I am not quite sure if I understand difference between direct and sequential file accessing in C. Lets say we have a structure in some file:
struct students_s {
int points[5];
int number[10];
int id;
}student;
In this case we can use direct access to that structure since all of the variables are the same type, therefore they are same fixed size. This is my thinking, and I am not quite sure if it is correct. Can someone please help me with this?
Another question I have is, is it necessary that students in that file are arranged from 1 to n with the id variable or their arrangemenet does not matter?
I tried searching for it online but I can not seem to find detailed answer.
Related
I was reading the chapter in Beautiful Code on the Linux kernel and the author discusses how Linux kernel implements inheritance in the C language (amongst other topics). In a nutshell, a 'base' struct is defined and in order to inherit from it the 'subclass' struct places a copy of the base at the end of the subclass struct definition. The author then spends a couple pages explaining a clever and complicated macro to figure out how many bytes to back in order to convert from the base part of the object to the subclass part of the object.
My question: Within the subclass struct, why not declare the base struct as the first thing in the struct, instead of the last thing?
The main advantage of putting the base struct stuff first is when casting from the base to the subclass you wouldn't need to move the pointer at all - essentially, doing the cast just means telling the compiler to let your code use the 'extra' fields that the subclass struct has placed after the stuff that the base defines.
Just to clarify my question a little bit let me throw some code out:
struct device { // this is the 'base class' struct
int a;
int b;
//etc
}
struct usb_device { // this is the 'subclass' struct
int usb_a;
int usb_b;
struct device dev; // This is what confuses me -
// why put this here, rather than before usb_a?
}
If one happens to have a pointer to the "dev" field inside of a usb_device object then in order to cast it back to that usb_device object one needs to subtract 8 from that pointer. But if "dev" was the first thing in a usb_device casting the pointer wouldn't need to move the pointer at all.
Any help on this would be greatly appreciated. Even advice on where to find an answer would be appreciated - I'm not really sure how to Google for the architectural reason behind a decision like this. The closest I could find here on StackOverflow is:
why to use these weird nesting structure
And, just to be clear - I understand that a lot of bright people have worked on the Linux kernel for a long time so clearly there's a good reason for doing it this way, I just can't figure out what it is.
The Amiga OS uses this "common header" trick in a lot of places and it looked like a good idea at the time: Subclassing by simply casting the pointer type. But there are drawbacks.
Pro:
You can extend existing data structures
You can use the same pointer in all places where the base type is expected, no pointer arithmetic needed, saving precious cycles
It feels natural
Con:
Different compilers tend to align data structures differently. If the base structure ended with char a;, then you could have 0, 1 or 3 pad bytes afterwards before the next field of the subclass starts. This led to quite nasty bugs, especially when you had to maintain backwards compatibility (i.e. for some reason, you have to have a certain padding because an ancient compiler version had a bug and now, there is lots of code which expects the buggy padding).
You don't notice quickly when you pass the wrong structure around. With the code in your question, fields get trashed very quickly if the pointer arithmetic is wrong. That is a good thing since it raises chances that a bug is discovered more early.
It leads to an attitude "my compiler will fix it for me" (which it sometimes won't) and all the casts lead to a "I know better than the compiler" attitude. The latter one would make you automatically insert casts before understanding the error message, which would lead to all kinds of odd problems.
The Linux kernel is putting the common structure elsewhere; it can be but doesn't have to be at the end.
Pro:
Bugs will show early
You will have to do some pointer arithmetic for every structure, so you're used to it
You don't need casts
Con:
Not obvious
Code is more complex
I'm new to the Linux kernel code, so take my ramblings here with a grain of salt. As far as I can tell, there is no requirement as to where to put the "subclass" struct. That is exactly what the macros provide: You can cast to the "subclass" structure, regardless of its layout. This provides robustness to your code (the layout of a structure can be changed, without having to change your code.
Perhaps there is a convention of placing the "base class" struct at the end, but I'm not aware of it. I've seen lots of code in drivers, where different "base class" structs are used to cast back to the same "subclass" structure (from different fields in the "subclass" of course).
I don't have fresh experience from the Linux kernel, but from other kernels. I'd say that this doesn't matter at all.
You are not supposed to cast from one to the other. Allowing casts like that should only be done in very specific situations. In most cases it reduces the robustness and flexibility of the code and is considered quite sloppy. So the deepest "architectural reason" you're looking for might just be "because that's the order someone happened to write it in". Or alternatively, that's what the benchmarks showed would be the best for performance of some important code path in that code. Or alternatively, the person who wrote it thinks it looks pretty (I always build upside-down pyramids in my variable declarations and structs if I have no other constraints). Or someone happened to write it this way 20 years ago and since then everyone else has been copying it.
There might be some deeper design behind this, but I doubt it. There's just no reason to design those things at all. If you want to find out from an authoritative source why it's done this way, just submit a patch to linux that changes it and see who yells at you.
It's for multiple inheritance. struct dev isn't the only interface you can apply to a struct in the linux kernel, and if you have more than one, just casting the sub class to a base class wouldn't work. For example:
struct device {
int a;
int b;
// etc...
};
struct asdf {
int asdf_a;
};
struct usb_device {
int usb_a;
int usb_b;
struct device dev;
struct asdf asdf;
};
I'm seeking to fully understand the following code snippet from kernel/notifier.c. I have read and built simple link lists and think I get the construct from K&R's C programming. However this is slightly more complex. The second line below which begins with the 'int' appears to be two items together which is unclear.
The first is the (*notifier_call) which I believe has independent but related significance with the second containing a 'notifier block' term.
Can you explain how it works in detail? I understand that there is a function pointer and multiple subscribers possible. But I lack the way to tie these facts together, and could use a primer or key so I exactly understand how the code works. The third line looks to contain the linking structure, or recursive nature. Forgive my terms, and correct them as fit as I am a new student of computer science terminology.
struct notifier_block {
int (*notifier_call)(struct notifier_block *, unsigned long, void *);
struct notifier_block *next;
int priority;
};
This is a simple pointer to a function returning int, taking as its arguments a pointer to a notifier_block structure, an unsigned long, and a pointer to void.
It has nothing to do with the linked list, that is linked by the next member.
I know following approach may not be portable but that is exactly what I want to find out now.
Imagine I have some data structure
struct student
{
char name[20];
int age;
} x;
Now I want to write it to a file like this:
fwrite(&x, sizeof(student), 1, filePointer);
Read similarly:
fread(voidPointer, sizeof(student), 1, filePointer);
// Now possibly do a memcpy
memcpy(studentObjectPointer, voidPointer, sizeof(student));
My question is: Say I don't want to copy this file to another computer, and I will read it from the same computer that created this file.
Will the portability (endianness, packed data structure) issues still apply to above approach? Or it will work fine?
If the file would be copied on other machines, you will have to build your own serializer and deserializer. With the structure you gave it is quite simple.
You have to define which endianness to adopt when writing numbers (here, the int age).
Here, you could process like this :
Open the file in binary mode
Write the 20 bytes of the name string
Write the age in a CHOSEN endianness (big endian)
When reading it back, you will certainly have to convert the Big-Endianness to the local endianness of the machine.
There is still a remaining issue : if sizeof (int) is not the same between the two machines. In that case things get more vicious and I think the above is not sufficient.
If you really need to be portable across a wide range of machines, consider the use of specific length types defined in #include <stdint.h> such as int32_t for instance.
Remember, we're living in a 4-dimension world. You said, the saved file will not move in the x, y and z axis, it will be used on the same computer. But you did not mentioned the 4th dimension, the time. If the structure changes, it will fail.
At least, you should put a signature field in the structure. Fill it with a constant value before write(), check it right after read(), and change the constant value when the structure gets modified.
hope you can help me with this, it's kinda tricky...
i have txt file which contents names and marks of peple
for an example
Carl,Johnson,123xxx123,9;
like in order: name,surname,certificate no,mark;
struct xxxxx
{
char name[20];
char surname[20];
char certificate No[10];
int mark;
};
the problem starts when i need to read ten records from txt...
don't have clue how to duplicate struct, do i have to define it like ten times
struct xxxx sk1[ ],sk2[ ]... and so on...?
Once you have defined a struct you can declare several variables of that type.
struct xxxx sk1;
struct xxxx sk2;
or even an array of them
struct xxxx sk[10];
To populate this structure with data from the text file you need to write some code, formatted as the input is I would think fscanf() would not be of much help as is.
The approach I would take is probably reading the entire the line using fscanf() then do some splitting on ',' to get the fields. The first three are plain strcpy, the last one would take use of atoi
My grasp of C input functions are not all that good as I much prefer C++ so you should perhaps look for a second opinion in this matter.
Edit: I found some perfectly good info on how to proceed from your old questions
I. Just implemented a kind of bitwise trie (based on nedtries), but my code does lot
Of memory allocation (for each node).
Contrary to my implemetation, nedtries are claimed to be fast , among othet things,
Because of their small number of memory allocation (if any).
The author claim his implementation to be "in-place", but what does it really means in this context ?
And how does nedtries achieve such a small number of dynamic memory allocation ?
Ps: I know that the sources are available, but the code is pretty hard to follow and I cannot figure how it works
I'm the author, so this is for the benefit of the many according to Google who are similarly having difficulties in using nedtries. I would like to thank the people here on stackflow for not making unpleasant comments about me personally which some other discussions about nedtries do.
I am afraid I don't understand the difficulties with knowing how to use it. Usage is exceptionally easy - simply copy the example in the Readme.html file:
typedef struct foo_s foo_t;
struct foo_s {
NEDTRIE_ENTRY(foo_t) link;
size_t key;
};
typedef struct foo_tree_s foo_tree_t;
NEDTRIE_HEAD(foo_tree_s, foo_t);
static foo_tree_t footree;
static size_t fookeyfunct(const foo_t *RESTRICT r)
{
return r->key;
}
NEDTRIE_GENERATE(static, foo_tree_s, foo_s, link, fookeyfunct, NEDTRIE_NOBBLEZEROS(foo_tree_s));
int main(void)
{
foo_t a, b, c, *r;
NEDTRIE_INIT(&footree);
a.key=2;
NEDTRIE_INSERT(foo_tree_s, &footree, &a);
b.key=6;
NEDTRIE_INSERT(foo_tree_s, &footree, &b);
r=NEDTRIE_FIND(foo_tree_s, &footree, &b);
assert(r==&b);
c.key=5;
r=NEDTRIE_NFIND(foo_tree_s, &footree, &c);
assert(r==&b); /* NFIND finds next largest. Invert the key function to invert this */
NEDTRIE_REMOVE(foo_tree_s, &footree, &a);
NEDTRIE_FOREACH(r, foo_tree_s, &footree)
{
printf("%p, %u\n", r, r->key);
}
NEDTRIE_PREV(foo_tree_s, &footree, &a);
return 0;
}
You declare your item type - here it's struct foo_s. You need the NEDTRIE_ENTRY() inside it otherwise it can contain whatever you like. You also need a key generating function. Other than that, it's pretty boilerplate.
I wouldn't have chosen this system of macro based initialisation myself! But it's for compatibility with the BSD rbtree.h so nedtries is very easy to swap in to anything using BSD rbtree.h.
Regarding my usage of "in place"
algorithms, well I guess my lack of
computer science training shows
here. What I would call "in place"
is when you only use the memory
passed into a piece of code, so if
you hand 64 bytes to an in place
algorithm it will only touch that 64
bytes i.e. it won't make use of
extra metadata, or allocate some
extra memory, or indeed write to
global state. A good example is an
"in place" sort implementation where
only the collection being sorted
(and I suppose the thread stack)
gets touched.
Hence no, nedtries doesn't need a
memory allocator. It stores all the
data it needs in the NEDTRIE_ENTRY
and NEDTRIE_HEAD macro expansions.
In other words, when you allocate
your struct foo_s, you do all the
memory allocation for nedtries.
Regarding understanding the "macro
goodness", it's far easier to
understand the logic if you compile
it as C++ and then debug it :). The
C++ build uses templates and the
debugger will cleanly show you state
at any given time. In fact, all
debugging from my end happens in a
C++ build and I meticulously
transcribe the C++ changes into
macroised C.
Lastly, before a new release, I
search Google for people having
problems with my software to see if
I can fix things and I am typically
amazed what someone people say about
me and my free software. Firstly,
why didn't those people having
difficulties ask me directly for
help? If I know that there is
something wrong with the docs, then
I can fix them - equally, asking on
stackoverflow doesn't let me know
immediately that there is a docs
problem bur rather relies on me to
find it next release. So all I would
say is that if anyone finds a
problem with my docs, please do
email me and say so, even if there
is a discussion say like here on
stackflow.
Niall
I took a look at the nedtrie.h source code.
It seems that the reason it is "in-place" is that you have to add the trie bookkeeping data to the items that you want to store.
You use the NEDTRIE_ENTRY macro to add parent/child/next/prev links to your data structure, and you can then pass that data structure to the various trie routines, which will extract and use those added members.
So it is "in-place" in the sense that you augment your existing data structures and the trie code piggybacks on that.
At least that's what it looks like. There's lots of macro goodness in that code so I could have gotten myself confused (:
In-place means you operate on the original (input) data, so the input data becomes the output data. Not-in-place means that you have separate input and output data, and the input data is not modified. In-place operations have a number of advantages - smaller cache/memory footprint, lower memory bandwidth, hence typically better performance, etc, but they have the disadvantage that they are destructive, i.e. you lose the original input data (which may or may not matter, depending on the use case).
In-place means to operate on the input data and (possibly) update it. The implication is that there no copying and/moving of the input data. This may result in loosing the input data original values which you will need to consider if it is relevant for your particular case.