Redisの文字列はどのように実装されているか

typedef char *sds;

/* Note: sdshdr5 is never used, we just access the flags byte directly.
 * However is here to document the layout of type 5 SDS strings.
struct __attribute__ ((__packed__)) sdshdr5 {
    unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr8 {
    uint8_t len; /* used */
    uint8_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr16 {
    uint16_t len; /* used */
    uint16_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr32 {
    uint32_t len; /* used */
    uint32_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};
struct __attribute__ ((__packed__)) sdshdr64 {
    uint64_t len; /* used */
    uint64_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};

typedef char *sds;

hisds sdsnewlen(const void *init, size_t initlen) {
   //pointer to the SDS structure
    void *sh;
    //variable of type sds, which is an alias of char*
    sds s;
    //get the type of SDS based on size
    char type = hi_sdsReqType(initlen);
    /* Empty strings are usually created in order to append. use type 8
     Use type 8. * since type 5 is not good at this. */
    if (type == SDS_TYPE_5 && initlen == 0) type = SDS_TYPE_8;
    int hdrlen = sdsHdrSize(type);
    unsigned char *fp; /* flags pointer. */

     // allocate memory for the newly created sds structure
    sh = s_malloc(hdrlen+initlen+1);
    if (sh == NULL) return NULL;
    if (!init)
        memset(sh, 0, hdrlen+initlen+1);
    
    // point to the buf array in the SDS structure, sh points to the starting position of the SDS structure, hdrlen indicates the length of the metadata    
    s = (char*)sh+hdrlen;
    fp = ((unsigned char*)s)-1;
    //initialize len,alloc according to type
    switch(type) {
        case SDS_TYPE_5: {
            *fp = type | (initlen << HI_SDS_TYPE_BITS);
            break;
        }
        case SDS_TYPE_8: {
            SDS_HDR_VAR(8,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_16: {
            SDS_HDR_VAR(16,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_32: {
            SDS_HDR_VAR(32,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
        case SDS_TYPE_64: {
            SDS_HDR_VAR(64,s);
            sh->len = initlen;
            sh->alloc = initlen;
            *fp = type;
            break;
        }
    }
    if (initlen && init)
    // copy the string to the sds variable s
        memcpy(s, init, initlen);
     //add "\0" at the end of the string variable to indicate the end of the string  
    s[initlen] = '\0';
    return s;
}

sds sdscatlen(sds s, const void *t, size_t len) {
//Get the length of the target string 
 size_t curlen = sdslen(s);
//determine if new space needs to be added based on append length and target string length
  s = sdsMakeRoomFor(s,len);
  if (s == NULL) return NULL;
  // copy the data of length len from the source string t to the end of the target string
  memcpy(s+curlen, t, len);
  //set the latest length of the target string
  sdssetlen(s, curlen+len);
  //after the copy is done, add "\0" at the end of the string;
  s[curlen+len] = '\0';
  return s;
}

static inline size_t sdslen(const sds s) {
    unsigned char flags = s[-1];
    switch(flags&SDS_TYPE_MASK) {
        case SDS_TYPE_5:
            return SDS_TYPE_5_LEN(flags);
        case SDS_TYPE_8:
            return SDS_HDR(8,s)->len;
        case SDS_TYPE_16:
            return SDS_HDR(16,s)->len;
        case SDS_TYPE_32:
            return SDS_HDR(32,s)->len;
        case SDS_TYPE_64:
            return SDS_HDR(64,s)->len;
    }
    return 0;
}

#define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))

SDS_HDR(8,s);
((struct sdshdr8 *)((s)-(sizeof(struct sdshdr8))))

sds sdsMakeRoomFor(sds s, size_t addlen) {
    void *sh, *newsh;
    //Get the space currently available in sds
    size_t avail = sdsavail(s);
    size_t len, newlen;
    char type, oldtype = s[-1] & SDS_TYPE_MASK;
    int hdrlen;
    size_t usable;

    // enough free space, no need to expand
    if (avail >= addlen) return s;

    len = sdslen(s);
    sh = (char*)s-sdsHdrSize(oldtype);
    newlen = (len+addlen);
    assert(newlen > len); /* Catch size_t overflow */
    // If the length of the new character array is less than SDS_MAX_PREALLOC
    // allocate 2 times the required length
    if (newlen < SDS_MAX_PREALLOC)
        newlen *= 2;
    // otherwise allocate the new length plus the length of SDS_MAX_PREALLOC
    else
        newlen += SDS_MAX_PREALLOC;
   
   //Retrieve the SDS type
    type = sdsReqType(newlen);

    /* Don't use type 5: the user is appending to the string and type 5 is
     /* Don't use type 5: the user is appending to the string and type 5 is * not able to remember empty space, so sdsMakeRoomFor() must be called
     * at every appending operation.
    if (type == SDS_TYPE_5) type = SDS_TYPE_8;

    hdrlen = sdsHdrSize(type);
    assert(hdrlen + newlen + 1 > len); /* Catch size_t overflow */
    if (oldtype==type) {
        newsh = s_realloc_usable(sh, hdrlen + newlen + 1, &usable);
        if (newsh == NULL) return NULL;
        s = (char*)newsh+hdrlen;
    } else {
        / / if the header size changes just move the character array forward without using realloc
        newsh = s_malloc_usable(hdrlen+newlen+1, &usable);
        if (newsh == NULL) return NULL;
        memcpy((char*)newsh+hdrlen, s, len+1);
        s_free(sh);
        s = (char*)newsh+hdrlen;
        s[-1] = type;
        sdssetlen(s, len);
    }
    usable = usable-hdrlen-1;
    if (usable > sdsTypeMaxSize(type))
        usable = sdsTypeMaxSize(type);
     //Update SDS capacity  
    sdssetalloc(s, usable);
    return s;
}

#define SDS_MAX_PREALLOC (1024*1024)

struct __attribute__ ((__packed__)) sdshdr16 {
    uint16_t len; /* used */
    uint16_t alloc; /* excluding the header and null terminator */
    unsigned char flags; /* 3 lsb of type, 5 unused bits */
    char buf[];
};

#include <string.h>
#include <iostream>

using namespace std;

typedef struct MyStruct
{
 char a;
 int b;   
} MyStruct;


int
main()
{
    cout << sizeof(MyStruct) << endl;
    
    return 0;
}

#include <string.h>
#include <iostream>

using namespace std;

typedef struct MyStruct
{
 char a;
 int b;   
} __attribute__ ((__packed__))MyStruct;


int
main()
{
    cout << sizeof(MyStruct) << endl;
    
    return 0;
}