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Variables, data types, operators, control flow, I/O with printf/scanf, and program structure.

Pointer declaration, dereferencing, pointer arithmetic, NULL, dynamic memory with malloc/free/calloc/realloc.

Function declarations, prototypes, pass-by-value, recursion, scope, storage classes, and function pointers.

C strings (null-terminated char arrays), string.h functions, multi-dimensional arrays, and array decay.

struct, union, enum, typedef, bit fields, nested types, and struct alignment/padding.

fopen/fclose/fread/fwrite/fgets, #define macros, #include, header guards, conditional compilation, and make basics.