Error in C: array type has incomplete element type

Question

I am working on a project that modifies part of the Android OS, and when I tried to implement a new system call to return the status of all the processes, I encountered the following error:

kernel/sys.c: error: array tpye has incomplete element type.

What I did in sys.c is the following:

/***previous code in sys.c***/

SYSCALL_DEFINE2(new_syscall, struct info __user*, buf, int __user*, nr){
    if(!buf || !nr) return -EINVAL;

    int nr_copy;
    int success;

    success = copy_from_user(&nr_copy, nr, sizeof(int));
    if(success == 0) return -EINVAL;
    if(nr_copy < 1) return -EINVAL;

    struct info buf_copy[nr_copy];  /*************

    return 0;
}

Complier complained about the line marked with stars.

My info.h file looks like this:

    #ifndef __LINUX_INFO_H
    #define __LINUX_INFO_H

    struct info {
        long state;         
        pid_t pid;          
        pid_t parent_pid;       
    };

    #endif

I tried including the info.h in sys.c, but it didn't help at all. Can anyone provide me some directions on what I should look into?

Thanks

Possible duplicate of array type has incomplete element type error when compiling — Kusalananda
– Kusalananda, Commented Jul 22, 2016 at 9:55

vonbrand · Accepted Answer · 2013-02-05 03:51:36Z

2

What compiler are you using? Dynamically sized arrays are a newish C feature that might not be supported. OTOH, they are allocated on the stack, and that is a very scarce commodity in-kernel. You should allocate memory for the array via kmalloc or one of its ilk.

answered Feb 5, 2013 at 3:51

vonbrand

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4 Comments

ragingcorgi Over a year ago

It's not dynamically allocated array, it's a fixed size array of length nr_copy. Good point for kmalloc

vonbrand Over a year ago

@user1926344, you are getting the size for the array the line above it's definition, unless my kernel-C is way off by now...

Bob Murphy Over a year ago

nr_copy is a variable declared as int, so buf_copy[nr_copy] is not a fixed size array at compile time.

ragingcorgi Over a year ago

oh i see the problem now. Thanks!

rjt · Accepted Answer · 2013-02-05 03:58:36Z

0

Have you tried explicitly declaring state as a long int.

long state;

to

long int state;

answered Feb 5, 2013 at 3:58

rjt

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4 Comments

rjt Over a year ago

There are two __user* pointers. info __user* and int __user*

vonbrand Over a year ago

Both long state and long int state mean exactly the same thing in C.

rjt Over a year ago

@vonbrand, Check that assumption with code. Depends on the hardware AND operating sytem: sizeof(int) on a 286 returns 2 sizeof(int) on a 16bitOS on a 386 returns 2, but 4 on a 32bit OS. sizeof(int) on my x86_64 returns 4, but sizeof(long int) returns 8.

vonbrand Over a year ago

completely irrelevant. As I said, in C long is exactly the same as long int, if you omit the type it defaults to int (yes, this comes from C's prehistory).

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