Where are header (``#include'') files searched for?
The exact behavior is implementation-defined (which means that it is supposed to be documented; see question 11.33). Typically, headers named with <> syntax are searched for in one or more standard places. Header files named with "" syntax are first searched for in the ``current directory,'' then (if not found) in the same standard places.
Traditionally (especially under Unix compilers), the current directory is taken to be the directory containing the file containing the #include directive. Under other compilers, however, the current directory (if any) is the directory in which the compiler was initially invoked. Check your compiler documentation.
References:
K&R2 Sec. A12.4 p. 231
ANSI Sec. 3.8.2
ISO Sec. 6.8.2
H&S Sec. 3.4 p. 55
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