Variably modified 'variable_name' at file scope error?

Question

I am new to C. I'm getting the following error when compiling:

error: variably modified 'header' at file scope
error: variably modified 'sequence' at file scope

Code:

struct list{
  char header[list_header_size];
  char sequence[list_sequence_size];
  struct list *next;
};

I thought the error meant that the compiler needed to know what these variables were from the beginning. So, I moved main(), which is where the struct is called, to the end of the program. I also tried declaring the variables at beginning of the program, but I'm not sure if I did that correctly. I tried char header; and char header[];.

Possible duplicate: Variably modified array at file scope in C. — Peter Mortensen
– Peter Mortensen, Commented Jul 29, 2023 at 9:13
Close @Peter, but subtly different, in that this is within declaration of a new type. — Toby Speight
– Toby Speight, Commented Jul 29, 2023 at 12:06

Theodoros Chatzigiannakis · Accepted Answer · 2013-02-17 20:42:59Z

4

You are right that the compiler needs to know the types of the members of the struct. One reason why it needs to know the types is so that it can calculate sizes. In your case, however, it can't know the sizes because in your struct you have defined two arrays that are not of a constant size. Therefore, the compiler doesn't know the total size of the struct and this defeats the purpose of knowing the types.

The closest to what you want is to replace the two char arrays with two char pointers and allocate the memory they will point to dynamically.

answered Feb 17, 2013 at 20:42

Theodoros Chatzigiannakis

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7 Comments

Kerrek SB Over a year ago

"Calculating sizes" is missing the interesting point: In C, array types are types, and the size of part of the type.

user1472747 Over a year ago

Alright, I did this. Now I get a weird error: expected ':', ',', ';', '}' or 'attribute' before '=' token ` int list_header_size = 200; int list_sequence_size = 2000; struct list{ char header = (char) malloc(sizeof(list_header_size)); char sequence = (char) malloc(sizeof(list_sequence_size)); struct list *next; }; `

Theodoros Chatzigiannakis Over a year ago

@user1472747 Please post your modified code so that we can see what's wrong.

user1472747 Over a year ago

I'm trying... but I can't figure out how to format code in the comment. I'll post it in the main post.

user1472747 Over a year ago

oh, wow. You're not supposed to set fields in the struct, are you... I'm an idiot >.<

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