)</code>.
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</p>
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</dd>
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<dt><code>O</code></dt>
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<dd><p>Reads an octal integer into the corresponding <var>arg</var>: <code>(long *arg)</code>.
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</p>
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</dd>
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<dt><code>u</code></dt>
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<dd><p>Reads an unsigned decimal integer into the corresponding
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<var>arg</var>: <code>(unsigned int *arg)</code>.
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</p>
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</dd>
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<dt><code>U</code></dt>
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<dd><p>Reads an unsigned decimal integer into the corresponding <var>arg</var>:
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<code>(unsigned long *arg)</code>.
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</p>
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</dd>
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<dt><code>x,X</code></dt>
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<dd><p>Read a hexadecimal integer into the corresponding <var>arg</var>:
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<code>(int *arg)</code>.
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</p>
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</dd>
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<dt><code>e, f, g</code></dt>
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<dd><p>Read a floating-point number into the corresponding <var>arg</var>:
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<code>(float *arg)</code>.
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</p>
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</dd>
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<dt><code>E, F, G</code></dt>
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<dd><p>Read a floating-point number into the corresponding <var>arg</var>:
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<code>(double *arg)</code>.
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</p>
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</dd>
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<dt><code>i</code></dt>
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<dd><p>Reads a decimal, octal or hexadecimal integer into the
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corresponding <var>arg</var>: <code>(int *arg)</code>.
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</p>
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</dd>
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<dt><code>I</code></dt>
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<dd><p>Reads a decimal, octal or hexadecimal integer into the
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corresponding <var>arg</var>: <code>(long *arg)</code>.
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</p>
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</dd>
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<dt><code>n</code></dt>
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<dd><p>Stores the number of characters read in the corresponding
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<var>arg</var>: <code>(int *arg)</code>.
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</p>
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</dd>
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<dt><code>p</code></dt>
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<dd><p>Stores a scanned pointer. ANSI C leaves the details
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to each implementation; this implementation treats
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<code>%p</code> exactly the same as <code>%U</code>. Corresponding
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<var>arg</var>: <code>(void **arg)</code>.
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</p></dd>
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</dl>
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<p>A <var>pattern</var> of characters surrounded by square brackets can be used
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instead of the <code>s</code> type character. <var>pattern</var> is a set of
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characters which define a search set of possible characters making up
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the <code>scanf</code> input field. If the first character in the brackets is a
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caret (<code>^</code>), the search set is inverted to include all ASCII characters
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except those between the brackets. There is also a range facility
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which you can use as a shortcut. <code>%[0-9] </code> matches all decimal digits.
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The hyphen must not be the first or last character in the set.
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The character prior to the hyphen must be lexically less than the
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character after it.
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</p>
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<p>Here are some <var>pattern</var> examples:
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</p><dl compact="compact">
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<dt><code>%[abcd]</code></dt>
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<dd><p>matches strings containing only <code>a</code>, <code>b</code>, <code>c</code>, and <code>d</code>.
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</p>
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</dd>
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<dt><code>%[^abcd]</code></dt>
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<dd><p>matches strings containing any characters except <code>a</code>, <code>b</code>,
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<code>c</code>, or <code>d</code>
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</p>
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</dd>
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<dt><code>%[A-DW-Z]</code></dt>
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<dd><p>matches strings containing <code>A</code>, <code>B</code>, <code>C</code>, <code>D</code>, <code>W</code>,
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<code>X</code>, <code>Y</code>, <code>Z</code>
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</p>
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</dd>
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<dt><code>%[z-a]</code></dt>
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<dd><p>matches the characters <code>z</code>, <code>-</code>, and <code>a</code>
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</p></dd>
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</dl>
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|
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<p>Floating point numbers (for field types <code>e</code>, <code>f</code>, <code>g</code>, <code>E</code>,
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<code>F</code>, <code>G</code>) must correspond to the following general form:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> [+/-] ddddd[.]ddd [E|e[+|-]ddd]
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</pre></div>
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<p>where objects inclosed in square brackets are optional, and <code>ddd</code>
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represents decimal, octal, or hexadecimal digits.
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</p></li></ul>
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<br>
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<p><strong>Returns</strong><br>
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<code>scanf</code> returns the number of input fields successfully
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scanned, converted and stored; the return value does
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not include scanned fields which were not stored.
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</p>
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<p>If <code>scanf</code> attempts to read at end-of-file, the return
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value is <code>EOF</code>.
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</p>
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<p>If no fields were stored, the return value is <code>0</code>.
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</p>
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<p><code>scanf</code> might stop scanning a particular field before
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reaching the normal field end character, or may
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terminate entirely.
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</p>
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<p><code>scanf</code> stops scanning and storing the current field
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and moves to the next input field (if any)
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in any of the following situations:
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</p>
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<ul>
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<li> The assignment suppressing character (<code>*</code>) appears
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after the <code>%</code> in the format specification; the current
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input field is scanned but not stored.
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</li><li> <var>width</var> characters have been read (<var>width</var> is a
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width specification, a positive decimal integer).
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</li><li> The next character read cannot be converted
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under the the current format (for example,
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if a <code>Z</code> is read when the format is decimal).
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</li><li> The next character in the input field does not appear
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in the search set (or does appear in the inverted search set).
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</li></ul>
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|
|
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<p>When <code>scanf</code> stops scanning the current input field for one of
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these reasons, the next character is considered unread and
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used as the first character of the following input field, or the
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first character in a subsequent read operation on the input.
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</p>
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<p><code>scanf</code> will terminate under the following circumstances:
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</p>
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<ul>
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<li> The next character in the input field conflicts
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with a corresponding non-whitespace character in the
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format string.
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</li><li> The next character in the input field is <code>EOF</code>.
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</li><li> The format string has been exhausted.
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</li></ul>
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<p>When the format string contains a character sequence that is
|
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not part of a format specification, the same character
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sequence must appear in the input; <code>scanf</code> will
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scan but not store the matched characters. If a
|
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conflict occurs, the first conflicting character remains in the input
|
|
as if it had never been read.
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</p>
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<br>
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<p><strong>Portability</strong><br>
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<code>scanf</code> is ANSI C.
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</p>
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<p>Supporting OS subroutines required: <code>close</code>, <code>fstat</code>, <code>isatty</code>,
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|
<code>lseek</code>, <code>read</code>, <code>sbrk</code>, <code>write</code>.
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</p>
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<br>
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<hr>
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<div class="header">
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<p>
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Next: <a href="stdio_005fext.html#stdio_005fext" accesskey="n" rel="next">stdio_ext</a>, Previous: <a href="sprintf.html#sprintf" accesskey="p" rel="prev">sprintf</a>, Up: <a href="Stdio.html#Stdio" accesskey="u" rel="up">Stdio</a> [<a href="Document-Index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Document-Index.html#Document-Index" title="Index" rel="index">Index</a>]</p>
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