Dynamically grown array of strings and subroutines [duplicate]

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Dynamically grown array of strings

I have a program that needs to return a list of strings. Neither the number of strings nor the length or strings is known at compile time.

Moreover, each string "grows" over a few iterations before the next string is created: I use realloc() and strcat() to add words to each string. There are several subroutines that can either add a string to the array of strings, or expand the strings.

My program is way too large to paste here, so here's a small sample just to demonstrate how I do it:

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>

    char **result = NULL;

    void function_1();
    void function_2();

    int main ()
    {
        result = (char **)realloc (result,sizeof(char *) * 1);

        result[0]= (char *)realloc(result[0],5 * sizeof(char));
        strcat(result[0],"hello");

        result[0]= (char *)realloc(result[0],10 * sizeof(char));
        strcat(result[0]," world");


        result = (char **)realloc (result, sizeof(char *) * 2);
        function_1();

        printf ("%s \n", result[0]);
        printf ("%s \n", result[1]);


      return 0;
    }

    void function_1(){

        function_2();

        result[1]= (char *)realloc(result[1],20 * sizeof(char));
        strcat(result[1],"12345");
    }

    void function_2(){
        result[1]= (char *)realloc(result[1],10 * sizeof(char));
        strcat(result[1],"0123456789");

        result[1]= (char *)realloc(result[1],15 * sizeof(char));
        strcat(result[1],"asdfg");
    }

so basically, every time I want to create a new string I use

    result = (char **)realloc (result,sizeof(char *) * TOTAL_NUMBER_OF_STRINGS);

and every time I want to expand a string I use

    result[STRING_NUMBER]= (char *)realloc(result[0],sizeof(char) * (current_length_of_string + length_of_new_word));

The small section of code I've provided works fine, but in my program when I'm using the same approach, eventually I get either something like:

    *** glibc detected *** ./uvicfmt3: realloc(): invalid next size: 0x081d1170 ***

or a segmentation fault.

Could someone suggest what's wrong with my approach? I've rewritten the part of my program that deals with dynamic memory allocation several times from scratch, but the problem still persist.

PS Here's my whole program: http://pastebin.com/7WhehW18 The format_file function get's called from the outside.

Don't cast the result of malloc() etc. in C. It does nothing but obscure diagnostics. Also, "hello" requires six bytes. — Kerrek SB
– Kerrek SB, Commented Nov 30, 2011 at 16:12

Teudimundo · Accepted Answer · 2011-11-30 17:14:30Z

1

Here you have for sure several problems

result[0]= (char *)realloc(result[0],5 * sizeof(char));
strcat(result[0],"hello");

result[0]= (char *)realloc(result[0],10 * sizeof(char));
strcat(result[0]," world");

you cannot know that the first result[0] is NULL, because you never set it to NULL;
same problem for result[0] string that might contain garbage because not initialized;
in the first realloc you alloc 5 chars and then you write 6 chars in it ('\0' for string termination);
the second realloc you alloc 10 chars in total while "hello world\0" is 12 chars (not considering that you probably have already some problem because of the previous strcat).

While the first 2 points might not be an issue in your case (you might accidentally find them already zeroed), the point 3,4 are a problem.

About points 1 and 2: When you allocate with malloc or realloc, the memory is not initialized, and it might contain anything in it. This means that you need to do a memset to 0 if you expect to find 0 there (if you do a realloc you should take care to zero only the newly allocated bytes).

While when you allocate an array of pointers you probably want to set all of them to 0 (the first time). For a string it is enough you set the first char to 0. this means that you should have done something like:

    result = (char **)realloc (result,sizeof(char *) * 1);
    memset(result, 0, sizeof(char *) * 1);
    result[0]= (char *)realloc(result[0],6 * sizeof(char));
    result[0][0] = '\0';
    strcat(result[0],"hello");

    result[0]= (char *)realloc(result[0],12 * sizeof(char));
    strcat(result[0]," world");

edited Nov 30, 2011 at 17:14

answered Nov 30, 2011 at 16:40

Teudimundo

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2 Comments

user1073407 Over a year ago

Thank you that is very helpful. Could you explain what you mean by point 2 though? I'm not entirely sure what you are referring to there.

Teudimundo Over a year ago

I edited the answer for answering your comment.

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