@node Error reporting
@chapter Error reporting

Many functions, especially those involving
system calls, detect and report exceptional
conditions, including error conditions and
interruptions. Exception conditions must often
be checked and handled, often by retrying,
doing something else, or reporting the error
to the user and exit.

Unless otherwise specified, exceptional
conditions are reporting using the facilities
described in this chapter.


@menu
* Error checking::                            Checking whether an exception condition occurred.
@end menu



@node Error checking
@section Error checking

@cpindex Checking for errors
@cpindex Errors checking
@cpindex Exception checking
Most functions return a sentinel value when an
exceptional condition occurs, typically a negative
value (actually @code{-1}), a constant such as
@code{EOF}, or @code{NULL}. This return value only
tells you that an exeception condition has occurred.

@hfindex errno.h
@lvindex errno
By including the header file @file{<errno.h>}, the
modifiable lvalue @code{errno} is made available.
@sc{ISO}@tie{}C specifies that @code{errno} is not
necessarily a variable, but rather just an modifiable
lvalue. In fact, must @code{libc}:s, including
@code{slibc} defines it as a macro that calls a
function and uses the unary @code{*}-operator on the
return.

@code{errno} is often thought of as a variable
declared as @code{int errno}, or @code{volatile int errno}.
But the value is thread-local, and is thus more
like @code{_Thread_local volatile int errno}. Because
of this, the @code{errno} is defined as a macro that
calls a function. Otherwise, it would not be implementable
without @sc{ISO}@tie{}C11.