I am porting some code to an 8051. (header.h)
typedef int (Writer) (int , u_char * , u_char , size_t , u_char * , int *) reentrant;
Writer bob;
. . . int bob(int val1, u_char * dat1, u_char dat2, size_t val2, u_char * dat3, int *pval) reentrant { }
//Comment out Writer bob; int (bob)(int val1, u_char * dat1, u_char dat2, size_t val2, u_char * dat3, int *pval) reentrant;
The .h file provides a "prototype" of a function to be defined elsewhere. The redefinition warning means that when you actually define the function in the .c file, it does not match a previously seen declaration (typically in a .h file). While you've run into this problem with the reentrant attribute, it applies to any change in function signature. That is, .h: void MyFunction (long a); .c: void MyFunction (short a); would produce a redefinition warning. The .h file should always match the actual definition of the function, lest you confuse seperately-compiled callers.
The .h file provides a "prototype" of a function to be defined elsewhere. That what it should do, for ordinary functions. But the case at hand is different, because
"the proper way of making sure of this is to always include the module's header file into its own .c file." Yep - that's what I always say!