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<h1 id="firstHeading" class="firstHeading">strcat, strcat_s</h1> <table class="t-dcl-begin"> <tr class="t-dsc-header"> <th> Defined in header <code><string.h></code> </th> <th> </th> <th> </th> </tr> <tr class="t-dcl-rev-aux"> <td></td> <td rowspan="3">(1)</td> <td></td> </tr> <tr class="t-dcl t-until-c99"> <td> <pre data-language="c">char *strcat( char *dest, const char *src );</pre>
</td> <td> <span class="t-mark-rev t-until-c99">(until C99)</span> </td> </tr> <tr class="t-dcl t-since-c99"> <td> <pre data-language="c">char *strcat( char *restrict dest, const char *restrict src );</pre>
</td> <td> <span class="t-mark-rev t-since-c99">(since C99)</span> </td> </tr> <tr class="t-dcl t-since-c11"> <td> <pre data-language="c">errno_t strcat_s(char *restrict dest, rsize_t destsz, const char *restrict src);</pre>
</td> <td> (2) </td> <td> <span class="t-mark-rev t-since-c11">(since C11)</span> </td> </tr> </table> <div class="t-li1">
<span class="t-li">1)</span> Appends a copy of the null-terminated byte string pointed to by <code>src</code> to the end of the null-terminated byte string pointed to by <code>dest</code>. The character <code>src[0]</code> replaces the null terminator at the end of <code>dest</code>. The resulting byte string is null-terminated.</div> <div class="t-li1">
The behavior is undefined if the destination array is not large enough for the contents of both <code>src</code> and <code>dest</code> and the terminating null character. The behavior is undefined if the strings overlap. The behavior is undefined if either <code>dest</code> or <code>src</code> is not a pointer to a null-terminated byte string.</div> <div class="t-li1">
<span class="t-li">2)</span> Same as <span class="t-v">(1)</span>, except that it may clobber the rest of the destination array (from the last character written to <code>destsz</code>) with unspecified values and that the following errors are detected at runtime and call the currently installed <a href="../../error/set_constraint_handler_s" title="c/error/set constraint handler s">constraint handler</a> function: <ul>
<li> <code>src</code> or <code>dest</code> is a null pointer </li>
<li> <code>destsz</code> is zero or greater than <code>RSIZE_MAX</code> </li>
<li> there is no null terminator in the first <code>destsz</code> bytes of <code>dest</code> </li>
<li> truncation would occur (the available space at the end of <code>dest</code> would not fit every character, including the null terminator, of <code>src</code>) </li>
<li> overlap would occur between the source and the destination strings</li>
</ul>
</div> <div class="t-li1">
The behavior is undefined if the size of the character array pointed to by <code>dest</code> < <code><a href="http://en.cppreference.com/w/c/string/byte/strlen"><span class="kw557">strlen</span></a><span class="br0">(</span>dest<span class="br0">)</span><span class="sy2">+</span><a href="http://en.cppreference.com/w/c/string/byte/strlen"><span class="kw557">strlen</span></a><span class="br0">(</span>src<span class="br0">)</span><span class="sy2">+</span><span class="nu0">1</span></code> <= <code>destsz</code>; in other words, an erroneous value of <code>destsz</code> does not expose the impending buffer overflow. As with all bounds-checked functions, <code>strcat_s</code> only guaranteed to be available if <code>__STDC_LIB_EXT1__</code> is defined by the implementation and if the user defines <code>__STDC_WANT_LIB_EXT1__</code> to the integer constant <code>1</code> before including <a href="../byte" title="c/string/byte"><code><string.h></code></a>.</div> <h3 id="Parameters"> Parameters</h3> <table class="t-par-begin"> <tr class="t-par"> <td> dest </td> <td> - </td> <td> pointer to the null-terminated byte string to append to </td>
</tr> <tr class="t-par"> <td> src </td> <td> - </td> <td> pointer to the null-terminated byte string to copy from </td>
</tr> <tr class="t-par"> <td> destsz </td> <td> - </td> <td> maximum number of characters to write, typically the size of the destination buffer </td>
</tr>
</table> <h3 id="Return_value"> Return value</h3> <div class="t-li1">
<span class="t-li">1)</span> returns a copy of <code>dest</code>
</div> <div class="t-li1">
<span class="t-li">2)</span> returns zero on success, returns non-zero on error. Also, on error, writes zero to <code>dest[0]</code> (unless <code>dest</code> is a null pointer or <code>destsz</code> is zero or greater than <code>RSIZE_MAX</code>).</div> <h3 id="Notes"> Notes</h3> <p>Because <code>strcat</code> needs to seek to the end of <code>dest</code> on each call, it is inefficient to concatenate many strings into one using <code>strcat</code>.</p>
<p><code>strcat_s</code> is allowed to clobber the destination array from the last character written up to <code>destsz</code> in order to improve efficiency: it may copy in multibyte blocks and then check for null bytes.</p>
<p>The function <code>strcat_s</code> is similar to the <a rel="nofollow" class="external text" href="https://www.freebsd.org/cgi/man.cgi?query=strlcat&sektion=3">BSD function <code>strlcat</code></a>, except that</p>
<ul>
<li> <code>strlcat</code> truncates the source string to fit in the destination </li>
<li> <code>strlcat</code> does not perform all the runtime checks that <code>strcat_s</code> does </li>
<li> <code>strlcat</code> does not make failures obvious by setting the destination to a null string or calling a handler if the call fails. </li>
</ul> <p>Although <code>strcat_s</code> prohibits truncation due to potential security risks, it's possible to truncate a string using bounds-checked <code><a href="strncat" title="c/string/byte/strncat">strncat_s</a></code> instead.</p>
<h3 id="Example"> Example</h3> <div class="t-example"> <div class="c source-c"><pre data-language="c">#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char str[50] = "Hello ";
char str2[50] = "World!";
strcat(str, str2);
strcat(str, " ...");
strcat(str, " Goodbye World!");
puts(str);
#ifdef __STDC_LIB_EXT1__
set_constraint_handler_s(ignore_handler_s);
int r = strcat_s(str, sizeof str, " ... ");
printf("str = \"%s\", r = %d\n", str, r);
r = strcat_s(str, sizeof str, " and this is too much");
printf("str = \"%s\", r = %d\n", str, r);
#endif
}</pre></div> <p>Possible output:</p>
<div class="text source-text"><pre data-language="c">Hello World! ... Goodbye World!
str = "Hello World! ... Goodbye World! ... ", r = 0
str = "", r = 22</pre></div> </div> <h3 id="References"> References</h3> <ul>
<li> C11 standard (ISO/IEC 9899:2011): </li>
<ul>
<li> 7.24.3.1 The strcat function (p: 364) </li>
<li> K.3.7.2.1 The strcat_s function (p: 617-618) </li>
</ul>
<li> C99 standard (ISO/IEC 9899:1999): </li>
<ul><li> 7.21.3.1 The strcat function (p: 327) </li></ul>
<li> C89/C90 standard (ISO/IEC 9899:1990): </li>
<ul><li> 4.11.3.1 The strcat function </li></ul>
</ul> <h3 id="See_also"> See also</h3> <table class="t-dsc-begin"> <tr class="t-dsc"> <td> <div><a href="strncat" title="c/string/byte/strncat"> <span class="t-lines"><span>strncat</span><span>strncat_s</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div> </td> <td> concatenates a certain amount of characters of two strings <br> <span class="t-mark">(function)</span> </td>
</tr> <tr class="t-dsc"> <td> <div><a href="strcpy" title="c/string/byte/strcpy"> <span class="t-lines"><span>strcpy</span><span>strcpy_s</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div> </td> <td> copies one string to another <br> <span class="t-mark">(function)</span> </td>
</tr> <tr class="t-dsc"> <td> <div><a href="memccpy" title="c/string/byte/memccpy"> <span class="t-lines"><span>memccpy</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-c23">(C23)</span></span></span></div> </td> <td> copies one buffer to another, stopping after the specified delimiter <br> <span class="t-mark">(function)</span> </td>
</tr> <tr class="t-dsc"> <td colspan="2"> <span><a href="https://en.cppreference.com/w/cpp/string/byte/strcat" title="cpp/string/byte/strcat">C++ documentation</a></span> for <code>strcat</code> </td>
</tr> </table> <div class="_attribution">
<p class="_attribution-p">
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