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three files/README.md

+#CPP
+- include file
+    -- ex) #include <stdio.h> or "file.h"
+
+- Conditional Complication
+    -- #if, #ifdef, #ifndef, #elif, #else, #endif
+
+#GCC compile option
+-c: generate .o file
+-g: for debeg
+-O: optimization
+
+#GCC compile - for multiple file
+- ex) func.h, func.c, main.c
+    --compile -gcc main.c func.c
+
+
+###project HW2###
+
+#GCC compile -- by using preprocessor
+- #ifdef, #else, #endif
+    -- func.c에서 int func_name_or_id() 구현 (no using #define PRINT_ID)
+    -- difference between gcc -DPRINT_ID main.c func.c and gcc main.c func.c
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