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   errno    ( 3 )

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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.

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