C error: array type has incomplete element type

Question

I'm trying to compile a program in Ubuntu 11.04 that works well in Windows but it gives the above error. I have added a comment to the line that is causing the error. Here's the code:

route_input() {
    int num_routes;//Variable to act as the loop counter for the loop getting route details
    int x;

    char route_id[3];
    char r_source[20];
    char r_destination[20];
    int r_buses;



    printf("Please enter the number of routes used: \n");
    scanf("%d", &num_routes);
    char routes_arr[num_routes][10];//An array to hold the details of each route

    printf("\nNumber of routes is %d\n", num_routes);

    struct route r[num_routes];//An array of structures of type route (This line causes the error)

    fflush(stdin);

    for (x = num_routes; x > 0; x--) {
         printf("\nEnter the route number: ");
         scanf("%s", r[x].route_num);
         printf("Route number is %s", r[x].route_num);


         printf("\nEnter the route source: ");
         fflush(stdin);
         scanf("%s", r[x].source);
         printf("Source = %s", r[x].source);


         printf("\nEnter the route destination: ");
         fflush(stdin);
         gets(r[x].destination);
         printf("Destination = %s", r[x].destination);

         printf("\nEnter the number of buses that use this route: ");
         scanf("%d", &r[x].num_of_buses);
         printf("Number of buses = %d", r[x].num_of_buses);


    }

    for (x = num_routes; x > 0; x--) {
        printf("\n\n+++Routes' Details+++\nRoute number = %s, Source = %s, Destination = %s, Number of buses for this route = %d\n", r[x].route_num, r[x].source, r[x].destination, r[x].num_of_buses);
    }

}

Solution 1

The error message is caused because you have an incomplete declaration of struct route. i.e. somewhere you have a line that says

struct route;

with no specification of what is in the struct. This is perfectly legal and allows the compiler to know the struct exists before it knows what is in it. That allows it to define pointers to items of type struct route for opaque types and for forward declarations.

However, the compiler cannot use an incomplete type as the elements for an array because it needs to know the size of the struct to calculate the amount of memory needed for the array and to calculate offsets from indexes.

I'd say you have forgotten to include the header that defines your route struct. Also, it's possible that Ubuntu has an opaque type called struct route in its library already, so you may have to rename your struct to avoid a clash.

Solution 2

You need to include the header file that defines struct route.
I'm not sure which header this is, and it may differ between Linux and Windows.

In Linux, net/route.h defines struct rtentry, which may be what you need.

Comments

[ouah]

Do not fflush(stdin), it is undefined behavior.

[deebee]

possible duplicate of stackoverflow.com/questions/2274550/…

[Mat]

What is struct route, where is it defined?

[dirkgently]

As @deebee pointed out, if your struct route is a recursive data structure with non-pointer self-referential members, you are likely to hit the incomplete type issue.

[Jens]

fflush(stdout) instead if you want to make sure the messages not ending in newlines do appear.

[Gandaro]

Full error messages are so underestimated…

[ugoren]

If that was the problem, char routes_arr[num_routes][10] would fail also. So he's probably using a c99 compiler.

[Kartik Anand]

as i said multidim arrays row variable is optional and as far as i know ubuntu11.04 GCC does give that error it isnt based on C99

[ugoren]

My point is that he's obviously using a compiler that supports a variable as dimension (or the routes_arr would fail). So his problem in defining r can't be what you say.

[Kartik Anand]

But 2D arrays dont need a row compulsarily that's why it isnt failing there...moreover route can be just any structure he has defined earlier it doesnt really need a header file

[Kartik Anand]

ive tested it check it urself

[Jens]

Kartik Anand, please educate yourself about variable length arrays. What your particular compiler accepts or doesn't accept is of no importance. The important part is, what the C language definition says.

[Kartik Anand]

@Jens before commenting why dont u try what i am saying and yes i have educated myself well in C(atleast)...C allows u to define 2D arrays with optional row..but it wont happen with 1D arrays there u have to specify index(constant)..seriously can u please give it a try before blaming other's education...and yes what ur compiler accepts or not does make a differene(Because its a compiler error)...and different versions do exist some accepting variable length arrays and some dont(GCC being one of them)

[Jens]

@Kartik Anand: "As far as I know C (at least GCC won't) doesn't allow you to have variables as array indexes." is just plain wrong since 1999. Why did you write this sentence? And seriously, could you try proper spelling? Thanks.

[Kartik Anand]

I know C99 added that support but GCC also follows ansi C..Try it on ubuntu and then question me...

[Kartik Anand]

moreover C99 support is an extension in GCC which can be checked if enabled or not..readthis(if these extentions are off or not used properly they will result in this error)gcc.gnu.org/onlinedocs/gcc/Standards.html

[Jens]

@Kartik, now please try compiling main(int argc, char**argv){int x[argc][argc];return 0;}. Gcc on Ubuntu compiles by default C99 (plus some other extensions).

[Kartik Anand]

what i am trying to say is if these extensions are turned off they will result in this error..try your code with -std..and then tell me the error you are facing...

[JeremyP]

In C99 it is legal to declare an array with a variable size. This is default behaviour for modern versions of GCC (well C99 + gnu extensions). Anyway, the error message says this is not the problem.

[ugoren]

Also, read the question headline: "array type has incomplete element type". The problem is with the element type, that's incomplete, not the dimensions.

[Trust Birungi]

Thanks for your help. I included the header file containing the route struct definition and it worked correctly.