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The igrep Program

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\f(BIigrep\fB is more properly speaking a tool for C programming
than it is strictly administrative.  
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It greps for a pattern in a C source file, but recursively 
follows any 
.T #include
directives. 
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It does this manually, rather than using the C pre-processor, 
because if you're trying to find out where \s-1ENOENT\s0 is defined, 
you can just grep the output of 
\f(BIcpp\fB and expect to find \s-1ENOENT\s0 there.
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The source shows a good example of opening files recursively
and keeping local file handles.
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$ igrep 2BIG /usr/include/errno.h
/usr/include/sys/errno.h 
 from /usr/include/errno.h: 
21: #define E2BIG 7 /* Arg list too long */
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