Why is the pointer changing for no reason? - c

Question

I am working on Assignment 2 of ops-class.
The following function bootstraps a file handler array for the process that is being created (eg. user processes for test programs provided here).
int _fh_bootstrap(struct fharray *fhs){
/* Initialize the file handle array of this process */
fharray_init(fhs);
/* String variables initialized for passage to vfs_open */
char* console_inp = kstrdup(CONSOLE); // CONSOLE = "con:"
char* console_out = kstrdup(console_inp);
char* console_err = kstrdup(console_inp);
/* Return variable */
int ret = 0;
/* Initialize the console files STDIN, STDOUT and STDERR */
struct vnode *stdin;
ret = vfs_open(console_inp,O_RDONLY,0,&stdin);
if(ret != 0){
return ret;
}
kfree(console_inp);
struct fh *stdinfh = kmalloc(sizeof(struct fh));
ret = _fh_create(O_RDONLY,stdin,stdinfh);
if(ret != 0){
return ret;
}
stdinfh->fd = STDIN_FILENO;
fharray_add(fhs,stdinfh,NULL);
struct vnode *stdout;
ret = vfs_open(console_out,O_WRONLY,0,&stdout);
if(ret != 0){
return ret;
}
kfree(console_out);
struct fh *stdoutfh = kmalloc(sizeof(struct fh));
ret = _fh_create(O_WRONLY,stdout,stdoutfh);
if(ret != 0){
return ret;
}
stdoutfh->fd = STDOUT_FILENO;
fharray_add(fhs,stdoutfh,NULL);
struct vnode *stderr;
ret = vfs_open(console_err,O_WRONLY,0,&stderr);
if(ret != 0){
return ret;
}
kfree(console_err);
struct fh *stderrfh = kmalloc(sizeof(struct fh));
ret = _fh_create(O_WRONLY,stderr,stderrfh);
if(ret != 0){
return ret;
}
stderrfh->fd = STDERR_FILENO;
fharray_add(fhs,stderrfh,NULL);
fharray_setsize(fhs,MAX_FD);
return 0;
/* Initialization of stdin, out and err filehandlers complete */
}
If I use os161-gdb to step through this function, I notice the following:
//*stdinfh after the call to _fh_create
{fd = 0, flag = 0, fh_seek = 0, fh_vnode = 0x80044ddc}
//**stdinfh->fh_vnode
{vn_refcount = 2, vn_countlock = {splk_lock = 0, splk_holder = 0x0}, vn_fs = 0x0,
vn_data = 0x8004ab60, vn_ops = 0x8003e690 <dev_vnode_ops>}
This is the strange part. After stepping through the second call to kmalloc (to init stdoutfh), the stdinfh->fh_vnode pointer changes value!
//**stdinfh->fh_vnode
(struct vnode *) 0x1
And even stranger, after proceeding to the following line
fharray_add(fhs,stdoutfh,NULL);
The value of *stdoutfh->fh_vnode and *stdinfh->fh_vnode IS THE SAME
1 possible explanation: Does the OS not have enough heap memory. I find it unlikely and even after assuming this, I can't exactly explain what is happening here.
Some extra code
_fh_create
struct fh definition
static int _fh_create(int flag, struct vnode *file, struct fh *handle){
KASSERT(file != NULL);
/* W , R , RW */
if (
((flag & O_RDONLY) && (flag & O_WRONLY)) ||
((flag & O_RDWR) && ((flag & O_RDONLY) || (flag & O_WRONLY)))
) {
handle = NULL;
return 1;
}
handle->flag = flag;
handle->fh_seek = 0;
handle->fh_vnode = &file;
return 0;
}
struct fh {
uint32_t fd; // file descriptor
int flag; // File handler mode
off_t fh_seek; // seek position in file
struct vnode **fh_vnode; // File object of the file
}
Definition of struct vnode can be found here.
Please let me know if you need more info and thanks for the help!

Answer 1

The code handle->fh_vnode is setting a pointer to automatic variable ("on the stack"), function parameters are automatic variables. After the function returns this will be a dangling pointer.
To fix this you will need to re-design your code a bit, e.g. perhaps the struct fh should just store file and not a pointer to file.