Appending a char to a char *foo;

This is a discussion on Appending a char to a char *foo; within the C Programming forums, part of the General Programming Boards category; Hello all, I am trying to append a char to a char* which is returned from a function -- I'll ...

  1. #1
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    Appending a char to a char *foo;

    Hello all,

    I am trying to append a char to a char* which is returned from a function -- I'll demonstrate this with an example:

    Code:
    typedef struct
    {
    	char key;
    	int  value;
    } charmap_t;
    
    charmap_t t[] =
    {
    	{'s', MaskS},
    	{'c', MaskC},
    	{'l', MaskL},
    	{'m', MaskM},
    	{'1', Mask1},
    	{'2', Mask2},
    	{'3', Mask3},
    	{0, 0}
    };
    
    char *modifier_to_string(int mask, charmap_t *t)
    {
    	char *mods = malloc(20);
    	char c;
    
    	for (c = 0; t->key !=0; t++)
    	{
    		if (mask & t->value)
    		{
    			c = table->key;
    			strcat(mods, (char *)&c);	
    		}
    	}
    	return allmods;
    }
    This piece of code works OK -- although when trying to print the result returned from the function above I get garbage, as in:

    Code:
    char *mod_string = modifier_to_string(2, modifiers);
    fprintf(stderr, "Modifier:\t%s\n", mod_string;
    I know that "2" exists, and that modifiers (of type charmap_t) is being iterated over, but still I get garbage.

    Any ideas?

    Thanks!

  2. #2
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    strcat starts at the first null character but there isn't one in mods. before the loop say *mods='\0'; or use calloc instead of malloc so the memory is zero filled.

  3. #3
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    The problem is that strcat has to receive null terminated strings as parameters. In your example function, neither mods nor c are necessarily null terminated (In fact, evaluating (char*)&c until it finds a '\0' may crash your program, as you're accessing memory you don't own).

    Furthermore, I'd use a different name for a global variable. 't' is a far too common name.

  4. #4
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    Quote Originally Posted by Meldreth View Post
    strcat starts at the first null character but there isn't one in mods. before the loop say *mods='\0'; or use calloc instead of malloc so the memory is zero filled.
    Gosh, that's it! Thank you. I feel so stupid, I forgot that step. In the case of doing that, don't I then have to free(mod_string) after I've printed it out?

    Thanks!

  5. #5
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    you should have been freeing it anyway. malloc and calloc are the same thing. the only difference is calloc zero fills the memory and malloc doesn't. ronix is right too. you need to turn c into a string also.
    Code:
    char c[2] = {0};
    for (c = 0; t->key !=0; t++)
    {
    	if (mask & t->value)
    	{
    		c[0] = table->key;
    		strcat(mods, c);

  6. #6
    CSharpener vart's Avatar
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    I think it is a little bit of overhead to use strcat for assigning just one char at a given location of the array
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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