номер последней ошибки (number of last error)
Примечание (Note)
A common mistake is to do
if (somecall() == -1) {
printf("somecall() failed\n");
if (errno == ...) { ... }
}
where errno no longer needs to have the value it had upon return
from somecall() (i.e., it may have been changed by the
printf(3)). If the value of errno should be preserved across a
library call, it must be saved:
if (somecall() == -1) {
int errsv = errno;
printf("somecall() failed\n");
if (errsv == ...) { ... }
}
Note that the POSIX threads APIs do not set errno on error.
Instead, on failure they return an error number as the function
result. These error numbers have the same meanings as the error
numbers returned in errno by other APIs.
On some ancient systems, <errno.h> was not present or did not
declare errno, so that it was necessary to declare errno manually
(i.e., extern int errno). Do not do this. It long ago ceased to
be necessary, and it will cause problems with modern versions of
the C library.
