#define HUGE_NUMBER ???
char string[HUGE_NUMBER];
do_something_with_the_string(string);
I was wondering what would be the maximum number that I could add to a char array without risking any potential memory problems, buffer overflows or the like. I wanted to get user input into it, and possibly the maximum possible.
See this response by Jack Klein (see original post):
The original C standard (ANSI 1989/ISO
1990) required that a compiler
successfully translate at least one
program containing at least one
example of a set of environmental
limits. One of those limits was being
able to create an object of at least
32,767 bytes.
This minimum limit was raised in the
1999 update to the C standard to be at
least 65,535 bytes.
No C implementation is required to
provide for objects greater than that
size, which means that they don't need
to allow for an array of ints greater
than (int)(65535 / sizeof(int)).
In very practical terms, on modern
computers, it is not possible to say
in advance how large an array can be
created. It can depend on things like
the amount of physical memory
installed in the computer, the amount
of virtual memory provided by the OS,
the number of other tasks, drivers,
and programs already running and how
much memory that are using. So your
program may be able to use more or
less memory running today than it
could use yesterday or it will be able
to use tomorrow.
Many platforms place their strictest
limits on automatic objects, that is
those defined inside of a function
without the use of the 'static'
keyword. On some platforms you can
create larger arrays if they are
static or by dynamic allocation.
Now, to provide a slightly more tailored answer, DO NOT DECLARE HUGE ARRAYS TO AVOID BUFFER OVERFLOWS. That's close to the worst practice one can think of in C. Rather, spend some time writing good code, and carefully make sure that no buffer overflow will occur. Also, if you do not know the size of your array in advance, look at malloc, it might come in handy :P
It depends on where char string[HUGE_NUMBER]; is placed.
Is it inside a function? Then the array will be on the stack, and if and how fast your OS can grow stacks depends on the OS. So here is the general rule: dont place huge arrays on the stack.
Is it ouside a function then it is global (process-memory), if the OS cannot allocate that much memory when it tries to load your program, your program will crash and your program will have no chance to notice that (so the following is better:)
Large arrays should be malloc'ed. With malloc, the OS will return a null-pointer if the malloc failed, depending on the OS and its paging-scheme and memory-mapping-scheme this will either fail when 1) there is no continuous region of free memory large enough for the array or 2) the OS cannot map enough regions of free physical memory to memory that appears to your process as continous memory.
So, with large arrays do this:
char* largeArray = malloc(HUGE_NUMBER);
if(!largeArray) { do error recovery and display msg to user }
Declaring arbitrarily huge arrays to avoid buffer overflows is bad practice. If you really don't know in advance how large a buffer needs to be, use malloc or realloc to dynamically allocate and extend the buffer as necessary, possibly using a smaller, fixed-sized buffer as an intermediary.
Example:
#define PAGE_SIZE 1024 // 1K buffer; you can make this larger or smaller
/**
* Read up to the next newline character from the specified stream.
* Dynamically allocate and extend a buffer as necessary to hold
* the line contents.
*
* The final size of the generated buffer is written to bufferSize.
*
* Returns NULL if the buffer cannot be allocated or if extending it
* fails.
*/
char *getNextLine(FILE *stream, size_t *bufferSize)
{
char input[PAGE_SIZE]; // allocate
int done = 0;
char *targetBuffer = NULL;
*bufferSize = 0;
while (!done)
{
if(fgets(input, sizeof input, stream) != NULL)
{
char *tmp;
char *newline = strchr(input, '\n');
if (newline != NULL)
{
done = 1;
*newline = 0;
}
tmp = realloc(targetBuffer, sizeof *tmp * (*bufferSize + strlen(input)));
if (tmp)
{
targetBuffer = tmp;
*bufferSize += strlen(input);
strcat(targetBuffer, input);
}
else
{
free(targetBuffer);
targetBuffer = NULL;
*bufferSize = 0;
fprintf(stderr, "Unable to allocate or extend input buffer\n");
}
}
}
If the array is going to be allocated on the stack, then you are limited by the stack size (typically 1MB on Windows, some of it will be used so you have even less). Otherwise I imagine the limit would be quite large.
However, making the array really big is not a solution to buffer overflow issues. Don't do it. Use functions that have a mechanism for limiting the amount of buffer they use to make sure you don't overstep your buffer, and make the size something more reasonable (1K for example).
You can use malloc() to get larger portions of memory than normally an array could handle.
Well, a buffer overflow wouldn't be caused by too large a value for HUGE_NUMBER so much as too small compared to what was written to it (write to index HUGE_NUMBER or higher, and you've overflown the buffer).
Aside from that it will depend upon the machine. There are certainly systems that could handle several millions in the heap, and a million or so on the stack (depending on other pressures), but there are also certainly some that couldn't handle more than a few hundred (small embedded devices would be an obvious example). While 65,535 is a standard-specified minimum, a really small device could specify that the standard was deliberately departed from for this reason.
In real terms, on a large machine, long before you actually run out of memory, you are needlessly putting pressure on the memory in a way that would affect performance. You would be better off dynamically sizing an array to an appropriate size.
Related
I'm running into a problem creating an array big enough to store words from a large text document (think books).
Usually, I would just do:
char wordList[1000000][30];
But, as expected the program crashes as soon as it tries to initialize the array. So, I tried a few different things, such as:
char *wordList[30]
int k=0;
while(k<1000000){
wordList[k]= malloc(sizeof(char)*30);
k++;
}
This, too, didn't work. So I'm wondering if there is an easier way. I know its possible. For the first option, my research as lead my to believe the the variable is initialized on the stack (which has small memory) and segfaults.
I'm not sure why the second one fails. Any suggestions? I've searched everywhere I could to find an answer, but most of the suggestions are in java or c++ where you just call new, or arraylist etc.
wordList is an array of 30 char* pointers. You are accessing way beyond the limit of this array. Specifically, you are accessing up to million spaces, but the array only has 30 spaces. This will cause undefined behaviour.
You need to instead make sure wordList has enough space for 1000000 pointers.
This should be instead:
char *wordList[1000000];
This allows flexibility on the length of the words. The only fixed size here is the array size.
If you use:
wordList[k]= malloc(sizeof(char)*30);
Moreover, this will run into issues if the words are more than 29 characters, excluding the \0 character at the end. Although, their are not many words longer than 29 characters. Words as long as:
supercalifragilisticexpialidocious
Are hard to come by.
Furthermore, it depends on how you are reading these words from your text document. If you parse the words, and instead use a temporary buffer to store them, then you can do this:
wordList[k]= malloc(strlen(word)+1); /* +1 for null-terminator */
Which will allocate memory for any sized word you copy into wordList[k]. This will be more efficient for smaller words like "the" and "or", instead of allocating 30 spaces for any word.
Note: Allocating a million pointers on the heap beforehand is also very wasteful, this process should be done on an as needed basis. It might even be better to use char **wordList, to allow more flexibility with how many words you allocate.
For example, you could allocate a starting size:
size_t start_size = 1000;
char **wordList = malloc(start_size * sizeof(*wordlist));
Then if more words are found, you can realloc() more space as needed.
realloc() resizes block of memory it points to, and returns a pointer.
An example would be:
if (start_size == word_count) {
start_size *= 2;
wordList = realloc(wordList, start_size * sizeof(*wordList));
if (wordList == NULL) {
/* handle exit */
Which would return a pointer which holds start_size * 2 spaces.
You should also check the return of malloc() and realloc(), as they can return NULL if unsuccessful. At the end of your program, you should also free() the pointers allocated from malloc().
In practice, a good rule of thumb when programming in C is that "big" data should be allocated in heap memory.
(I am guessing that you are coding for an ordinary laptop or desktop machine running some common operating system; I'm actually thinking of a desktop Linux computer, like the machine I am answering here; but you could adapt my answer to a desktop running some Windows, to a tablet running some Android, to an Apple computer running some MacOSX)
Notice that your current wordList is 30 megabytes (that is sizeof(wordList)==30000000). That is big! But in some cases not big enough (probably not enough for the whole Saint James bible, and certainly not for the entire laws, decrees, and juridictions of US or of France, or for the archive of a century-old newspaper). You can easily find textual data of more than several dozens of megabytes today, such as all the messages on StackOverflow.
You may need to understand more about current operating systems to understand all of my answer; I recommend reading Operating Systems: Three Easy Pieces and, if your computer runs Linux and you want to code for Linux, Advanced Linux Programming.
You don't want to allocate a big array (or any kind of big data) as local variables (or with alloca(3) ...) on your call stack, because the call stack is limited (typically to one megabyte, or a few of them). On some special computers (think of expensive servers running some specially configured Linux) you could raise that (machine call stack) limit, but perhaps not easily, to several dozens of megabytes. Expecting a gigabyte call stack is not reasonable.
You probably don't want to have huge global or static data (those allocated at compile time in your data segment) of fixed size. If you did that, your program might still lack of memory (because you under-estimated that fixed size) or could not even start on smaller computers (e.g. if your data segment had 20Gbytes, your executable might start on my desktop with 32Gbytes, but would fail to start -at execve(2) time- on your laptop with only 16Gbytes).
The remaining option is usual practice: allocate all "big" data in heap memory by indirectly using primitives growing your virtual address space. In standard C, you'll extensively use malloc and friends (e.g. calloc) - with free to release memory. FWIW, the underlying primitives (to grow the virtual address space) on Linux include mmap(2) and related system calls (and may be called by the malloc implementation on your system). But the standard C dynamic memory allocation techniques (that is malloc & free) are hiding these gory (implementation specific) details in the C standard library (so your code using malloc & free could become portable with efforts from your part).
Read some coding rules, e.g. the GNU ones, related to memory usage, and robust programs, notably:
Avoid arbitrary limits on the length or number of any data structure, including file names, lines, files, and symbols, by allocating all data structures dynamically
(emphasis is mine)
Practically speaking, your wordList (that is a poor name, it is not a list but a vector or a table) should probably be a dynamically allocated array of pointers to dynamically allocated strings. You could declare it as char**wordList; and you want to keep its allocated size and used length (perhaps in two other global variables, size_t allocatedSize, usedLength; ...). You might prefer to use a struct ending with a flexible array member
Don't forget to check against failure of malloc. Perhaps you want to initialize your data with something like:
allocatedSize=1000;
usedLength=0;
wordList= calloc(allocatedSize, sizeof(char*));
if (!wordList) { perror("initial calloc wordlist"); exit(EXIT_FAILURE); };
Here is a routine to add a new word to your wordList; that routine does not check if the word is indeed new (maybe you want to use some other data structure, like some hash-table, or some self balancing binary search tree); if you want to keep only unique words, read some Introduction to Algorithms. Otherwise, you could use:
void add_new_word(const char*w) {
if (usedLength >= allocatedSize) {
size_t newsize = 4*usedLength/3+10;
(heuristically, we don't want to re-allocate wordList too often; hence the "geometrical" growth above)
char**newlist = calloc(newsize*sizeof(char*));
if (!newlist) { perror("calloc newlist"); exit(FAILURE); };
memcpy (newlist, wordList, usedLength*sizeof(char*));
free (wordList);
wordList = newlist;
allocatedSize = newsize;
};
// here we are sure that wordList is not full,
// so usedLength < allocatedSize
char *dw = strdup(w);
if (!dw) { perror("strdup failure"); exit(EXIT_FAILURE); };
we are using the very common strdup(3) function to copy some string into a heap allocated one. If your system don't have that, it is really easy to write, using strlen, malloc, strcpy ...
wordList[usedLength++] = dw;
} // end of add_new_word
After a call to add_new_word you know that a new word has been added at index usedLength-1 of wordList.
Notice that my add_new_word is checking against failure of malloc (including the one called from strdup) and satisfy the "robustness" criteria: all data is heap allocated!
BTW, some computers are (IMHO wrongly) enabling memory overcommitment. This is a system administration issue. I dislike that feature (because when it is enabled, malloc would never fail, but programs would crash badly when memory resources are exhausted).
FWIW, here is the routine to release memory, to be called near end of program (or registered thru atexit(3))
void destroy_word_list(void) {
if (!wordList) return;
for (size_t ix=0; ix<usedLength; ix++) free(wordList[ix]);
free (wordList);
usedLength = 0;
allocatedSize = 0;
wordList = NULL;
} // end of destroy_word_list
You may want to use valgrind e.g. to debug memory leaks.
Is there a max size for a char buffer? I have a program that is collecting strings for a char buffer and writing it to a proc file. After a certain point it appears to stop writing things - is there too much in there? What is that max size so I can work around this?
Here is code. This is an LKM - is limits.h available from kernel space?
Foremost:
const char* input = "hooloo\n";
Next:
int read_info( char *page, char **start, off_t off, int count, int *eof, void *data )
{
unsigned int mem;
char answer_buf[strlen(input) + 1 + 14];
name_added = vmalloc(strlen(input) + 1 + 14);
strcpy(name_added, input);
strcat(name_added, extension);
mem = sprintf(answer_buf, "%s\n", name_added);
memcpy(page, answer_buf, mem);
return strlen(answer_buf) + 1;
}
All in my code are things like this are things that remalloc the buffer and add to it. Also, that read_info is for the procfile. This issue is I keep adding to that buffer with the code above over and over and over - eventually I ca my procfile and the text cuts off - it doesn't go on forever like i want )-=.
There's no concrete maximum size "in C" specifically. Theoretical (or "potential") maximum size of any object on a C platform is determined by the implementation and is usually derived from the properties of the underlying machine platform and OS.
On platforms with flat memory model it will typically be limited by the size of the address space in theory, and by the size of the available free memory (or that specific kind) in practice.
On platforms with segmented memory model it might be limited by the segment size, which is smaller than the address space size. Although implementations are free to breach that limit by "emulating" flat memory model in the code. For that reason on such platforms the maximum object size can also depend on compilation settings.
The only maximum size for a dynamically allocated char buffer will be available system memory.
A buffer on the stack will have its size constrained by maximum stack size. This will vary greatly depending on host OS.
When writing data to file, are you checking the size returned by fwrite and calling it repeatedly to write the remainder of the buffer if necessary?
You have a memory leak in your code!
The following memory is never freed:
name_added = vmalloc(strlen(input) + 1 + 14);
I don't understand why you allocate memory for the output at all.
And you do it twice, both on the stack and on the heap.
The caller has provided a buffer for the output. Use it!
Don't create copies!
I'd say it's at least able to handle 1,000 unique characters
Surprisingly simple/stupid/basic question, but I have no idea: Suppose I want to return the user of my function a C-string, whose length I do not know at the beginning of the function. I can place only an upper bound on the length at the outset, and, depending on processing, the size may shrink.
The question is, is there anything wrong with allocating enough heap space (the upper bound) and then terminating the string well short of that during processing? i.e. If I stick a '\0' into the middle of the allocated memory, does (a.) free() still work properly, and (b.) does the space after the '\0' become inconsequential? Once '\0' is added, does the memory just get returned, or is it sitting there hogging space until free() is called? Is it generally bad programming style to leave this hanging space there, in order to save some upfront programming time computing the necessary space before calling malloc?
To give this some context, let's say I want to remove consecutive duplicates, like this:
input "Hello oOOOo !!" --> output "Helo oOo !"
... and some code below showing how I'm pre-computing the size resulting from my operation, effectively performing processing twice to get the heap size right.
char* RemoveChains(const char* str)
{
if (str == NULL) {
return NULL;
}
if (strlen(str) == 0) {
char* outstr = (char*)malloc(1);
*outstr = '\0';
return outstr;
}
const char* original = str; // for reuse
char prev = *str++; // [prev][str][str+1]...
unsigned int outlen = 1; // first char auto-counted
// Determine length necessary by mimicking processing
while (*str) {
if (*str != prev) { // new char encountered
++outlen;
prev = *str; // restart chain
}
++str; // step pointer along input
}
// Declare new string to be perfect size
char* outstr = (char*)malloc(outlen + 1);
outstr[outlen] = '\0';
outstr[0] = original[0];
outlen = 1;
// Construct output
prev = *original++;
while (*original) {
if (*original != prev) {
outstr[outlen++] = *original;
prev = *original;
}
++original;
}
return outstr;
}
If I stick a '\0' into the middle of the allocated memory, does
(a.) free() still work properly, and
Yes.
(b.) does the space after the '\0' become inconsequential? Once '\0' is added, does the memory just get returned, or is it sitting there hogging space until free() is called?
Depends. Often, when you allocate large amounts of heap space, the system first allocates virtual address space - as you write to the pages some actual physical memory is assigned to back it (and that may later get swapped out to disk when your OS has virtual memory support). Famously, this distinction between wasteful allocation of virtual address space and actual physical/swap memory allows sparse arrays to be reasonably memory efficient on such OSs.
Now, the granularity of this virtual addressing and paging is in memory page sizes - that might be 4k, 8k, 16k...? Most OSs have a function you can call to find out the page size. So, if you're doing a lot of small allocations then rounding up to page sizes is wasteful, and if you have a limited address space relative to the amount of memory you really need to use then depending on virtual addressing in the way described above won't scale (for example, 4GB RAM with 32-bit addressing). On the other hand, if you have a 64-bit process running with say 32GB of RAM, and are doing relatively few such string allocations, you have an enormous amount of virtual address space to play with and the rounding up to page size won't amount to much.
But - note the difference between writing throughout the buffer then terminating it at some earlier point (in which case the once-written-to memory will have backing memory and could end up in swap) versus having a big buffer in which you only ever write to the first bit then terminate (in which case backing memory is only allocated for the used space rounded up to page size).
It's also worth pointing out that on many operating systems heap memory may not be returned to the Operating System until the process terminates: instead, the malloc/free library notifies the OS when it needs to grow the heap (e.g. using sbrk() on UNIX or VirtualAlloc() on Windows). In that sense, free() memory is free for your process to re-use, but not free for other processes to use. Some Operating Systems do optimise this - for example, using a distinct and independently releasble memory region for very large allocations.
Is it generally bad programming style to leave this hanging space there, in order to save some upfront programming time computing the necessary space before calling malloc?
Again, it depends on how many such allocations you're dealing with. If there are a great many relative to your virtual address space / RAM - you want to explicitly let the memory library know not all the originally requested memory is actually needed using realloc(), or you could even use strdup() to allocate a new block more tightly based on actual needs (then free() the original) - depending on your malloc/free library implementation that might work out better or worse, but very few applications would be significantly affected by any difference.
Sometimes your code may be in a library where you can't guess how many string instances the calling application will be managing - in such cases it's better to provide slower behaviour that never gets too bad... so lean towards shrinking the memory blocks to fit the string data (a set number of additional operations so doesn't affect big-O efficiency) rather than having an unknown proportion of the original string buffer wasted (in a pathological case - zero or one character used after arbitrarily large allocations). As a performance optimisation you might only bother returning memory if unusued space is >= the used space - tune to taste, or make it caller-configurable.
You comment on another answer:
So it comes down to judging whether the realloc will take longer, or the preprocessing size determination?
If performance is your top priority, then yes - you'd want to profile. If you're not CPU bound, then as a general rule take the "preprocessing" hit and do a right-sized allocation - there's just less fragmentation and mess. Countering that, if you have to write a special preprocessing mode for some function - that's an extra "surface" for errors and code to maintain. (This trade-off decision is commonly needed when implementing your own asprintf() from snprintf(), but there at least you can trust snprintf() to act as documented and don't personally have to maintain it).
Once '\0' is added, does the memory just get returned, or is it
sitting there hogging space until free() is called?
There's nothing magical about \0. You have to call realloc if you want to "shrink" the allocated memory. Otherwise the memory will just sit there until you call free.
If I stick a '\0' into the middle of the allocated memory, does (a.)
free() still work properly
Whatever you do in that memory free will always work properly if you pass it the exact same pointer returned by malloc. Of course if you write outside it all bets are off.
\0 is just one more character from malloc and free perspective, they don't care what data you put in the memory. So free will still work whether you add \0 in the middle or don't add \0 at all. The extra space allocated will still be there, it won't be returned back to the process as soon as you add \0 to the memory. I personally would prefer to allocate only the required amount of memory instead of allocating at some upper bound as that will just wasting the resource.
As soon as you get memory from heap by calling malloc(), the memory is yours to use. Inserting \0 is like inserting any other character. This memory will remain in your possession until you free it or until OS claims it back.
The \0is a pure convention to interpret character arrays as stings - it is independent of the memory management. I.e., if you want to get your money back, you should call realloc. The string does not care about memory (what is a source of many security problems).
malloc just allocates a chunk of memory .. Its upto you to use however you want and call free from the initial pointer position... Inserting '\0' in the middle has no consequence...
To be specific malloc doesnt know what type of memory you want (It returns onle a void pointer) ..
Let us assume you wish to allocate 10 bytes of memory starting 0x10 to 0x19 ..
char * ptr = (char *)malloc(sizeof(char) * 10);
Inserting a null at 5th position (0x14) does not free the memory 0x15 onwards...
However a free from 0x10 frees the entire chunk of 10 bytes..
free() will still work with a NUL byte in memory
the space will remain wasted until free() is called, or unless you subsequently shrink the allocation
Generally, memory is memory is memory. It doesn't care what you write into it. BUT it has a race, or if you prefer a flavor (malloc, new, VirtualAlloc, HeapAlloc, etc). This means that the party that allocates a piece of memory must also provide the means to deallocate it. If your API comes in a DLL, then it should provide a free function of some sort.
This of course puts a burden on the caller right?
So why not put the WHOLE burden on the caller?
The BEST way to deal with dynamically allocated memory is to NOT allocate it yourself. Have the caller allocate it and pass it on to you. He knows what flavor he allocated, and he is responsible to free it whenever he is done using it.
How does the caller know how much to allocate?
Like many Windows APIs have your function return the required amount of bytes when called e.g. with a NULL pointer, then do the job when provided with a non-NULL pointer (using IsBadWritePtr if it is suitable for your case to double-check accessibility).
This can also be much much more efficient. Memory allocations COST a lot. Too many memory allocations cause heap fragmentation and then the allocations cost even more. That's why in kernel mode we use the so called "look-aside lists". To minimize the number of memory allocations done, we reuse the blocks we have already allocated and "freed", using services that the NT Kernel provides to driver writers.
If you pass on the responsibility for memory allocation to your caller, then he might be passing you cheap memory from the stack (_alloca), or passing you the same memory over and over again without any additional allocations. You don't care of course, but you DO allow your caller to be in charge of optimal memory handling.
To elaborate on the use of the NULL terminator in C:
You cannot allocate a "C string" you can allocate a char array and store a string in it, but malloc and free just see it as an array of the requested length.
A C string is not a data type but a convention for using a char array where the null character '\0' is treated as the string terminator.
This is a way to pass strings around without having to pass a length value as a separate argument. Some other programming languages have explicit string types that store a length along with the character data to allow passing strings in a single parameter.
Functions that document their arguments as "C strings" are passed char arrays but have no way of knowing how big the array is without the null terminator so if it is not there things will go horribly wrong.
You will notice functions that expect char arrays that are not necessarily treated as strings will always require a buffer length parameter to be passed.
For example if you want to process char data where a zero byte is a valid value you can't use '\0' as a terminator character.
You could do what some of the MS Windows APIs do where you (the caller) pass a pointer and the size of the memory you allocated. If the size isn't enough, you're told how many bytes to allocate. If it was enough, the memory is used and the result is the number of bytes used.
Thus the decision about how to efficiently use memory is left to the caller. They can allocate a fixed 255 bytes (common when working with paths in Windows) and use the result from the function call to know whether more bytes are needed (not the case with paths due to MAX_PATH being 255 without bypassing Win32 API) or whether most of the bytes can be ignored...
The caller could also pass zero as the memory size and be told exactly how much needs to be allocated - not as efficient processing-wise, but could be more efficient space-wise.
You can certainly preallocate to an upperbound, and use all or something less.
Just make sure you actually use all or something less.
Making two passes is also fine.
You asked the right questions about the tradeoffs.
How do you decide?
Use two passes, initially, because:
1. you'll know you aren't wasting memory.
2. you're going to profile to find out where
you need to optimize for speed anyway.
3. upperbounds are hard to get right before
you've written and tested and modified and
used and updated the code in response to new
requirements for a while.
4. simplest thing that could possibly work.
You might tighten up the code a little, too.
Shorter is usually better. And the more the
code takes advantage of known truths, the more
comfortable I am that it does what it says.
char* copyWithoutDuplicateChains(const char* str)
{
if (str == NULL) return NULL;
const char* s = str;
char prev = *s; // [prev][s+1]...
unsigned int outlen = 1; // first character counted
// Determine length necessary by mimicking processing
while (*s)
{ while (*++s == prev); // skip duplicates
++outlen; // new character encountered
prev = *s; // restart chain
}
// Construct output
char* outstr = (char*)malloc(outlen);
s = str;
*outstr++ = *s; // first character copied
while (*s)
{ while (*++s == prev); // skip duplicates
*outstr++ = *s; // copy new character
}
// done
return outstr;
}
I'm new to c. Just have a question about the character arrays (or string) in c: When I want to create a character array in C, do I have to give the size at the same time?
Because we may not know the size that we actually need. For example of client-server program, if we want to declare a character array for the server program to receive a message from the client program, but we don't know the size of the message, we could do it like this:
char buffer[1000];
recv(fd,buffer, 1000, 0);
But what if the actual message is only of length 10. Will that cause a lot of wasted memory?
Yes, you have to decide the dimension in advance, even if you use malloc.
When you read from sockets, as in the example, you usually use a buffer with a reasonable size, and dispatch data in other structure as soon you consume it. In any case, 1000 bytes is not a so much memory waste and is for sure faster than asking a byte at a time from some memory manager :)
Yes, you have to give the size if you are not initializing the char array at the time of declaration. Better approach for your problem is to identify the optimum size of the buffer at run time and dynamically allocate the memory.
What you're asking about is how to dynamically size a buffer. This is done with a dynamic allocation such as using malloc() -- a memory allocator. Using it gives you an important responsibility though: when you're done using the buffer you must return it to the system yourself. If using malloc() [or calloc()], you return it with free().
For example:
char *buffer; // pointer to a buffer -- essentially an unsized array
buffer = (char *)malloc(size);
// use the buffer ...
free(buffer); // return the buffer -- do NOT use it any more!
The only problem left to solve is how to determine the size you'll need. If you're recv()'ing data that hints at the size, you'll need to break the communication into two recv() calls: first getting the minimum size all packets will have, then allocating the full buffer, then recv'ing the rest.
When you don't know the exact amount of input data, do as follows:
Create a small buffer
Allocate some memory for a "storage" (e.g. twice of buffer size)
Fill the buffer with the data from the input stream (e.g. socket, file etc.)
Copy the data from the buffer to the storage
4.1 If there is not enough place in storage, re-allocate the memory (e.g. with a size twice bigger than it is at this point)
Do steps 3 and 4 unless the "END OF STREAM"
Your storage contains the data now.
If you don't know the size a-priori, then you have no choice but to create it dynamically using malloc (or whatever equivalent mechanism in your language of choice.)
size_t buffer_size = ...; /* read from a DEFINE or from a config file */
char * buffer = malloc( sizeof( char ) * (buffer_size + 1) );
Creating a buffer of size m, but only receiving an input string of size n with n < m is not a waste of memory, but an engineering compromise.
If you create your buffer with a size close to the intended input, you risk having to refill the buffer many, many times for those cases where m >> n. Typically, iterations over the buffer are tied up with I/O operations, so now you might be saving some bytes (which is really nothing in today's hardware) at the expense of potentially increasing the problems in some other end. Specially for client-server apps. If we were talking about resource-constrained embedded systems, that'd be another thing.
You should be worrying about getting your algorithms right and solid. Then you worry, if you can, about shaving off a few bytes here and there.
For me, I'd rather create a buffer that is 2 to 10 times greater than the average input (not the smallest input as in your case, but the average), assuming my input tends to have a slow standard deviation in size. Otherwise, I'd go 20 times the size or more (specially if memory is cheap and doing this minimizes hitting the disk or the NIC card.)
At the most basic setup, one typically gets the size of the buffer as a configuration item read off a file (or passed as an argument), and defaulting to a default compile time value if none is provided. Then you can adjust the size of your buffers according to the observed input sizes.
More elaborate algorithms (say TCP) adjust the size of their buffers at run-time to better accommodate input whose size might/will change over time.
Even if you use malloc you also must define the size first! So instead you give a large number that is capable of accepting the message like:
int buffer[2000];
In case of small message or large you can reallocate it to release the unused locations or to occupy the unused locations
example:
int main()
{
char *str;
/* Initial memory allocation */
str = (char *) malloc(15);
strcpy(str, "tutorialspoint");
printf("String = %s, Address = %u\n", str, str);
/* Reallocating memory */
str = (char *) realloc(str, 25);
strcat(str, ".com");
printf("String = %s, Address = %u\n", str, str);
free(str);
return(0);
}
Note: make sure to include stdlib.h library
For a small to-be-embedded application, I wrote a few functions + struct that work as String Buffer (similar to std::stringstream in C++).
While the code as such works fine, There are a few not-so-minor problems:
I never before wrote functions in C that manually allocate and use growing memory, thus I'm afraid there are still some quirks that yet need to be adressed
It seems the code allocates far more memory than it actually needs, which is VERY BAD
Due to warnings reported by valgrind I have switched from malloc to calloc in one place in the code, which sucessfully removed the warning, but I'm not entirely sure if i'm actually using it correctly
Example of what I mean that it allocates more than it really needs (using a 56k file):
==23668== HEAP SUMMARY:
==23668== in use at exit: 0 bytes in 0 blocks
==23668== total heap usage: 49,998 allocs, 49,998 frees, 1,249,875,362 bytes allocated
... It just doesn't look right ...
The code in question is here (too large to copy it in a <code> field on SO): http://codepad.org/LQzphUzd
Help is needed, and I'm grateful for any advice!
The way you are growing your buffer is rather inefficient. For each little piece of string, you realloc() memory, which can mean new memory is allocated and the contents of the "old" memory are copied over. That is slow and fragments your heap.
Better is to grow in fixed amounts, or in fixed percentages, i.e. make the new size 1.5 or 2 times the size of the old size. That also wastes some memory, but will keep the heap more usable and not so many copies are made.
This means you'll have to keep track of two values: capacity (number of bytes allocated) and length (actual length of the string). But that should not be too hard.
I would introduce a function "FstrBuf_Grow" which takes care of all of that. You just call it with the amount of memory you want to add, and FstrBuf_Grow will take care that the capacity matches the requirements by reallocing when necessary and at least as much as necessary.
...
void FstrBuf_Grow(FstringBuf *buf, size_t more)
{
while (buf->length + more) > buf->capacity
buf->capacity = 3 * buf->capacity / 2;
buf->data = realloc(buf->data, buf->capacity + 1);
}
That multiplies capacity by 1.5 until data is large enough. You can choose different strategies, depending on your needs.
The strncat(ptr->data, str, len);, move before the ptr->length = ((ptr->length) + len); and use strncpy(ptr->data+ptr->length.... And the ptr = NULL; in the Destroy is useless.
The code of the "library" seems to be correct BUT be aware that you are continously reallocating the buffer. Normally you should try to grow the buffer only rarely (for example every time you need to grow the buffer you use max(2* the current size, 4) as the new size) because growing the buffer is O(n). The big memory allocation is probably because the first time you allocate a small buffer. Then you realloc it in a bigger buffer. Then you need to realloc it in a buffer even bigger and so the heap grows.
It looks like you're re-allocating the buffer on every append. Shouldn't you grow it only when you want to append more than it can hold?
When reallocating you want to increase the size of the buffer using a strategy that gives you the best trade off between the number of allocations and the amount of memory allocated. Just doubling the size of the buffer every time you hit the limit might not be ideal for an embedded program.
Generally for embedded applications it is much better to allocate a circular FIFO buffer 1-3 times the maximum message size.