Question

I was wanting a simple string table that will store a bunch of constants and I thought “Hey! Lua does that, let me use some of there functions!”

This is mainly in the lstring.h/lstring.c files (I am using 5.2)

I will show the code I am curious about first. Its from lobject.h

/*
** Header for string value; string bytes follow the end of this structure
*/
typedef union TString {
  L_Umaxalign dummy;  /* ensures maximum alignment for strings */
  struct {
    CommonHeader;
    lu_byte reserved;
    unsigned int hash;
    size_t len;  /* number of characters in string */
  } tsv;
} TString;


/* get the actual string (array of bytes) from a TString */
#define getstr(ts)  cast(const char *, (ts) + 1)

/* get the actual string (array of bytes) from a Lua value */
#define svalue(o)       getstr(rawtsvalue(o))

As you see, the data is stored outside of the structure. To get the byte stream, you take the size of TString, add 1, and you got the char* pointer.

Isn’t this bad coding though? Its been DRILLED into m in my C classes to make clearly defined structures. I know I might be stirring a nest here, but do you really lose that much speed/space defining a structure as header for data rather than defining a pointer value for that data?

Answer 1

The idea is probably that you allocate the header and the data in one big chunk of data instead of two:

TString *str = (TString*)malloc(sizeof(TString) + <length_of_string>);

In addition to having just one call to malloc/free, you also reduce memory fragmentation and increase memory localization.

But answering your question, yes, these kind of hacks are usually a bad practice, and should be done with extreme care. And if you do, you’ll probably want to hide them under a layer of macros/inline functions.