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
errno. This variable is declared in the header file
errnocontains the system error number. You can change the value of
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
errnoat 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
errnowhen they succeed; thus, the value of
errnoafter a successful call is not necessarily zero, and you should not use
errnoto determine whether a call failed. The proper way to do that is documented for each function. If the call failed, you can examine
Many library functions can set
errnoto a nonzero value as a result of calling other library functions which might fail. You should assume that any library function might alter
errnowhen the function returns an error.
Portability Note: ISO C specifies
errnoas 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
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
errnoto 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:
EAGAIN are the same.
Since the values are distinct, you can use them as labels in a
switch statement; just don't use both
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
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
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.