redis内存分配使用了zmalloc函数对不同的系统进行了封装.针对不同的系统调用，统一提供一套内存函数接口。

封装位于zmalloc.h文件中

//如果使用tcmalloc
#if defined(USE_TCMALLOC)
#define malloc(size) tc_malloc(size)
#define calloc(count,size) tc_calloc(count,size)
#define realloc(ptr,size) tc_realloc(ptr,size)
#define free(ptr) tc_free(ptr)
#elif defined(USE_JEMALLOC)
//如果使用jemalloc
#define malloc(size) je_malloc(size)
#define calloc(count,size) je_calloc(count,size)
#define realloc(ptr,size) je_realloc(ptr,size)
#define free(ptr) je_free(ptr)
#endif


void *zmalloc(size_t size);
void *zcalloc(size_t size);
void *zrealloc(void *ptr, size_t size);
void zfree(void *ptr);
char *zstrdup(const char *s);
size_t zmalloc_used_memory(void);
void zmalloc_enable_thread_safeness(void);
void zmalloc_set_oom_handler(void (*oom_handler)(size_t));
float zmalloc_get_fragmentation_ratio(size_t rss);
size_t zmalloc_get_rss(void);
size_t zmalloc_get_private_dirty(void);
void zlibc_free(void *ptr);

//分配函数
void *zmalloc(size_t size) {
    void *ptr = malloc(size+PREFIX_SIZE);

    if (!ptr) zmalloc_oom_handler(size);
#ifdef HAVE_MALLOC_SIZE
    //如果接口同自带内存大小
    update_zmalloc_stat_alloc(zmalloc_size(ptr));
    return ptr;
#else
    //接口内无自带内存大小的长度，需要接口保存
    *((size_t*)ptr) = size;
    //增加内存占用情况。
    update_zmalloc_stat_alloc(size+PREFIX_SIZE);
    return (char*)ptr+PREFIX_SIZE;
#endif
}
//calloc封装
void *zcalloc(size_t size) {
    void *ptr = calloc(1, size+PREFIX_SIZE);

    if (!ptr) zmalloc_oom_handler(size);
#ifdef HAVE_MALLOC_SIZE
    update_zmalloc_stat_alloc(zmalloc_size(ptr));
    return ptr;
#else
    *((size_t*)ptr) = size;
    update_zmalloc_stat_alloc(size+PREFIX_SIZE);
    return (char*)ptr+PREFIX_SIZE;
#endif
}

void *zrealloc(void *ptr, size_t size) {
#ifndef HAVE_MALLOC_SIZE
    void *realptr;
#endif
    size_t oldsize;
    void *newptr;
    //若ptr为空，则分配新的
    if (ptr == NULL) return zmalloc(size);
#ifdef HAVE_MALLOC_SIZE
    oldsize = zmalloc_size(ptr);
    newptr = realloc(ptr,size);
    if (!newptr) zmalloc_oom_handler(size);

    update_zmalloc_stat_free(oldsize);
    update_zmalloc_stat_alloc(zmalloc_size(newptr));
    return newptr;
#else
    realptr = (char*)ptr-PREFIX_SIZE;
    oldsize = *((size_t*)realptr);
    newptr = realloc(realptr,size+PREFIX_SIZE);
    if (!newptr) zmalloc_oom_handler(size);

    *((size_t*)newptr) = size;
    update_zmalloc_stat_free(oldsize);
    update_zmalloc_stat_alloc(size);
    return (char*)newptr+PREFIX_SIZE;
#endif
}
//获取物理内存的大小
size_t zmalloc_get_rss(void) {
    int page = sysconf(_SC_PAGESIZE);
    size_t rss;
    char buf[4096];
    char filename[256];
    int fd, count;
    char *p, *x;

    snprintf(filename,256,"/proc/%d/stat",getpid());
    if ((fd = open(filename,O_RDONLY)) == -1) return 0;
    if (read(fd,buf,4096) <= 0) {
        close(fd);
        return 0;
    }
    close(fd);

    p = buf;
    count = 23; /* RSS is the 24th field in /proc/<pid>/stat */
    while(p && count--) {
        p = strchr(p,' ');
        if (p) p++;
    }
    if (!p) return 0;
    x = strchr(p,' ');
    if (!x) return 0;
    *x = '\0';

    rss = strtoll(p,NULL,10);
    rss *= page;
    return rss;
}
//获取redis占用的内存
size_t zmalloc_used_memory(void) {
    size_t um;

    if (zmalloc_thread_safe) {
#if defined(__ATOMIC_RELAXED) || defined(HAVE_ATOMIC)
        um = update_zmalloc_stat_add(0);
#else
        pthread_mutex_lock(&used_memory_mutex);
        um = used_memory;
        pthread_mutex_unlock(&used_memory_mutex);
#endif
    }
    else {
        um = used_memory;
    }

    return um;
}
//返回占用的物理内存/分配的内存
float zmalloc_get_fragmentation_ratio(size_t rss) {
    return (float)rss/zmalloc_used_memory();
}