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| <h3 class="section">2.1 Checking for Errors</h3> |
| |
| <p>Most library functions return a special value to indicate that they have |
| failed. The special value is typically <code>-1</code>, a null pointer, or a |
| constant such as <code>EOF</code> that is defined for that purpose. But this |
| return value tells you only that an error has occurred. To find out |
| what kind of error it was, you need to look at the error code stored in the |
| variable <code>errno</code>. This variable is declared in the header file |
| <samp><span class="file">errno.h</span></samp>. |
| <a name="index-errno_002eh-59"></a> |
| <!-- errno.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Variable: volatile int <b>errno</b><var><a name="index-errno-60"></a></var><br> |
| <blockquote><p>The variable <code>errno</code> contains the system error number. You can |
| change the value of <code>errno</code>. |
| |
| <p>Since <code>errno</code> is declared <code>volatile</code>, it might be changed |
| asynchronously by a signal handler; see <a href="Defining-Handlers.html#Defining-Handlers">Defining Handlers</a>. |
| However, a properly written signal handler saves and restores the value |
| of <code>errno</code>, so you generally do not need to worry about this |
| possibility except when writing signal handlers. |
| |
| <p>The initial value of <code>errno</code> at program startup is zero. Many |
| library functions are guaranteed to set it to certain nonzero values |
| when they encounter certain kinds of errors. These error conditions are |
| listed for each function. These functions do not change <code>errno</code> |
| when they succeed; thus, the value of <code>errno</code> after a successful |
| call is not necessarily zero, and you should not use <code>errno</code> to |
| determine <em>whether</em> a call failed. The proper way to do that is |
| documented for each function. <em>If</em> the call failed, you can |
| examine <code>errno</code>. |
| |
| <p>Many library functions can set <code>errno</code> to a nonzero value as a |
| result of calling other library functions which might fail. You should |
| assume that any library function might alter <code>errno</code> when the |
| function returns an error. |
| |
| <p><strong>Portability Note:</strong> ISO C<!-- /@w --> specifies <code>errno</code> as a |
| “modifiable lvalue” rather than as a variable, permitting it to be |
| implemented as a macro. For example, its expansion might involve a |
| function call, like <code>*_errno ()</code><!-- /@w -->. In fact, that is what it is |
| on the GNU system itself. The GNU library, on non-GNU systems, does |
| whatever is right for the particular system. |
| |
| <p>There are a few library functions, like <code>sqrt</code> and <code>atan</code>, |
| that return a perfectly legitimate value in case of an error, but also |
| set <code>errno</code>. For these functions, if you want to check to see |
| whether an error occurred, the recommended method is to set <code>errno</code> |
| to zero before calling the function, and then check its value afterward. |
| </p></blockquote></div> |
| |
| <p><a name="index-errno_002eh-61"></a>All the error codes have symbolic names; they are macros defined in |
| <samp><span class="file">errno.h</span></samp>. The names start with ‘<samp><span class="samp">E</span></samp>’ and an upper-case |
| letter or digit; you should consider names of this form to be |
| reserved names. See <a href="Reserved-Names.html#Reserved-Names">Reserved Names</a>. |
| |
| <p>The error code values are all positive integers and are all distinct, |
| with one exception: <code>EWOULDBLOCK</code> and <code>EAGAIN</code> are the same. |
| Since the values are distinct, you can use them as labels in a |
| <code>switch</code> statement; just don't use both <code>EWOULDBLOCK</code> and |
| <code>EAGAIN</code>. Your program should not make any other assumptions about |
| the specific values of these symbolic constants. |
| |
| <p>The value of <code>errno</code> doesn't necessarily have to correspond to any |
| of these macros, since some library functions might return other error |
| codes of their own for other situations. The only values that are |
| guaranteed to be meaningful for a particular library function are the |
| ones that this manual lists for that function. |
| |
| <p>On non-GNU systems, almost any system call can return <code>EFAULT</code> if |
| it is given an invalid pointer as an argument. Since this could only |
| happen as a result of a bug in your program, and since it will not |
| happen on the GNU system, we have saved space by not mentioning |
| <code>EFAULT</code> in the descriptions of individual functions. |
| |
| <p>In some Unix systems, many system calls can also return <code>EFAULT</code> if |
| given as an argument a pointer into the stack, and the kernel for some |
| obscure reason fails in its attempt to extend the stack. If this ever |
| happens, you should probably try using statically or dynamically |
| allocated memory instead of stack memory on that system. |
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