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| <h4 class="subsection">12.14.2 Input Conversion Syntax</h4> |
| |
| <p>A <code>scanf</code> template string is a string that contains ordinary |
| multibyte characters interspersed with conversion specifications that |
| start with ‘<samp><span class="samp">%</span></samp>’. |
| |
| <p>Any whitespace character (as defined by the <code>isspace</code> function; |
| see <a href="Classification-of-Characters.html#Classification-of-Characters">Classification of Characters</a>) in the template causes any number |
| of whitespace characters in the input stream to be read and discarded. |
| The whitespace characters that are matched need not be exactly the same |
| whitespace characters that appear in the template string. For example, |
| write ‘<samp><span class="samp"> , </span></samp>’ in the template to recognize a comma with optional |
| whitespace before and after. |
| |
| <p>Other characters in the template string that are not part of conversion |
| specifications must match characters in the input stream exactly; if |
| this is not the case, a matching failure occurs. |
| |
| <p>The conversion specifications in a <code>scanf</code> template string |
| have the general form: |
| |
| <pre class="smallexample"> % <var>flags</var> <var>width</var> <var>type</var> <var>conversion</var> |
| </pre> |
| <p>In more detail, an input conversion specification consists of an initial |
| ‘<samp><span class="samp">%</span></samp>’ character followed in sequence by: |
| |
| <ul> |
| <li>An optional <dfn>flag character</dfn> ‘<samp><span class="samp">*</span></samp>’, which says to ignore the text |
| read for this specification. When <code>scanf</code> finds a conversion |
| specification that uses this flag, it reads input as directed by the |
| rest of the conversion specification, but it discards this input, does |
| not use a pointer argument, and does not increment the count of |
| successful assignments. |
| <a name="index-flag-character-_0028_0040code_007bscanf_007d_0029-1078"></a> |
| <li>An optional flag character ‘<samp><span class="samp">a</span></samp>’ (valid with string conversions only) |
| which requests allocation of a buffer long enough to store the string in. |
| (This is a GNU extension.) |
| See <a href="Dynamic-String-Input.html#Dynamic-String-Input">Dynamic String Input</a>. |
| |
| <li>An optional decimal integer that specifies the <dfn>maximum field |
| width</dfn>. Reading of characters from the input stream stops either when |
| this maximum is reached or when a non-matching character is found, |
| whichever happens first. Most conversions discard initial whitespace |
| characters (those that don't are explicitly documented), and these |
| discarded characters don't count towards the maximum field width. |
| String input conversions store a null character to mark the end of the |
| input; the maximum field width does not include this terminator. |
| <a name="index-maximum-field-width-_0028_0040code_007bscanf_007d_0029-1079"></a> |
| <li>An optional <dfn>type modifier character</dfn>. For example, you can |
| specify a type modifier of ‘<samp><span class="samp">l</span></samp>’ with integer conversions such as |
| ‘<samp><span class="samp">%d</span></samp>’ to specify that the argument is a pointer to a <code>long int</code> |
| rather than a pointer to an <code>int</code>. |
| <a name="index-type-modifier-character-_0028_0040code_007bscanf_007d_0029-1080"></a> |
| <li>A character that specifies the conversion to be applied. |
| </ul> |
| |
| <p>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 allow. |
| |
| <p>With the ‘<samp><span class="samp">-Wformat</span></samp>’ option, the GNU C compiler checks calls to |
| <code>scanf</code> 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>scanf</code>-style format string. |
| See <a href="../gcc/Function-Attributes.html#Function-Attributes">Declaring Attributes of Functions</a>, for more information. |
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