Q: I had the definition char a in one source file, and in another I declared extern char *a. Why didn't it work?
A: In one source file you defined an array of characters and in the other you declared a pointer to characters. The declaration extern char *a does not declare an array and therefore does not match the actual definition. The type pointer-to-type-T is not the same as array-of-type-T. Use extern char a.
ISO Sec. 22.214.171.124
CT&P Sec. 3.3 pp. 33-4, Sec. 4.5 pp. 64-5