From what I've read on the internet and in books, when editing a text file, or any file for that matter, the file must be entirely rewritten; You can't just insert data into a file and save it.
If this is so, how can there be arrays in programming languages? In C++ I can explicitly modify certain values in the middle of arrays. To me, this seems like a demonstration of the modification of one or two bytes in the middle of a group of bytes.
The only two possible solutions I have thought of are
There is some funky stuff going on behind-the-scenes in C++, so it seems like only one or two bytes are being modified, but the array is actually entirely rewritten.
Then, after thinking about it, and especially after typing it out, I realized the the aformentioned solution seems really really dumb and totally not true, because there are things like addresses and pointers and the performance sounds awful. So I thought that maybe files are not entirely rewritten; only the everything after the first point in new data was inserted is rewritten. This seems much more plausible to me, and makes sense.
What is the difference between writing data to a file and writing data to an array?
You can change the values of particular locations in an array without needing to re-write the whole thing. However, you cannot insert new values into the middle of an array without shifting everything following the new values in order to make room.
Similarly, you can overwrite sections of a file without copying it (although the underlying storage technology may need to re-write an entire storage block in order to change a single byte within that block), but you can't insert new data without somehow making room for it. Text editors (and editors for more complicated file formats) are designed for random-access modifications that do not preserve length, so they will typically re-write the entire file regardless of what changed rather than trying to optimize for length-preserving edits.
Related
I am a beginning C++ student. I have a structure array that holds employee info.
I can put values into the structure, write those values to a binary dat file and
read the values back into the program so that it can be displayed to the console.
Here is my problem. Once I close the program, I can't get the file to read the data from the file back into memory - instead it reads "garbage."
I tried a few things and then read this in my book:
NOTE: Structures containing pointers cannot be correctly stored to
disk using the techniques of this section. This is because if the
structure is read into memory on a subsequent run of the program, it
cannot be guaranteed that all program variables will be at the same
memory locations.
I am pretty sure this is what is going on when I try to open a .dat file with previously stored information and try to read it into a structure array.
I can send my code examples if that would help clarify my question.
Any suggestions would be appreciated.
Speaking generally (since I don't have your code) there's two reasons you generally shouldn't just write the bytes of a struct or class to a file:
As your book mentioned, writing a pointer to disk is pointless, since you're just storing a random address and not the data at that address. You need to write out the data itself.
You especially should not attempt to write a struct/class all at once with something like
fwrite(file, myStruct, sizeof(myStruct)). Compilers sometimes put empty bytes between variables in structs in order to let the processor read them faster - this is called padding. Using a different compiler or compiling for a different computer architecture can pad structures differently, so a file that opens correctly on one computer might not open correctly on another.
There's lots of ways you can write out data to a file, be it in a binary format or in some human-readable format like XML. Regardless of what method you choose to use (each has strengths and weaknesses), every one of them involves writing each piece of data you care to save one by one, then reading them back in one by one. Higher level languages like Java or C# have ways to do this automagically by storing metadata about objects, but this comes at the cost of more memory usage and slower program execution.
I want to erase lines within a file. I know you can store the content of the file (in a vector for example), erase the line and write again. However, it feels very cumbersome, and not very efficient if the file gets bigger.
Anyone knows of a better, more efficient, more elegant way of doing it?
On most file-systems, this is the only option you have, short of switching to an actual database.
However, if you find yourself in this situation (i.e. very large files, with inserts/deletes in the middle), consider whether you can do something like maintaining a bitmap at the top of the file, where each bit represents one line of your file. To "delete" a line, simply flip the corresponding bit value.
There's nothing particularly magical about disk files. They still like to store their data in contiguous areas (typically called something like "blocks"). They don't have ways of leaving data-free holes in the middle of these areas. So if you want to "remove" three bytes from the middle of one of these areas, something somewhere is going to have to accomplish this by moving everything else in that area back by three bytes. No, it is not efficient.
This is why text editors (which have to do this kind of thing a lot) tend to load as much of the file as possible (if not all of it) into RAM, where moving data around is much faster. They typically only write changes back to disk when requested (or periodically). If you are going to have to make lots of changes like this, I'd suggest taking a page from their book and doing something similar.
The BerkeleyDB (dbopen(3)) has an access method called DB_RECNO. This allows one to manipulate files with arbitrary lengths using any sort of record delimiter. The default uses variable-length records with unix newlines as delimiters. You then access each "record" using an integer index. Using this, you can delete arbitrary lines from your text file. This isn't specific to C++, but if you are on most Unix/Linux systems, this API is already available to you.
I need to Erase the file contents from a selected Point (C++ fstream) which function should i use ?
i have written objects , i need to delete these objects in middle of the file
C++ has no standard mechanism to truncate a file at a given point. You either have to recreate the file (open with ios::trunc and write the contents you want to keep) or use OS-specific API calls (SetEndOfFile on Windows, truncate or ftruncate on Unix).
EDIT: Deleting stuff in the middle of a file is an exceedingly precarious business. Long before considering any other alternatives, I would try to use a server-less database engine like SQLite to store serialised objects. Better still, I would use SQLite as intended by storing the data needed by those objects in a proper schema.
EDIT 2: If the problem statement requires raw file access...
As a general rule, you don't delete data from the middle of a file. If the objects can be serialised to a fixed size on disk, you can work with them as records, and rather than trying to delete data, you use a table that indexes records within the file. E.g., if you write four records in sequence, the table will hold [0, 1, 2, 3]. In order to delete the second record, you simply remove its entry from the table: [0, 2, 3]. There are at least two ways to reuse the holes left behind by the table:
On each insertion, scan for the first unused index and write the object out at the corresponding record location. This will become more expensive, though, as the file grows.
Maintain a free list. Store, as a separate variable, the index of the most recently freed record. In the space occupied by that record encode the index of the record freed before it, and so on. This maintains a handy linked-list of free records while only requiring space fo one additional number. It is more complicated to work with, however, and requires an extra disk I/O when deleting and inserting.
If the objects can't be serialised to a fixed-length, then this approach becomes much, much harder. Variable-length record management code is very complex.
Finally, if the problem statement requires keeping records in order on disk, then it's a stupid problem statement, because insertion/removal in the middle of a file is ridiculously expensive; no sane design would require this.
The general method is to open the file for read access, open a new file for write access, read the content of the first file and write the data you want retained to the second file. When complete, you delete the first file and rename the second to that of the first.
I have a some large data structure (N > 10,000) that usually only needs to be created once (at runtime), and can be reused many times afterwards, but it needs to be loaded very quickly. (It is used for user input processing on iPhoneOS.) mmap-ing a file seems to be the best choice.
Are there any data structure libraries for C++ (or C)? Something along the line
ReadOnlyHashTable<char, int> table ("filename.hash");
// mmap(...) inside the c'tor
...
int freq = table.get('a');
...
// munmap(...); inside the d'tor.
Thank you!
Details:
I've written a similar class for hash table myself but I find it pretty hard to maintain, so I would like to see if there's existing solutions already. The library should
Contain a creation routine that serialize the data structure into file. This part doesn't need to be fast.
Contain a loading routine that mmap a file into read-only (or read-write) data structure that can be usable within O(1) steps of processing.
Use O(N) amount of disk/memory space with a small constant factor. (The device has serious memory constraint.)
Small time overhead to accessors. (i.e. the complexity isn't modified.)
Assumptions:
Bit representation of data (e.g. endianness, encoding of float, etc.) does not matter since it is only used locally.
So far the possible types of data I need are integers, strings, and struct's of them. Pointers do not appear.
P.S. Can Boost.intrusive help?
You could try to create a memory mapped file and then create the STL map structure with a customer allocator. Your customer allocator then simply takes the beginning of the memory of the memory mapped file, and then increments its pointer according to the requested size.
In the end all the allocated memory should be within the memory of the memory mapped file and should be reloadable later.
You will have to check if memory is free'd by the STL map. If it is, your customer allocator will lose some memory of the memory mapped file but if this is limited you can probably live with it.
Sounds like maybe you could use one of the "perfect hash" utilities out there. These spend some time opimising the hash function for the particular data, so there are no hash collisions and (for minimal perfect hash functions) so that there are no (or at least few) empty gaps in the hash table. Obviously, this is intended to be generated rarely but used frequently.
CMPH claims to cope with large numbers of keys. However, I have never used it.
There's a good chance it only generates the hash function, leaving you to use that to generate the data structure. That shouldn't be especially hard, but it possibly still leaves you where you are now - maintaining at least some of the code yourself.
Just thought of another option - Datadraw. Again, I haven't used this, so no guarantees, but it does claim to be a fast persistent database code generator.
WRT boost.intrusive, I've just been having a look. It's interesting. And annoying, as it makes one of my own libraries look a bit pointless.
I thought this section looked particularly relevant.
If you can use "smart pointers" for links, presumably the smart pointer type can be implemented using a simple offset-from-base-address integer (and I think that's the point of the example). An array subscript might be equally valid.
There's certainly unordered set/multiset support (C++ code for hash tables).
Using cmph would work. It does have the serialization machinery for the hash function itself, but you still need to serialize the keys and the data, besides adding a layer of collision resolution on top of it if your query set universe is not known before hand. If you know all keys before hand, then it is the way to go since you don't need to store the keys and will save a lot of space. If not, for such a small set, I would say it is overkill.
Probably the best option is to use google's sparse_hash_map. It has very low overhead and also has the serialization hooks that you need.
http://google-sparsehash.googlecode.com/svn/trunk/doc/sparse_hash_map.html#io
GVDB (GVariant Database), the core of Dconf is exactly this.
See git.gnome.org/browse/gvdb, dconf and bv
and developer.gnome.org/glib/2.30/glib-GVariant.html
I do embedded software, but this isn't really an embedded question, I guess. I don't (can't for technical reasons) use a database like MySQL, just C or C++ structs.
Is there a generic philosophy of how to handle changes in the layout of these structs from version to version of the program?
Let's take an address book. From program version x to x+1, what if:
a field is deleted (seems simple enough) or added (ok if all can use some new default)?
a string gets longer or shorter? An int goes from 8 to 16 bits of signed / unsigned?
maybe I combine surname/forename, or split name into two fields?
These are just some simple examples; I am not looking for answers to those, but rather for a generic solution.
Obviously I need some hard coded logic to take care of each change.
What if someone doesn't upgrade from version x to x+1, but waits for x+2? Should I try to combine the changes, or just apply x -> x+ 1 followed by x+1 -> x+2?
What if version x+1 is buggy and we need to roll-back to a previous version of the s/w, but have already "upgraded" the data structures?
I am leaning towards TLV (http://en.wikipedia.org/wiki/Type-length-value) but can see a lot of potential headaches.
This is nothing new, so I just wondered how others do it....
I do have some code where a longer string is puzzled together from two shorter segments if necessary. Yuck. Here's my experience after 12 years of keeping some data compatible:
Define your goals - there are two:
new versions should be able to read what old versions write
old versions should be able to read what new versions write (harder)
Add version support to release 0 - At least write a version header. Together with keeping (potentially a lot of) old reader code around that can solve the first case primitively. If you don't want to implement case 2, start rejecting new data right now!
If you need only case 1, and and the expected changes over time are rather minor, you are set. Anyway, these two things done before the first release can save you many headaches later.
Convert during serialization - at run time, only keep the data in the "new format" in memory. Do necessary conversions and tests at persistence limits (convert to newest when reading, implement backward compatibility when writing). This isolates version problems in one place, helping to avoid hard-to-track-down bugs.
Keep a set of test data from all versions around.
Store a subset of available types - limit the actually serialized data to a few data types, such as int, string, double. In most cases, the extra storage size is made up by reduced code size supporting changes in these types. (That's not always a tradeoff you can make on an embedded system, though).
e.g. don't store integers shorter than the native width. (you might need to do that when you need to store long integer arrays).
add a breaker - store some key that allows you to intentionally make old code display an error message that this new data is incompatible. You can use a string that is part of the error message - then your old version could display an error message it doesn't know about - "you can import this data using the ConvertX tool from our web site" is not great in a localized application but still better than "Ungültiges Format".
Don't serialize structs directly - that's the logical / physical separation. We work with a mix of two, both having their pros and cons. None of these can be implemented without some runtime overhead, which can pretty much limit your choices in an embedded environment. At any rate, don't use fixed array/string lengths during persistence, that should already solve half of your troubles.
(A) a proper serialization mechanism - we use a bianry serializer that allows to start a "chunk" when storing, which has its own length header. When reading, extra data is skipped and missing data is default-initialized (which simplifies implementing "read old data" a lot in the serializationj code.) Chunks can be nested. That's all you need on the physical side, but needs some sugar-coating for common tasks.
(B) use a different in-memory representation - the in-memory reprentation could basically be a map<id, record> where id woukld likely be an integer, and record could be
empty (not stored)
a primitive type (string, integer, double - the less you use the easier it gets)
an array of primitive types
and array of records
I initially wrote that so the guys don't ask me for every format compatibility question, and while the implementation has many shortcomings (I wish I'd recognize the problem with the clarity of today...) it could solve
Querying a non existing value will by default return a default/zero initialized value. when you keep that in mind when accessing the data and when adding new data this helps a lot: Imagine version 1 would calculate "foo length" automatically, whereas in version 2 the user can overrride that setting. A value of zero - in the "calculation type" or "length" should mean "calculate automatically", and you are set.
The following are "change" scenarios you can expect:
a flag (yes/no) is extended to an enum ("yes/no/auto")
a setting splits up into two settings (e.g. "add border" could be split into "add border on even days" / "add border on odd days".)
a setting is added, overriding (or worse, extending) an existing setting.
For implementing case 2, you also need to consider:
no value may ever be remvoed or replaced by another one. (But in the new format, it could say "not supported", and a new item is added)
an enum may contain unknown values, other changes of valid range
phew. that was a lot. But it's not as complicated as it seems.
There's a huge concept that the relational database people use.
It's called breaking the architecture into "Logical" and "Physical" layers.
Your structs are both a logical and a physical layer mashed together into a hard-to-change thing.
You want your program to depend on a logical layer. You want your logical layer to -- in turn -- map to physical storage. That allows you to make changes without breaking things.
You don't need to reinvent SQL to accomplish this.
If your data lives entirely in memory, then think about this. Divorce the physical file representation from the in-memory representation. Write the data in some "generic", flexible, easy-to-parse format (like JSON or YAML). This allows you to read in a generic format and build your highly version-specific in-memory structures.
If your data is synchronized onto a filesystem, you have more work to do. Again, look at the RDBMS design idea.
Don't code a simple brainless struct. Create a "record" which maps field names to field values. It's a linked list of name-value pairs. This is easily extensible to add new fields or change the data type of the value.
Some simple guidelines if you're talking about a structure use as in a C API:
have a structure size field at the start of the struct - this way code using the struct can always ensure they're dealing only with valid data (for example, many of the structures the Windows API uses start with a cbCount field so these APIs can handle calls made by code compiled against old SDKs or even newer SDKs that had added fields
Never remove a field. If you don't need to use it anymore, that's one thing, but to keep things sane for dealing with code that uses an older version of the structure, don't remove the field.
it may be wise to include a version number field, but often the count field can be used for that purpose.
Here's an example - I have a bootloader that looks for a structure at a fixed offset in a program image for information about that image that may have been flashed into the device.
The loader has been revised, and it supports additional items in the struct for some enhancements. However, an older program image might be flashed, and that older image uses the old struct format. Since the rules above were followed from the start, the newer loader is fully able to deal with that. That's the easy part.
And if the struct is revised further and a new image uses the new struct format on a device with an older loader, that loader will be able to deal with it, too - it just won't do anything with the enhancements. But since no fields have been (or will be) removed, the older loader will be able to do whatever it was designed to do and do it with the newer image that has a configuration structure with newer information.
If you're talking about an actual database that has metadata about the fields, etc., then these guidelines don't really apply.
What you're looking for is forward-compatible data structures. There are several ways to do this. Here is the low-level approach.
struct address_book
{
unsigned int length; // total length of this struct in bytes
char items[0];
}
where 'items' is a variable length array of a structure that describes its own size and type
struct item
{
unsigned int size; // how long data[] is
unsigned int id; // first name, phone number, picture, ...
unsigned int type; // string, integer, jpeg, ...
char data[0];
}
In your code, you iterate through these items (address_book->length will tell you when you've hit the end) with some intelligent casting. If you hit an item whose ID you don't know or whose type you don't know how to handle, you just skip it by jumping over that data (from item->size) and continue on to the next one. That way, if someone invents a new data field in the next version or deletes one, your code is able to handle it. Your code should be able to handle conversions that make sense (if employee ID went from integer to string, it should probably handle it as a string), but you'll find that those cases are pretty rare and can often be handled with common code.
I have handled this in the past, in systems with very limited resources, by doing the translation on the PC as a part of the s/w upgrade process. Can you extract the old values, translate to the new values and then update the in-place db?
For a simplified embedded db I usually don't reference any structs directly, but do put a very light weight API around any parameters. This does allow for you to change the physical structure below the API without impacting the higher level application.
Lately I'm using bencoded data. It's the format that bittorrent uses. Simple, you can easily inspect it visually, so it's easier to debug than binary data and is tightly packed. I borrowed some code from the high quality C++ libtorrent. For your problem it's so simple as checking that the field exist when you read them back. And, for a gzip compressed file it's so simple as doing:
ogzstream os(meta_path_new.c_str(), ios_base::out | ios_base::trunc);
Bencode map(Bencode::TYPE_MAP);
map.insert_key("url", url.get());
map.insert_key("http", http_code);
os << map;
os.close();
To read it back:
igzstream is(metaf, ios_base::in | ios_base::binary);
is.exceptions(ios::eofbit | ios::failbit | ios::badbit);
try {
torrent::Bencode b;
is >> b;
if( b.has_key("url") )
d->url = b["url"].as_string();
} catch(...) {
}
I have used Sun's XDR format in the past, but I prefer this now. Also it's much easier to read with other languages such as perl, python, etc.
Embed a version number in the struct or, do as Win32 does and use a size parameter.
if the passed struct is not the latest version then fix up the struct.
About 10 years ago I wrote a similar system to the above for a computer game save game system. I actually stored the class data in a seperate class description file and if i spotted a version number mismatch then I coul run through the class description file, locate the class and then upgrade the binary class based on the description. This, obviously required default values to be filled in on new class member entries. It worked really well and it could be used to auto generate .h and .cpp files as well.
I agree with S.Lott in that the best solution is to separate the physical and logical layers of what you are trying to do. You are essentially combining your interface and your implementation into one object/struct, and in doing so you are missing out on some of the power of abstraction.
However if you must use a single struct for this, there are a few things you can do to help make things easier.
1) Some sort of version number field is practically required. If your structure is changing, you will need an easy way to look at it and know how to interpret it. Along these same lines, it is sometimes useful to have the total length of the struct stored in a structure field somewhere.
2) If you want to retain backwards compatibility, you will want to remember that code will internally reference structure fields as offsets from the structure's base address (from the "front" of the structure). If you want to avoid breaking old code, make sure to add all new fields to the back of the structure and leave all existing fields intact (even if you don't use them). That way, old code will be able to access the structure (but will be oblivious to the extra data at the end) and new code will have access to all of the data.
3) Since your structure may be changing sizes, don't rely on sizeof(struct myStruct) to always return accurate results. If you follow #2 above, then you can see that you must assume that a structure may grow larger in the future. Calls to sizeof() are calculated once (at compile time). Using a "structure length" field allows you to make sure that when you (for example) memcpy the struct you are copying the entire structure, including any extra fields at the end that you aren't aware of.
4) Never delete or shrink fields; if you don't need them, leave them blank. Don't change the size of an existing field; if you need more space, create a new field as a "long version" of the old field. This can lead to data duplication problems, so make sure to give your structure a lot of thought and try to plan fields so that they will be large enough to accommodate growth.
5) Don't store strings in the struct unless you know that it is safe to limit them to some fixed length. Instead, store only a pointer or array index and create a string storage object to hold the variable-length string data. This also helps protect against a string buffer overflow overwriting the rest of your structure's data.
Several embedded projects I have worked on have used this method to modify structures without breaking backwards/forwards compatibility. It works, but it is far from the most efficient method. Before long, you end up wasting space with obsolete/abandoned structure fields, duplicate data, data that is stored piecemeal (first word here, second word over there), etc etc. If you are forced to work within an existing framework then this might work for you. However, abstracting away your physical data representation using an interface will be much more powerful/flexible and less frustrating (if you have the design freedom to use such a technique).
You may want to take a look at how Boost Serialization library deals with that issue.