指针，有效的内存地址以及对sds库的了解

Question

I am trying to learn how to use the sds functions. I have the code sds.h and sds.c code files in my project folder and the little exploration program that i wrote compiles and runs just fine. However I am having a hard time understanding some of what I am seeing in the code from the sds.h and sds.c files. I can't tell why it compiles let alone works.

The code in question is:

typedef char *sds;

struct sdshdr {
  int len;
  int free;
  char buf[];
};

static inline size_t sdslen(const sds s) {
  struct sdshdr *sh = (void*)(s-(sizeof(struct sdshdr)));
  return sh->len;
}

The sdslen() function is called numerous times in the sds.c file and I can even use it in my own program after including the sds.h. I know that the typedef makes sds a type that is just a char pointer. The static keyword restricts the scope of the function. Inline means that the function will be pasted into the code by the compiler when it is called rather then using the stack and function call mechanisms. It looks to me like the *sh pointer in the sdslen() function is assigned an address sizeof(struct sdshdr) memory addresses before the address stored in s and then without initializing any of the variables in the struct the len member variable is passed back. Any help in understanding this, what is really happening, would be appreciated.

Answer 1

I think I figured it out. The answer is in another function from the sds.c file:

sds sdsnewlen(const void *init, size_t initlen) {
  struct sdshdr *sh;

  if (init) {
    sh = zmalloc(sizeof(struct sdshdr)+initlen+1);
  } else {
    sh = zcalloc(sizeof(struct sdshdr)+initlen+1);
  }
  if (sh == NULL) return NULL;
  sh->len = initlen;
  sh->free = 0;
  if (initlen && init)
    memcpy(sh->buf, init, initlen);
  sh->buf[initlen] = '\0';
  return (char*)sh->buf;
}

When a new sds is created and initialized by the sdsnewlen() function memory is dynamically allocated for the entire struct and the c-string but the address of the c-string is what gets passed back. If sdslen() were called with a sds variable that had been allocated using malloc without using the sdsnewlen() function it would cause a problem. As long as the sds variables are initialized using the provided functions then the memory is valid, the member variables have been initialized and the the sds variable can be used in things like printf() the same as any c-string could be.