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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_location (). In fact, that is what it is on GNU/Linux and GNU/Hurd systems. The GNU C Library, on each system, 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.

Except on GNU/Hurd 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 GNU/Hurd systems, 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.