Date: 2010nov25 Updated: 2019mar18 Language: C++ Product: Visual C++ Q. Debug Assertion fails: Expression:(unsigned)(c+1) <= 256 Why is my program getting this debug assertion failed:

Debug Assertion failed! File: isctype.c Line: 56 Expression:(unsigned)(c+1) <= 256

A. Probably because you are reading beyond your buffer. This isn't a problem with VC++ ... it is helping you out. For example this will cause this problem:

char buf[] = "abc"; for (char *p = buf;;p++) // Loop forever (bug on purpose) { if (isalnum(*p)) printf("alnum"); }

As you can see this faulty code will do isalnum(buf[5]) which is beyond what we have allocated. isalnum() or isdigit() or other is* function will assert letting us know.