Error-prone knowledge points in C language

1. Calculation of structural bodies

struct Bbb{//long a;//char c1;//char c2;//long b;//long c;//}*p;//sizeof (struct BBB) =16;//int main ()//{//p = (struct BB b*) 0x100000;//printf ("0x%x", p + 0x1),//plus the entire structure size is 0x10010//printf ("0x%x", (unsigned long) p + 0x1);//integral type plus 1,0x100001// printf ("0x%x", (unsigned long*) p + 0x1);//plus sizeof (unsigned long) *, for 0x100004//printf ("0x%x", (char *) p + 0x1);//Plus size of (Char) * *, for 0x100001//system ("pause");//return 0;//}

2. Structure structure, the size of the structure in the common body

Union aaa{//struct {//char c1;//short sl;//char c2;//}half;//short kk;//}number;//struct BBB{//char ucFirst;//short Us;//char c2;//short uo;//}half;//struct tagccc//{//struct//{//char c1;//short sl;//char c2;//}half;//long kk;//};//knot The structure is a whole in 4 aligned 6+4=10,short and the largest occupied byte in Long is the integer multiples of 12//int main ()//{//printf ("%d%d%d\n", sizeof (union AAA), sizeof (struct BBB), sizeof (struct TAGCCC));//system ("pause");//return 0;//}

Execution Result:

1-byte alignment: 4 6 8

4-byte Alignment: 6 8 12

3. If there is no break in the case statement, then the statement after it will be executed.

4. Pointers and const declarations:

(1) The object referred to by the pointer is readable

const int *p;

int const *P;

(2) The pointer is readable

int *const p;

(3) Pointers and pointers refer to objects that are readable

const int * const P;

int const *CONSTP;

5. Dynamic opening up

void GetMemory (char *p,int len) {p= (char *) malloc (len);} int main () {char *p;   GetMemory (p,10);   strcpy (p, "bit");   printf ("%s", p);  Free (p); }

The above code has three errors!!!

A. At this time the call function P dynamic opening and the main function p is not the same place, because P is only a temporary copy, no address passed.

b.strcpy, there was an error, and P is not pointing at this time.

C.free does not open it (temporary variable) and should have the pointer empty after free.

The correct code is:

void GetMemory (char **p,int len) {*p= (char *) malloc (len);} int main () {char *p;   GetMemory (&p,10);   strcpy (p, "bit");   printf ("%s", p);   Free (p);  P=null; }

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Error-prone knowledge points in C language