Printing out an array of pointers defined in header file

Question

I'm currently working on a DBMS as a college project, I'm having issues at printing out an array of strings which is defined in an header file.

I've tried playing around with pointers but I guess the problem is within the definition of the array, which being in another file can't be retrieved properly.

Here's metacommand.h file

typedef enum {NORMAL, DB, TABLE} Mode;

const char* normalModeMetaCommands[] = {".help", ".exit", ".db-mode", ".table-mode"};
const char* dbModeMetaCommands[] = {".help", ".exit", ".workingdir", ".dbs", ".db"};
const char* tableModeMetaCommands[] = {".help", ".exit", ".tables", ".table", ".index", ".select"};

const char* getModeName(Mode mode);
const char* getModeInputPrompt(Mode mode);
const char* getModeMetaCommands(Mode mode);

static void print_metacommands();

And here's some snippets from database.c file

static Mode mode = NORMAL;

const char* getModeMetaCommands(Mode mode) 
{
   switch (mode) 
   {
      case NORMAL: return *normalModeMetaCommands;
      case DB: return *dbModeMetaCommands;
      case TABLE: return *tableModeMetaCommands;
   }
}

static void print_metacommands(){

    size_t size = sizeof(getModeMetaCommands(mode))/sizeof(getModeMetaCommands(mode)[0]);
    for(int i=0; i<size; i++){
        printf("%s\n", getModeMetaCommands(mode)[i]);
    }

}

I have defined some functions that change the enum value in the database.c file, so that in each mode (NORMAL, DB and TABLE) I can print out an exclusive list of metacommands.

EDIT

My function print_metacommands() doesn't print those char pointers array (*normalModeMetaCommands[], *dbModeMetaCommands[], *tableModeMetaCommands[]). Output is a segFault.

If I change:

const char* getModeMetaCommands(Mode mode);

to:

const char** getModeMetaCommands(Mode mode);

And:

const char* getModeMetaCommands(Mode mode) 
{
   switch (mode) 
   {
      case NORMAL: return *normalModeMetaCommands;
      case DB: return *dbModeMetaCommands;
      case TABLE: return *tableModeMetaCommands;
   }
}

to:

const char** getModeMetaCommands(Mode mode) 
{
   switch (mode) 
   {
      case NORMAL: return normalModeMetaCommands;
      case DB: return dbModeMetaCommands;
      case TABLE: return tableModeMetaCommands;
   }
}

The output is only ".help" because it seems like:

size_t size = sizeof(getModeMetaCommands(mode))/sizeof(getModeMetaCommands(mode)[0]);
//evaluated to: size=1

So, how can I print out those char pointers array via static void print_metacommands();?

What is your question? What happens and how does it differ from your expectation? — walnut, Sep 03 '19 at 22:00
maybe related: https://stackoverflow.com/questions/5499504/shared-c-constants-in-a-header — walnut, Sep 03 '19 at 22:08
Gabriele Biagini, what JL2210 is trying to tell you is that making an answer would be much easier and probably more to the point, if you would make a [mcve] to demonstrate your problem. (@JL2210 try typing `[mcve]` in a comment like that) — Yunnosch, Sep 04 '19 at 06:46
Sorry for not being much clear, I've reformulated the question entirely, hope this helps! — Gabriele Biagini, Sep 04 '19 at 07:59

score 0 · Answer 1 · edited Jun 20 '20 at 09:12

There's many basic errors here.

Never declare global variables anywhere, period.
Never define variables (or functions) in header files. This always leads to various linking hiccups and bugs where different instances of variables with the same name co-exist in different translation units.

Instead, move all variable definitions and initializations to file scope inside a .c file, make them static and only let the caller access them through setter/getter functions. They will no longer be global, but note that they are still not thread safe.
You cannot use sizeof on a pointer, because pointers are not arrays. It gives the size of the pointer, not the length of the pointed-at string.

Printing out an array of pointers defined in header file

1 Answers1