Array of undefined length of structures generates segmentation fault

Question

I am writing a compiler project. I created an AST tree, where every node is defined by structure:

typedef struct node {
   char type[10];
   char *string;
   int number_of_children;

   struct node *children[];
} node;

I wanted to create an array of symbols of undefined length. I created structure, to store name of the symbol and it's address in the memory:

typedef struct symbol {
   char *name;
   int address;
} symbol;

and then I created array of undefined length:

symbol *symbols_table[];

I created function to add new symbol:

void add_symbol(node *p) {
    symbols_table[number_of_symbols] = malloc(sizeof(symbol));
    symbols_table[number_of_symbols]->name = malloc(sizeof(p->string));

    strcpy(symbols_table[number_of_symbols]->name, p->string);
    symbols_table[number_of_symbols]->address = memory_pointer;

    memory_pointer++;
    number_of_symbols++;
}

where p is node from the AST tree and memory_pointer is needed to give address in the memory to each symbol.

And here is the problem. When I want to add only 2 symbols to the symbols_table everything is working fine. But when I want to add 3 and more it gives segmentation fault. Do you have any ideas, why is it happening?

What do you think the declaration symbol *symbols_table[] creates? If you think it creates an array that can hold any number of elements you might choose to store in it, you are sadly mistaken. — Peter
– Peter, Commented Jan 3, 2016 at 9:38
I thought as you said. So this must be it. I have to change this array. Thank you very much :) — brzepkowski
– brzepkowski, Commented Jan 3, 2016 at 9:40
This definition symbol *symbols_table[];, if done globally, should make the compiler issue a warning, if given local to a function it should not even compile. Take warnings serious. — alk
– alk, Commented Jan 3, 2016 at 13:02

Sourav Ghosh · Accepted Answer · 2016-01-03 09:19:18Z

2

The problem, as I see here is with

 symbols_table[number_of_symbols]->name = malloc(sizeof(p->string));

in your code, string is a pointer. So, sizeof(p->string)) will not give you the size of the allocated memory, it will give you the size of the pointer itself. At later point, when you do strcpy() it overruns the allocated memory an creates undefined behavior.

What you want instead is

 symbols_table[number_of_symbols]->name = malloc(strlen(p->string) + 1);

to get the proper memory allocation.

On a differnt approach, you can also have a look at strdup() to avoid the whole malloc() + strcpy() thing.

answered Jan 3, 2016 at 9:19

Sourav Ghosh

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3 Comments

user4520 Over a year ago

+1, although strdup is not standard C, so it depends on OP's platform whether they can use it or not.

Sourav Ghosh Over a year ago

@szczurcio Right. That is why I've put that as a note, rather. :)

brzepkowski Over a year ago

Thank you for quick answer. I tried both solutions, but it didn't work :/ I don't think, that the problem is in the other part of program, but I may be wrong.

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