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Message: [QUOTE="Pascal J. Bourguignon, post: 4872114"] Well, since C is weakly-to-not typed, you cannot enforce it at the variable site, however, it is specified indeed that it is invalid or undefined to derefer a pointer that doesn't point to allocated memory, and to compare pointers that don't point to elements of the same array or one beyond. This can be enforced by heavy pointers and run-time checks. Of course, since they prefer to have their results fast than correct, this is rarely enforced. One way is to define pointers as: typedef struct { address arrayBase; int elementSize; int elementCount; int index; } Pointer; Pointer NULL={0,0,0,0}; void Pointer_incr(Pointer* p){ if(p.index<p.elementCount){ p.index++; }else{ error("Trying to increment a pointer out of bounds"); }} void Pointer_decr(Pointer* p){ if(0<p.index){ p.index--; }else{ error("Trying to decrement a pointer out of bounds"); }} int Pointer_minus(Pointer p,Pointer q){ if(p.arrayBase!=q.arrayBase){ error("Incompatible pointers"); }else{ return(p.index-q.index); }} bool Pointer_equalp(Pointer p,Pointer q){ return(Pointer_minus(p,q)==0); } bool Pointer_lessp (Pointer p,Pointer q){ return(Pointer_minus(p,q)<0); } T Pointer_deref(Type T,Pointer p){ if(p.index<p.elementCount){ return(deref(T,p.arrayBase+p.index)); }else{ error("Trying to dereference out of bound pointer."); }} char a; char b; int c[10]; int d[10]; char* p1=&a; /* <=> p1.arraybase=address_of(a); p1.elementSize=1; p1.elementCount=1; p1.index=0; */ char* p2=&b; /* <=> p2.arraybase=address_of(b); p2.elementSize=1; p2.elementCount=1; p2.index=0; */ int* p3=&c; /* <=> p3.arraybase=address_of(c); p3.elementSize=sizeof(int); p3.elementCount=10; p3.index=0; */ int* p4=&(d[9]); /* <=> p4.arraybase=address_of(d); p4.elementSize=sizeof(int); p4.elementCount=10; p4.index=9; */ int* p5=&d; /* <=> p5.arraybase=address_of(d); p5.elementSize=sizeof(int); p5.elementCount=10; p5.index=9; */ char* n=NULL; p2++; /* <=> Pointer_incr(&p2); */ p3++; /* <=> Pointer_incr(&p3); */ p1==p2; /* <=> Pointer_equalp(p1,p2); <=> error */ p3<p4; /* <=> Pointer_lessp(p3,p4); <=> error */ p4<p5; /* <=> Pointer_lessp(p5,p4); returns false. */ *p4=*p5; /* <=> copies the int from d[0] to d[9]. */ *n=0; /* <=> error (dereferencing NULL) */ [/QUOTE]

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