#c#memory

test.c

// 在64机器上，返回16字节
// 存在 pading
// http://www.catb.org/esr/structure-packing/
struct foo3 {
    char *p;     /* 8 bytes */
    char c;      /* 1 byte */
};

struct foo3 singleton;
struct foo3 quad[4];


// 等价于
struct foo3 {
    char *p;     /* 8 bytes */
    char c;      /* 1 byte */
    char pad[7];
};


// 如果你的结构有结构成员，内部结构也希望有最长标量的对齐方式。
struct foo5 {
    char c;
    struct foo5_inner {
        char *p;
        short x;
    } inner;
};

// 等价于
struct foo5 {
    char c;           /* 1 byte*/
    char pad1[7];     /* 7 bytes */
    struct foo5_inner {
        char *p;      /* 8 bytes */
        short x;      /* 2 bytes */
        char pad2[6]; /* 6 bytes */
    } inner;
};
 
// Bitfields
struct foo6 {
    short s;
    char c;
    int flip:1;
    int nybble:4;
    int septet:7;
};


// 等价于, c99
struct foo6 {
    short s;       /* 2 bytes */
    char c;        /* 1 byte */
    int flip:1;    /* total 1 bit */
    int nybble:4;  /* total 5 bits */
    int pad1:3;    /* pad to an 8-bit boundary */
    int septet:7;  /* 7 bits */
    int pad2:25;   /* pad to 32 bits */
};

// 也可能是下面的模式，有可能是填充在有效负载之前
//  C99 rules 没有保证padding的都是0,可能是垃圾值
// 位域不能跨越机器字边界的限制意味着，虽然以下结构的前两个结构如你所期望的那样被装入一个和两个32位字
struct foo6 {
    short s;       /* 2 bytes */
    char c;        /* 1 byte */
    int pad1:3;    /* pad to an 8-bit boundary */
    int flip:1;    /* total 1 bit */
    int nybble:4;  /* total 5 bits */
    int pad2:25;   /* pad to 32 bits */
    int septet:7;  /* 7 bits */
};


// 
struct foo7 {
    int bigfield:31;      /* 32-bit word 1 begins */
    int littlefield:1;
};

struct foo8 {
    int bigfield1:31;     /* 32-bit word 1 begins /*
    int littlefield1:1;
    int bigfield2:31;     /* 32-bit word 2 begins */
    int littlefield2:1;
};

// C11 and C++14 may pack foo9 tighter
struct foo9 {
    int bigfield1:31;     /* 32-bit word 1 begins */
    int bigfield2:31;     /* 32-bit word 2 begins */
    int littlefield1:1;
    int littlefield2:1;   /* 32-bit word 3 begins */
};


// 需要24个字节
struct foo10 {
    char c;
    struct foo10 *p;
    short x;
};

// 只有16个字节
struct foo11 {
    struct foo11 *p; /* 8 bytes */
    short x;         /* 2 bytes */
    char c;          /* 1 byte */
    char pad[5];     /* 5 bytes */
};