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| <h3 class="section">5.3 String Length</h3> |
| |
| <p>You can get the length of a string using the <code>strlen</code> function. |
| This function is declared in the header file <samp><span class="file">string.h</span></samp>. |
| <a name="index-string_002eh-473"></a> |
| <!-- string.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: size_t <b>strlen</b> (<var>const char *s</var>)<var><a name="index-strlen-474"></a></var><br> |
| <blockquote><p>The <code>strlen</code> function returns the length of the null-terminated |
| string <var>s</var> in bytes. (In other words, it returns the offset of the |
| terminating null character within the array.) |
| |
| <p>For example, |
| <pre class="smallexample"> strlen ("hello, world") |
| ⇒ 12 |
| </pre> |
| <p>When applied to a character array, the <code>strlen</code> function returns |
| the length of the string stored there, not its allocated size. You can |
| get the allocated size of the character array that holds a string using |
| the <code>sizeof</code> operator: |
| |
| <pre class="smallexample"> char string[32] = "hello, world"; |
| sizeof (string) |
| ⇒ 32 |
| strlen (string) |
| ⇒ 12 |
| </pre> |
| <p>But beware, this will not work unless <var>string</var> is the character |
| array itself, not a pointer to it. For example: |
| |
| <pre class="smallexample"> char string[32] = "hello, world"; |
| char *ptr = string; |
| sizeof (string) |
| ⇒ 32 |
| sizeof (ptr) |
| ⇒ 4 /* <span class="roman">(on a machine with 4 byte pointers)</span> */ |
| </pre> |
| <p>This is an easy mistake to make when you are working with functions that |
| take string arguments; those arguments are always pointers, not arrays. |
| |
| <p>It must also be noted that for multibyte encoded strings the return |
| value does not have to correspond to the number of characters in the |
| string. To get this value the string can be converted to wide |
| characters and <code>wcslen</code> can be used or something like the following |
| code can be used: |
| |
| <pre class="smallexample"> /* <span class="roman">The input is in </span><code>string</code><span class="roman">.</span> |
| <span class="roman">The length is expected in </span><code>n</code><span class="roman">.</span> */ |
| { |
| mbstate_t t; |
| char *scopy = string; |
| /* In initial state. */ |
| memset (&t, '\0', sizeof (t)); |
| /* Determine number of characters. */ |
| n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t); |
| } |
| </pre> |
| <p>This is cumbersome to do so if the number of characters (as opposed to |
| bytes) is needed often it is better to work with wide characters. |
| </p></blockquote></div> |
| |
| <p>The wide character equivalent is declared in <samp><span class="file">wchar.h</span></samp>. |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: size_t <b>wcslen</b> (<var>const wchar_t *ws</var>)<var><a name="index-wcslen-475"></a></var><br> |
| <blockquote><p>The <code>wcslen</code> function is the wide character equivalent to |
| <code>strlen</code>. The return value is the number of wide characters in the |
| wide character string pointed to by <var>ws</var> (this is also the offset of |
| the terminating null wide character of <var>ws</var>). |
| |
| <p>Since there are no multi wide character sequences making up one |
| character the return value is not only the offset in the array, it is |
| also the number of wide characters. |
| |
| <p>This function was introduced in Amendment 1<!-- /@w --> to ISO C90<!-- /@w -->. |
| </p></blockquote></div> |
| |
| <!-- string.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: size_t <b>strnlen</b> (<var>const char *s, size_t maxlen</var>)<var><a name="index-strnlen-476"></a></var><br> |
| <blockquote><p>The <code>strnlen</code> function returns the length of the string <var>s</var> in |
| bytes if this length is smaller than <var>maxlen</var> bytes. Otherwise it |
| returns <var>maxlen</var>. Therefore this function is equivalent to |
| <code>(strlen (</code><var>s</var><code>) < n ? strlen (</code><var>s</var><code>) : </code><var>maxlen</var><code>)</code> but it |
| is more efficient and works even if the string <var>s</var> is not |
| null-terminated. |
| |
| <pre class="smallexample"> char string[32] = "hello, world"; |
| strnlen (string, 32) |
| ⇒ 12 |
| strnlen (string, 5) |
| ⇒ 5 |
| </pre> |
| <p>This function is a GNU extension and is declared in <samp><span class="file">string.h</span></samp>. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: size_t <b>wcsnlen</b> (<var>const wchar_t *ws, size_t maxlen</var>)<var><a name="index-wcsnlen-477"></a></var><br> |
| <blockquote><p><code>wcsnlen</code> is the wide character equivalent to <code>strnlen</code>. The |
| <var>maxlen</var> parameter specifies the maximum number of wide characters. |
| |
| <p>This function is a GNU extension and is declared in <samp><span class="file">wchar.h</span></samp>. |
| </p></blockquote></div> |
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