Originální popis anglicky: 
isascii - test for a 7-bit US-ASCII character
Návod, kniha: POSIX Programmer's Manual
#include <ctype.h>
 
 
int isascii(int 
 c);  
 
The 
isascii() function shall test whether 
c is a 7-bit US-ASCII
  character code.
The 
isascii() function is defined on all integer values.
The 
isascii() function shall return non-zero if 
c is a 7-bit
  US-ASCII character code between 0 and octal 0177 inclusive; otherwise, it
  shall return 0.
No errors are defined.
The following sections are informative.
None.
None.
None.
None.
The Base Definitions volume of IEEE Std 1003.1-2001,
  
<ctype.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
  Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
  Operating System Interface (POSIX), The Open Group Base Specifications Issue
  6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
  Engineers, Inc and The Open Group. In the event of any discrepancy between
  this version and the original IEEE and The Open Group Standard, the original
  IEEE and The Open Group Standard is the referee document. The original
  Standard can be obtained online at http://www.opengroup.org/unix/online.html
  .