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c 语言中的bit field 是一种节省内存的方式, 用于struct 或者 union 的成员变量的声明。基本的语法是:
struct BF { ... type_specifier [declarator] : constant_expression; ...};
采用这样一种方式的好处是,我们可以明确地指定某一个成员变量在内存中占用的空间(constant_expression bits). 比如:
struct BitArray { unsigned a : 1; unsigned b : 1;};// sizeof(BitArray) == 4struct BitArray2{ unsigned a; unsigned b;};// sizeof(BitArray2) == 8
上面的例子可以看出,使用了Bit field, 我们可以将相邻的变量a
和 b
紧凑的排列到一个unsigned
中。 而如果不适用Bit field, 很明显编译器将会单独为a
和 b
创建一个 unsigned
成员变量。
但是在使用bit field的时候,我们发现有很多限制或者注意点,这里罗列出大部分可能会碰到的问题。
enum E { E1, E2, E3, E4, E5, E6 = 200 };struct SMixture { char a : 1, b : 1 ; //ok E e1 : 2, e2 : 2; //ok float f : 3 ; //compiler error};
struct SMixture { char a : 1; //start of first char char b : 3; //continue in the first char char c : 5; //start of second char char d : 4; //start of third char};// sizeof(SMixture) == 3;
struct SMixture { char c : 1; char d : 0; //compiler error char : 0; //okay};
width 为0的域是特殊域,表示该0-width bit field的下一个成员的地址必须以该0-width bit field 的类型的对齐方式对齐。
struct SMixture { char c : 1; //start of char int : 0; short d : 1; //start of short after alignment at type int boundary};//sizeof(SMixture) == 6;struct SMixture2 { char c : 1; //start of char int : 0; short d; //start of short after alignment at type int boundary};//sizeof(SMixture2) == 6
int main(){ SMixture s; short* ps = &s.d; //compiler error short& rs = s.d; //compiler error return 0;}
struct SMixture { static char c : 1; //compiler error};
/* from xnu/bsd/netinet/bootp.h *//* * Bootstrap Protocol (BOOTP). RFC 951. *//* * HISTORY * * 14 May 1992 ? at NeXT * Added correct padding to struct nextvend. This is * needed for the i386 due to alignment differences wrt * the m68k. Also adjusted the size of the array fields * because the NeXT vendor area was overflowing the bootp * packet. *//* . . . */struct nextvend { u_char nv_magic[4]; /* Magic number for vendor specificity */ u_char nv_version; /* NeXT protocol version */ /* * Round the beginning * of the union to a 16 * bit boundary due to * struct/union alignment * on the m68k. */ unsigned short :0; union { u_char NV0[58]; struct { u_char NV1_opcode; /* opcode - Version 1 */ u_char NV1_xid; /* transcation id */ u_char NV1_text[NVMAXTEXT]; /* text */ u_char NV1_null; /* null terminator */ } NV1; } nv_U;};
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