How to define a system call in Linux with non-default return type?

Question

I am trying to define a custom system call that traverses all the processes currently running. The function declaration would look something like this: ssize_t traverse_process(struct customStructure *list, size_t length);

I've found out that in order to define a new system call, we use the SYSCALL_DEFINEx macro (in my case 2).

However, I also read that the macro is actually equivalent to asmlinkage long sys_traverse_process(struct customStructure *list, size_t length); .

However, as you can see, my traverse_process' function declaration returns a ssize_t. Is there any way to define a system call without using the fore-mentioned macro?

Any help would be well appreciated.! Thank you.

Answer 1

Returning long is the only proper way for a system call in Linux.

There is no way for a system call to return a value, which size differs from the size of long .

Do you expect ssize_t to have the same size as long on all platforms ? If yes (this is a correct expectation), then there is no reason to prefer ssize_t over long . If you are not sure that your return type will fit to long on every platform, then you simply cannot use this return type.

For example, from the C standard you knows, that read function returns ssize_t . But read system call has long as return type (it is defined using DEFINE_SYSCALL macro):

SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
{
    struct fd f = fdget(fd);
    ssize_t ret = -EBADF;

    if (f.file) {
        loff_t pos = file_pos_read(f.file);
        ret = vfs_read(f.file, buf, count, &pos);
        file_pos_write(f.file, pos);
        fdput(f);
    }
    return ret;
}

Note, that despite on long being return type of the system call, the above implementation returns a value of type ssize_t .