segmention fault >> culprit is isdigit() c library function call

[vartan]

dear maintainer:
the fallowing is a small sample of the code that contains all the info
to reproduce the problem.
the C library function isdigit() fails and causes segmention fault when
the input is is not numeric like xx.

COMPILE COMMAND : >> gcc -ansi --version -Wall  -o ch2-13.out    -lm
ch2-13.c

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