errno contains the system error number. You can
change the value of
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
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
when they succeed; thus, the value of
errno after a successful
call is not necessarily zero, and you should not use
determine whether a call failed. The proper way to do that is
documented for each function. If the call failed, you can
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
that return a perfectly legitimate value in case of an error, but also
errno. For these functions, if you want to check to see
whether an error occurred, the recommended method is to set
to zero before calling the function, and then check its value afterward.