length of dynamic array

This is a discussion on length of dynamic array within the C Programming forums, part of the General Programming Boards category; how do I get the length (or size) of a dynamic array? tried this but I get 0 Code: #include ...

  1. #1
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    length of dynamic array

    how do I get the length (or size) of a dynamic array? tried this but I get 0

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    
    typedef struct a
    {
    	int age;
    	char name[10];
    } el;
    
    int main (int argc, const char * argv[]) 
    {
    	int i,n;
    	el *array;
    	int d;
    
    	printf("length :> ");
    	scanf("%d",&n);	
    
    	array = (el *) calloc (n,sizeof(el));
    
    	
    
    
    	printf("\nINPUT\n\n");
    	for (i=0; i<n; i++)
    	{
    			printf("name %d. el :> ",i);
    			scanf("%s", array[i]. name);
    			printf("age %d. el :> ",i);
    			scanf("%d",& array[i]. age);
    	}
    	
    	printf("\nPRINTOUT\n\n");
    	for (i=0; i<n; i++)
    	{
    			printf("%d. el :> NAME:\t%s\tAGE:\t%d\n",i, array[i]. name, array[i]. age);		
    	}	
    	
    	d =  sizeof(array)/sizeof(el);
    	
    					printf("Length is :> %d",d);
    
    	return 0;
    }
    Last edited by budala; 09-02-2009 at 04:48 AM.

  2. #2
    Jack of many languages Dino's Avatar
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    Variable n is not initialized. The value of n * sizeof(el) will be how large array is - you have to keep track of that.
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  3. #3
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    Code:
    d =  sizeof(array)/sizeof(el);
    This will not work since sizeof(array) is just the size of a pointer on your machine (probably 4 or 8 bytes depending on your CPU architecture). As Dino said, the only way to get the size is to use what you passed to calloc() or malloc().
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    oops, I changed the order of instructions red and blue.

    I don't want to keep track of the size, I want to have a dynamic array and calculate its size

  5. #5
    C++まいる!Cをこわせ!
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    I'll put it in simple words:
    There is no (standard) way to calculate the length of a dynamic array in C.

    If you really, really, really, really must do it, there are non-portable solutions.
    Microsoft's compilers gives you the option of _msize.
    Then you could use the OS's allocation APIs to allocate and find the size of it.
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    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by budala View Post
    oops, I changed the order of instructions red and blue.

    I don't want to keep track of the size, I want to have a dynamic array and calculate its size
    You can't calculate the size of a malloc'ed piece of memory (unless you get your hands dirty with the grubby internals of malloc, but then that will only work on that one particular system). The whole point of malloc is that you know what you called it with.

  7. #7
    Registered User C_ntua's Avatar
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    Quote Originally Posted by budala View Post
    oops, I changed the order of instructions red and blue.

    I don't want to keep track of the size, I want to have a dynamic array and calculate its size
    If it would work of what you have in mind you could put an ending character to enable you to calculate its size.
    In most cases tracking the size would be the best way though. Since it is easy and the extra calculations not time consuming compared to malloc().
    If you plan on using malloc (or realloc) a lot of times and are afraid you might miss an update of its size you could wrap "the keep track of size" code and malloc in a single function

  8. #8
    Guest Sebastiani's Avatar
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    Another option is to store the size information in the data itself.

    Code:
    #include <stdlib.h>
    
    size_t* b_element_size_ptr_( void* buf )
    {
        return ( size_t* )( ( char* )buf - sizeof( size_t ) );
    }
    
    size_t* b_length_ptr_( void* buf )
    {
        return ( size_t* )( ( char* )buf - ( sizeof( size_t ) << 1 ) );
    }
    
    void* b_allocate( size_t siz, size_t len )
    {
        char*
            buf = malloc( siz * len + ( sizeof( size_t ) << 1 ) );
        if( buf )
        {
            buf += sizeof( size_t ) << 1;
            *b_element_size_ptr_( buf ) = siz;
            *b_length_ptr_( buf ) = len;
        }
        return buf;
    }
    
    void b_deallocate( void* buf )
    {
        if( buf )
            free( b_length_ptr_( buf ) );
    }
    
    size_t b_length( void* buf )
    {
        return *b_length_ptr_( buf );
    }
    
    size_t b_bytes( void* buf )
    {
        return b_length( buf ) * *b_element_size_ptr_( buf );
    }
    Example:

    Code:
    #include <stdio.h>
    
    int main( void )
    {
        int*
            data = b_allocate( sizeof( int ), 1024 );
        if( data )
        {
            printf( "Length of data: %d\n", b_length( data ) );
            printf( "Size (in bytes): %d\n", b_bytes( data ) );
        }
        b_deallocate( data );
        return 0;
    }
    Last edited by Sebastiani; 09-02-2009 at 10:53 AM.

  9. #9
    Jack of many languages Dino's Avatar
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    There's probably some OS specific system call you could make prior to the malloc, to determine your process's free memory, and then you could call it again after the malloc, and figure out the difference.

    Sounds like a contrived class exercise to me.
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  10. #10
    Guest Sebastiani's Avatar
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    Quote Originally Posted by Dino View Post
    There's probably some OS specific system call you could make prior to the malloc, to determine your process's free memory, and then you could call it again after the malloc, and figure out the difference.

    Sounds like a contrived class exercise to me.
    Except, at that point you obviously know how large the block is going to be.

  11. #11
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    Quote Originally Posted by Dino View Post
    . . .
    Sounds like a contrived class exercise to me.
    He he, you could call it that - it's my own research though and for practical application if I can ever get them dang pointers figured out.

    As far as the indexOf and strReplaceAll go, I really do need those functions pretty badly and a few more like them.
    I'm running in C# and I'm just sick of the lame performance.
    I'm pretty near convinced that if I can write funcs in C instead of using the C# funcs I'll improve my performance dramatically.

    Salem has me convinced that my biggest problem is, I don't understand memory allocation (no matter what the language). If I could understand what's really going on 'under the covers' for us 'managed code' guys, it'd help a great deal with performance. I've been dodging pointers, memory allocation and garbage collection for a *long* time.

    Performance has never been a major issue for me till lately, I find myself dealing with large quantities of data these days.

    I've been a full time programmer (mostly business/small business - database/internet) 15 years and I'm still learning . . .

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    This is just me thinking out loud... but perhaps you should just get a book on C and start with chapter 1 ... of course you will go through parts of it pretty quickly... but at least then you'll have all the required information...

    In deference to Salem's obvious smarts... In this case I might suggest that your real problem is that you're trying to understand C as some sort of mystical subset of C# ... Believe me the language similarities are completely absent beyond the totally disastrous choices of names. If the big guns had been smart they would have followed their predicessor's lead and moved up the alphabet C++ would be E (D already existed) and when they got to C# they would have said "Oh F it"...

    Learn C ... don't try to learn C as it applies to C#... Just learn the language on it's own merrits.
    Last edited by CommonTater; 02-12-2011 at 02:27 PM.

  13. #13
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    I like Sebastiani approach.
    But he left out b_realloc()

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