I have an array inside a structure as below
struct st
{
....
int index[1];
....
}
How can I increase size of the array present inside structure to say 6, when I want to use the array inside a function.
Probably you looking for struct hack. Struct hack is a technique which allows you allocate additional memory for an array inside struct. Here is an example
struct str {
int value;
char ar[0];
};
int main()
{
struct str *s = malloc( sizeof(struct str) + 20 );
strncpy( s->ar,"abcd", 5);
printf("%s",s->ar);
return 0;
}
As array defined at the end of the struct, s->ar will get those additional 20 bytes added to sizeof(struct str) in malloc.
Edit as Daan Timmer noted, this technique can be applied only to last member of a struct.
You may try this:
struct st { ....
int index[6];
....
}
Also you may check function malloc() and realloc() in C
On a side note:
You may check STL container like std::vector which encapsulate the associated memory management.
An array defined that way exists on the stack. In order to dynamically change the size you will need to use allocate on the heap using malloc realloc and free.
Related
I'm in a class right now that works with C and one of my assignments requires that I work with a struct that my professor wrote for us. It's actually two structs, with one struct basically containing an array of the first struct.
Here's what they look like:
typedef struct cityStruct
{
unsigned int zip;
char *town
} city;
typedef struct zipTownsStruct
{
int *towns;
city **zips;
city *cities
} zipTowns;
And here's my function for allocating memory for the zipTowns structure:
void getArrs(zipTowns *arrs, int size)
{
arrs->towns = malloc(sizeof(int) * size);
arrs->zips = malloc(sizeof(city **) * size);
arrs->cities = malloc(sizeof(city *) * size);
}
From what I understand, what I'm doing here is allocating space in memory for a certain number of ints, city pointers, and city structures, based on the size variable. I understand that this is basically what an array is.
I'm having trouble with understanding how I can access these arrays and manipulate items in it. Writing this gives me an error:
strcpy(arrs.cities[0]->town, "testTown\0");
You can see what I'm trying to do here. I want to access each "City" in the zipTowns struct by index and insert a value.
How can I access the items in these dynamically allocated array of structures?
Think of x->y as (*x).y.
arrs is not a structure, it's a pointer to a structure, and cities is not a pointer to a pointer to a structure, it's just a pointer to a structure.
Use arrs->cities[0].town instead of arrs.cities[0]->town.
However, you're still not allocating enough room for these structures. This should make it clearer what you're doing with the allocations, and should also give you enough room for your data:
arrs->towns = malloc(sizeof(*arrs->towns) * size);
arrs->zips = malloc(sizeof(*arrs->zips) * size);
arrs->cities = malloc(sizeof(*arrs->cities) * size);
With the second and third, you were only allocating enough room for a pointer to be stored instead of the actual data type.
With this approach, you will be able to access from arrs->cities[0] to arrs->cities[9] and you also will be able to access the members of each city by doing arrs->cities[<number>].<member>.
You also do not need to intentionally null-terminate your strings. This is already done for you. Therefore, you can replace "testTown\0" with "testTown".
I am trying to initialise a struct by allocating memory to it and it's pointer members using malloc:
typedef struct {
char *name;
prob_t *prob;
} name_t;
I understand that I need to allocate memory to the pointers separately once the struct is initialised:
name_t
*init_name_dict() {
name_t *name_dict;
name_dict = (name_t*)malloc(MAX_LINES*sizeof(*name_dict));
name_dict->name = (char*)malloc(MAX_LEN*sizeof(*name_dict->name));
name_dict->prob = (prob_t*)malloc(MAX_PROB*sizeof(*name_dict->prob));
return name_dict;
}
But when I do so, it allocates memory to the struct, but not to either of its member pointers (they just point to junk).
What am I doing wrong? Thanks
As explained here, malloc doesn't "clean" the memory, that then can be full of garbage (because for example the same memory was returned by another call to malloc(), used and then free()). The three classical solutions are:
Live with it. Set manually all the members of the struct (if you are
using the malloc to allocate a struct) before using the struct (or in general set the all the obtained memory to the value you want)
Use memset to zero all the memory before using it
Use calloc instead of malloc (note that it has a slightly different signature). calloc is similar to malloc + memset. As an example:
name_t *init_name_dict() {
name_t *name_dict;
name_dict = calloc(MAX_LINES, sizeof(*name_dict));
name_dict->name = calloc(MAX_LEN, sizeof(*name_dict->name));
name_dict->prob = calloc(MAX_PROB, sizeof(*name_dict->prob));
return name_dict;
}
As a sidenote, in C you don't need/shouldn't cast a pointer returned by malloc/calloc (but if in truth you are using a C++ compiler then you have to cast it...).
If you want cleared memory (as opposed to memory with junk in it), you need calloc
instead of malloc, but that's trivial.
You're bigger problems are:
1) no error checking
2) possibly needless malloc calls
3) you're allocating MAX_LINES of theses name_t structure but initializing
only one of them
If the .name and .prob fields won't be reallocated, you should change your name_t
definition to
typedef struct { char name[MAX_LEN]; prob_t prob[MAX_PROB]; } name_t;
and allocate all MAX_LINES name_t's in one go: calloc(MAX_LINES, sizeof(name_t)).
If you need the original name_t structure, then I'd have an initializer for one:
int init_name_dict (name_t *this)
{
if(0==(this->name=calloc(MAX_LEN, sizeof *this->name))) return -1;
if(0==(this->prob=calloc(MAX_PROB, sizeof *this->prob))){ free(this->name); return -1; }
return 0;
}
a destructor for it
void destroy_name_dict(name_t *this) { free(this->name); free(this->prob); }
and then an initializing allocator for the whole array:
name_t* new_name_dicts(void)
{
name_t *r = malloc(MAX_LINES*sizeof *r);
if(!r) return r;
int i;
for(i=0; i<MAX_LINES; i++)
if(0>init_name_dict(&r[i])) goto fail;
return r;
fail:
for(--i; i>=0; --i)
destructor_name_dict(&r[i]);
return NULL;
}
(Basically what would amount to a C++ vector constructor that picks up the constructor for the
cell type.)
Struct
typedef struct {
char *name;
prob_t *prob;
} name_t;
has two pointers as members. So on 32 bit OS, sizeof(name_t) is 8 bytes.
Dynamic creation of instance of name_t struct
name_t *name_dict = (name_t*)malloc(sizeof(name_dict));
allocates only 8 bytes to store two pointers. As xanatos said allocated memory is garbage and pointers will point random locations. You can use calloc() when allocation name_dict or manually nullify them name_dict->name = NULL;name_dict->prob = NULL;. You can also don't bother yourself by content of pointers and in next code line allocate memory to members
name_dict->name = (char*)malloc(MAX_LEN*sizeof(char));
name_dict->prob = (prob_t*)malloc(sizeof(prob_t));
You can also check if memory was allocated good and both pointers don't ppoint to NULL.
To sum up, properly written init_name_dict() method
name_t * init_name_dict()
{
name_t *name_dict = (name_t*)malloc(sizeof(name_t));
if (name_dict != NULL)
{
name_dict->name = (char*)malloc(MAX_LEN*sizeof(char)));
name_dict->prob = (prob_t*)malloc(sizeof(prob_t));
}
return name_dict;
}
Errors in your code was
MAX_LINES here(assume that you want to create only one structure here)
name_dict = (name_t*)malloc(MAX_LINES*sizeof(*name_dict));
MAX_PROB here (assume that you want to create only one structure
here)
name_dict->prob = (prob_t*)malloc(MAX_PROB*sizeof(*name_dict->prob));
I have looked around but have been unable to find a solution to what must be a well asked question.
Here is the code I have:
#include <stdlib.h>
struct my_struct {
int n;
char s[]
};
int main()
{
struct my_struct ms;
ms.s = malloc(sizeof(char*)*50);
}
and here is the error gcc gives me:
error: invalid use of flexible array member
I can get it to compile if i declare the declaration of s inside the struct to be
char* s
and this is probably a superior implementation (pointer arithmetic is faster than arrays, yes?)
but I thought in c a declaration of
char s[]
is the same as
char* s
The way you have it written now , used to be called the "struct hack", until C99 blessed it as a "flexible array member". The reason you're getting an error (probably anyway) is that it needs to be followed by a semicolon:
#include <stdlib.h>
struct my_struct {
int n;
char s[];
};
When you allocate space for this, you want to allocate the size of the struct plus the amount of space you want for the array:
struct my_struct *s = malloc(sizeof(struct my_struct) + 50);
In this case, the flexible array member is an array of char, and sizeof(char)==1, so you don't need to multiply by its size, but just like any other malloc you'd need to if it was an array of some other type:
struct dyn_array {
int size;
int data[];
};
struct dyn_array* my_array = malloc(sizeof(struct dyn_array) + 100 * sizeof(int));
Edit: This gives a different result from changing the member to a pointer. In that case, you (normally) need two separate allocations, one for the struct itself, and one for the "extra" data to be pointed to by the pointer. Using a flexible array member you can allocate all the data in a single block.
You need to decide what it is you are trying to do first.
If you want to have a struct with a pointer to an [independent] array inside, you have to declare it as
struct my_struct {
int n;
char *s;
};
In this case you can create the actual struct object in any way you please (like an automatic variable, for example)
struct my_struct ms;
and then allocate the memory for the array independently
ms.s = malloc(50 * sizeof *ms.s);
In fact, there's no general need to allocate the array memory dynamically
struct my_struct ms;
char s[50];
ms.s = s;
It all depends on what kind of lifetime you need from these objects. If your struct is automatic, then in most cases the array would also be automatic. If the struct object owns the array memory, there's simply no point in doing otherwise. If the struct itself is dynamic, then the array should also normally be dynamic.
Note that in this case you have two independent memory blocks: the struct and the array.
A completely different approach would be to use the "struct hack" idiom. In this case the array becomes an integral part of the struct. Both reside in a single block of memory. In C99 the struct would be declared as
struct my_struct {
int n;
char s[];
};
and to create an object you'd have to allocate the whole thing dynamically
struct my_struct *ms = malloc(sizeof *ms + 50 * sizeof *ms->s);
The size of memory block in this case is calculated to accommodate the struct members and the trailing array of run-time size.
Note that in this case you have no option to create such struct objects as static or automatic objects. Structs with flexible array members at the end can only be allocated dynamically in C.
Your assumption about pointer aritmetics being faster then arrays is absolutely incorrect. Arrays work through pointer arithmetics by definition, so they are basically the same. Moreover, a genuine array (not decayed to a pointer) is generally a bit faster than a pointer object. Pointer value has to be read from memory, while the array's location in memory is "known" (or "calculated") from the array object itself.
The use of an array of unspecified size is only allowed at the end of a structure, and only works in some compilers. It is a non-standard compiler extension. (Although I think I remember C++0x will be allowing this.)
The array will not be a separate allocation for from the structure though. So you need to allocate all of my_struct, not just the array part.
What I do is simply give the array a small but non-zero size. Usually 4 for character arrays and 2 for wchar_t arrays to preserve 32 bit alignment.
Then you can take the declared size of the array into account, when you do the allocating. I often don't on the theory that the slop is smaller than the granularity that the heap manager works in in any case.
Also, I think you should not be using sizeof(char*) in your allocation.
This is what I would do.
struct my_struct {
int nAllocated;
char s[4]; // waste 32 bits to guarantee alignment and room for a null-terminator
};
int main()
{
struct my_struct * pms;
int cb = sizeof(*pms) + sizeof(pms->s[0])*50;
pms = (struct my_struct*) malloc(cb);
pms->nAllocated = (cb - sizoef(*pms) + sizeof(pms->s)) / sizeof(pms->s[0]);
}
I suspect the compiler doesn't know how much space it will need to allocate for s[], should you choose to declare an automatic variable with it.
I concur with what Ben said, declare your struct
struct my_struct {
int n;
char s[1];
};
Also, to clarify his comment about storage, declaring char *s won't put the struct on the stack (since it is dynamically allocated) and allocate s in the heap, what it will do is interpret the first sizeof(char *) bytes of your array as a pointer, so you won't be operating on the data you think you are, and probably will be fatal.
It is vital to remember that although the operations on pointers and arrays may be implemented the same way, they are not the same thing.
Arrays will resolve to pointers, and here you must define s as char *s. The struct basically is a container, and must (IIRC) be fixed size, so having a dynamically sized array inside of it simply isn't possible. Since you're mallocing the memory anyway, this shouldn't make any difference in what you're after.
Basically you're saying, s will indicate a memory location. Note that you can still access this later using notation like s[0].
pointer arithmetic is faster than arrays, yes?
Not at all - they're actually the same. arrays translate to pointer arithmetics at compile-time.
char test[100];
test[40] = 12;
// translates to: (test now indicates the starting address of the array)
*(test+40) = 12;
Working code of storing array inside a structure in a c, and how to store value in the array elements Please leave comment if you have any doubts, i will clarify at my best
Structure Define:
struct process{
int process_id;
int tau;
double alpha;
int* process_time;
};
Memory Allocation for process structure:
struct process* process_mem_aloc = (struct process*) malloc(temp_number_of_process * sizeof(struct process));
Looping through multiple process and for each process updating process_time dyanamic array
int process_count = 0;
int tick_count = 0;
while(process_count < number_of_process){
//Memory allocation for each array of the process, will be containting size equal to number_of_ticks: can hold any value
(process_mem_aloc + process_count)->process_time = (int*) malloc(number_of_ticks* sizeof(int));
reading data from line by line from a file, storing into process_time array and then printing it from the stored value, next while loop is inside the process while loop
while(tick_count < number_of_ticks){
fgets(line, LINE_LENGTH, file);
*((process_mem_aloc + process_count)->process_time + tick_count) = convertToInteger(line);;
printf("tick_count : %d , number_of_ticks %d\n",tick_count,*((process_mem_aloc + process_count)->process_time + tick_count));
tick_count++;
}
tick_count = 0;
the code generated will be identical (array and ptr). Apart from the fact that the array one wont compile that is
and BTW - do it c++ and use vector
I have this struct:
typedef struct SomeStruct {
char someString[];
} SomeStruct;
This produces an error since someString's size is not defined when initialized.
I want to make someString an array of strings, but I will not know the size of the array at the time of initialization. (The elements that will be in the array will depend on user input later in the program).
Is it possible to initialize this as an array of strings without knowing the size of the array?
Yes, the C standard talks about this in 7.2.18-26. What you are describing is known as a flexible array member of a struct. From the standard:
As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member.
Essentially what it is saying is, if the last member of the struct is an array of undefined size (as might be the case for runtime sizes), then when using the struct, you would allocate the appropriate size of your struct including how large you want the string to be. For example:
typedef struct SomeStruct {
char someString[];
} SomeStruct;
has the flexible array member someString. A common way to use this is:
SomeStruct *p = malloc(sizeof (SomeStruct) + str_size);
Assuming that the call to malloc succeeds, the object pointed to by p behaves, for most purposes, as if p had been declared as:
struct {char someString[str_size]; } *p;
Read the standard for more detail. The buzzword flexible array member will show up a lot of information too. The wikipedia is a good place to start.
You can use a structure with flexible array. For example
typedef struct SomeStruct
{
size_t n;
char someString[];
} SomeStruct;
where n is used to store the number of elements in the array.
Then you can create objects of the structure the following way
SomeStruct *s = malloc( sizeof( SomeStruct ) + 10 * sizeof( char[100] ) );
s->n = 10;
If you can't use a dynamic array (it sounds like this, if you get a compile error for it), you can actually overrun the array, as long as it's at the end of the struct, and as long as you can actually access that memory. Example:
#include <stdio.h>
#include <stdlib.h>
typedef struct SomeStruct {
char someString[10];
} SomeStruct;
int main (void)
{
// Allocate 4x space, so we have room to overrun
SomeStruct *p = malloc(sizeof(SomeStruct) * 4);
p->someString[38] = 'a';
printf("%c\n", p->someString[38]);
}
Of course, you still have to actually allocate the space, so it may not be so useful to you depending on your case.
I currently have no code, because I don't know how to do this at all. Could I just, by myself, calculate how many bytes is needed for each lower-level struct and malloc it to it? That's really terrible coding, isn't it. Here's the two structs I'm trying to mash together:
struct property {
int d;
char name [111]; // I just malloc this like I would a normal array, right?
char descr [1025]; // Ditto.
}
struct category {
int d [413]; // The d's of all the child structs sorted highest to lowest.
char name [111];
struct property property [413]; // This. How do I allocate this?
}</code>
Do I have to do struct property* property = (struct property*) malloc(sizeof(struct property) * 413);? Will the malloc of the array within remain intact? How do mallocs in structs behave in general?
You don't have a pointer member inside your structure property so you don't need to malloc any of your structure members.
When you malloc for the structure it will give you enough memory to hold all the structure members including arrays, exceptions are pointer structure members(You don't have any).
Your malloc without the cast would do fine. It allocates contiguous memory for the whole array. The arrays inside the struct's are all allocated along with it, they are proper arrays and not pointers.
Sizeof will give you the size of your entire structure. It properly accounts for the size of arrays and structures.
However, 413 items seems arbitrary. Would a variable sized structure work better for you?
In that case, calculating the size ahead of time to avoid mallocs is a good performance idea. Malloc can be slow, it can require locks, and the heap can fragment over time. This example shows you how to make a "variable length" structure with a pointer instead of an array or a variable length array at the end of your structure:
struct category
{
int cItems; // need this if handling variable # of items now.
int *d; // ptr instead of array
char *name; // ptr again
struct property property[0]; // var length array
}
int cItems = 413; // or whatever
// this is a nifty trick to get the size of a variable length struct:
int cbCategory = (size_t)(&((struct category*)0)->property[cItems]);
int cbD = sizeof(int)*cItems;
int cbName = sizeof(char)*cItems;
struct category *pCategory = (struct category*)malloc(cbCategory + cbD + cbName);
// wire up d:
pCategory->d = (int*)((char*)pCategory + cbCategory);
// or wire d up this way:
pCategory->d = (int*)&pCategory->property[cItems];
// wire up name
pCategory->name = (char*)pCategory->d + cbD;
// or wire up name this way
pCategory->name = (char*)&pCategory->d[cItems];
// set items
pCategory->cItems = cItems;
Note, I assumed that d had 413 elements to it. I could have just as easily left it a an array.