Checking for Errors - The GNU C Library

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### 2.1 Checking for Errors

Most library functions return a special value to indicate that they have failed. The special value is typically `-1`, a null pointer, or a constant such as `EOF` 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 `errno`. This variable is declared in the header file `errno.h`.

— Variable: volatile int errno

> The variable `errno` contains the system error number. You can change the value of `errno`.

> Since `errno` is declared `volatile`, it might be changed asynchronously by a signal handler; see Defining Handlers. However, a properly written signal handler saves and restores the value of `errno`, so you generally do not need to worry about this possibility except when writing signal handlers.

> The initial value of `errno` 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 `errno` when they succeed; thus, the value of `errno` after a successful call is not necessarily zero, and you should not use `errno` to determine whether a call failed. The proper way to do that is documented for each function. If the call failed, you can examine `errno`.

> Many library functions can set `errno` to a nonzero value as a result of calling other library functions which might fail. You should assume that any library function might alter `errno` when the function returns an error.

> Portability Note: ISO C specifies `errno` 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 `*_errno ()`. 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.

> There are a few library functions, like `sqrt` and `atan`, that return a perfectly legitimate value in case of an error, but also set `errno`. For these functions, if you want to check to see whether an error occurred, the recommended method is to set `errno` to zero before calling the function, and then check its value afterward.

All the error codes have symbolic names; they are macros defined in `errno.h`. The names start with ‘`E`’ and an upper-case letter or digit; you should consider names of this form to be reserved names. See Reserved Names.

The error code values are all positive integers and are all distinct, with one exception: `EWOULDBLOCK` and `EAGAIN` are the same. Since the values are distinct, you can use them as labels in a `switch` statement; just don't use both `EWOULDBLOCK` and `EAGAIN`. Your program should not make any other assumptions about the specific values of these symbolic constants.

The value of `errno` 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.

On non-GNU systems, almost any system call can return `EFAULT` 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 `EFAULT` in the descriptions of individual functions.

In some Unix systems, many system calls can also return `EFAULT` 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.