C array/malloc/free problem

This is a discussion on C array/malloc/free problem within the C Programming forums, part of the General Programming Boards category; Hi all, Apologies for covering old ground AGAIN... I'm having trouble with a 2D array, pointers, malloc and free (a ...

  1. #1
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    C array/malloc/free problem

    Hi all,

    Apologies for covering old ground AGAIN... I'm having trouble with a 2D array, pointers, malloc and free (a classic combo). I'm getting a variety of core dumps when running the code below... can anyone spot the problem?

    The core dumps are for reasons such as: "free(): invalid next size (fast)". So I guess I'm freeing something I shouldn't be, or corrupting something with overflow. Help!

    Firstly, I have declared the structures in my software as follows:
    Code:
    typedef struct waitingtype {
            char* sym_name;
            char* bulk_name;
            char** alloc_names;
            int alloc_count;
    } port_wait_struct;
    
    typedef struct maptype {
            port_wait_struct** waiting_ports;
            int waiting_count;
    
    /* Other stuff here, removed to avoid confusion */
    } global_structure;
    I am creating the initial pointer as follows:
    Code:
    global_structure* bulks;
            bulks = (global_structure*)malloc(sizeof(global_structure));
            bulks->waiting_ports = NULL;
            bulks->waiting_count = 0;
    The problematic code follows (by the time I get here, I have char arrays param1, param2 and param3 which are all less than 256 characters and are null terminated) :

    Code:
            int found=-1;
            for( int i=0; i<bulks->waiting_count; i++ )
            {
                    if( strcmp( bulks->waiting_ports[i]->bulk_name, param1 ) == 0 )
                    {
                            found = i;
                    }
            }
    
            if( found == -1 )
            {
                    found = bulks->waiting_count;
                    port_wait_struct** new_struct = (port_wait_struct**)malloc( found * sizeof(port_wait_struct*) );
    
                    // Only copy old values across if there were any
                    if( bulks->waiting_count > 0 )
                    {
                            for( int i=0; i<bulks->waiting_count; i++ )
                            {
                                    new_struct[i] = bulks->waiting_ports[i];
                            }
                    }
    
                    // Put our new value on the end.
                    new_struct[found] = (port_wait_struct*)malloc( sizeof( port_wait_struct ) );
                    new_struct[found]->bulk_name = (char*)malloc( 256 * sizeof(char) );
                    new_struct[found]->sym_name = (char*)malloc( 256 * sizeof(char) );
    
                    strncpy( new_struct[found]->bulk_name, param1, 256 );
                    new_struct[found]->bulk_name[256 - 1] = '\0';
                    strncpy( new_struct[found]->sym_name, param3, 256 );
                    new_struct[found]->sym_name[256 - 1] = '\0';
    
                    new_struct[found]->alloc_count = 0;
                    new_struct[found]->alloc_names = NULL;
    
                    // Free old one and put our new one in its place.
                    if( bulks->waiting_ports NEQ NULL )
                    {
                            free( bulks->waiting_ports );
                    }
                    bulks->waiting_ports = new_struct;
                    bulks->waiting_count++;
    
            }
    
            // By here we have a 'found' value, whether it's a new one or an
            // old one.  We need to add ourselves onto the list of names.
            int old_count = bulks->waiting_ports[found]->alloc_count;
            char** new_names = (char**)malloc( (old_count+1) * sizeof(char*) );
    
            // Copy old ones across if there were any.
            if( old_count > 0 )
            {
                    for( int i=0; i<old_count; i++ )
                    {
                            new_names[i] = bulks->waiting_ports[found]->alloc_names[i];
                    }
            }
    
            new_names[old_count] = (char*)malloc( 256 * sizeof(char) );
            strcpy( new_names[old_count], param2 );
    
            free( bulks->waiting_ports[found]->alloc_names );
            bulks->waiting_ports[found]->alloc_names = new_names;
            bulks->waiting_ports[found]->alloc_count++;
    It's supposed to be an implementation of a map of char-arrays to 'lists' of other char-arrays. param1 is the 'key' and param2 is the char-array to be stored in the 'list'. param3 is kind of irrelevent, but needs to be stored beside the 'key'.

    It core dumps if I run the problematic chunk as a function with a whole bunch of test data, and it varies how far through the test data I get.

    Help! Much appreciated!

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    It seems to me that your job would be a lot easier if you used realloc().

    Anyway, I think this is your problem.
    Code:
    port_wait_struct** new_struct = (port_wait_struct**)malloc( found * sizeof(port_wait_struct*) );
    Since you soon afterwards access new_struct[found], you likely want (found+1):
    Code:
    port_wait_struct** new_struct = (port_wait_struct**)malloc( (found + 1) * sizeof(port_wait_struct*) );
    Oh, and you might want to consider not casting *alloc().
    dwk

    Seek and ye shall find. quaere et invenies.

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  3. #3
    Captain Crash brewbuck's Avatar
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    If you are running on Linux... Valgrind, Valgrind, Valgrind. It will spit out the EXACT LINE NUMBER where the bug occurs.

  4. #4
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    By Jove dwks, you've got it! If I could hug you, I would.

    One more question, you say "you might want to consider not casting *alloc()"... what alternatives are there?

  5. #5
    Kernel hacker
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    Quote Originally Posted by Weasel View Post
    One more question, you say "you might want to consider not casting *alloc()"... what alternatives are there?
    If your code is C (rather than C++), then the return type from malloc() [and friends] is void *, which according to the C standard is compatible with any other type of pointer, so for example, using
    Code:
    int *p;
    
    p = malloc(10);
    works just fine without warnings.

    Not putting a cast in there will tell you when you have forgotten to include <stdlib.h>, and thus not got a declaration on malloc - this could lead to mysterious problems in some systems (because some compilers will return pointers in a different type of register than integer values, and if you don't get the declaration of malloc included, it may think that it returns an integer).

    If you are using C++ to compile your code (e.g. your file is called xxx.cpp rather than xxx.c), then you should probably consider using new and delete rather than malloc.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  6. #6
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    I see your point. Thanks for the explanation.

    I'm using C, so I'm stuck with malloc and free!

    Thanks again,
    Weasel

  7. #7
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    Quote Originally Posted by Weasel View Post
    I see your point. Thanks for the explanation.

    I'm using C, so I'm stuck with malloc and free!

    Thanks again,
    Weasel
    Nothing wrong with C. Just different from C++.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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