Calculate header size(requested memory) of a item stored in memcached.

header_size.c

#include <stdio.h>
#include <stdint.h>

/** Time relative to server start. Smaller than time_t on 64-bit systems. */
typedef unsigned int rel_time_t;

typedef struct _stritem {
    struct _stritem *next;
    struct _stritem *prev;
    struct _stritem *h_next;    /* hash chain next */
    rel_time_t      time;       /* least recent access */
    rel_time_t      exptime;    /* expire time */
    int             nbytes;     /* size of data */
    unsigned short  refcount;
    uint8_t         nsuffix;    /* length of flags-and-length string */
    uint8_t         it_flags;   /* ITEM_* above */
    uint8_t         slabs_clsid;/* which slab class we're in */
    uint8_t         nkey;       /* key length, w/terminating null and padding */
    /* this odd type prevents type-punning issues when we do
     * the little shuffle to save space when not using CAS. */
    union {
        uint64_t cas;
        char end;
    } data[];
    /* if it_flags & ITEM_CAS we have 8 bytes CAS */
    /* then null-terminated key */
    /* then " flags length\r\n" (no terminating null) */
    /* then data with terminating \r\n (no terminating null; it's binary!) */
} item;

/**
 * Generates the variable-sized part of the header for an object.
 *
 * key     - The key
 * nkey    - The length of the key
 * flags   - key flags
 * nbytes  - Number of bytes to hold value and addition CRLF terminator
 * suffix  - Buffer for the "VALUE" line suffix (flags, size).
 * nsuffix - The length of the suffix is stored here.
 *
 * Returns the total size of the header.
 */
static size_t item_make_header(const uint8_t nkey, const int flags, const int nbytes,
                     char *suffix, uint8_t *nsuffix) {
    /* suffix is defined at 40 chars elsewhere.. */
    *nsuffix = (uint8_t) snprintf(suffix, 40, " %d %d\r\n", flags, nbytes - 2);

    printf("item\tnkey\tnsuffix\tbytes\n");
    printf("%d\t%d\t%d\t%d\n", sizeof(item), nkey, *nsuffix, nbytes);

    return sizeof(item) + nkey + *nsuffix + nbytes;
}

void main() {
    size_t nkey;
    int nbytes;

    int flags = 0;
    int use_cas = 1;

    char suffix[40];
    uint8_t nsuffix;
    size_t ntotal;

    printf("Key length: ");
    scanf("%d", &nkey);
    printf("Data length: ");
    scanf("%d", &nbytes);
    printf("\n");

    nbytes += 2;        // addition CRLF terminator

    ntotal = item_make_header(nkey + 1, flags, nbytes, suffix, &nsuffix);
    if (use_cas) {
        ntotal += sizeof(uint64_t);
    }

    printf("\n");
    printf("Header size: %d\n", ntotal);
}