how many null bytes, concatenating strings in C

Question

If i want to concatenate 2 strings in C, do i have to allocate an extra null char for every string or one is enough?

int main(){
    char *s1 = NULL;
    char *s2 = NULL;
    char *s1_s2 = NULL;

    s1 = malloc(sizeof(char) * strlen("string1") + 1);
    strcpy(s1, "string1");
    s2 = malloc(sizeof(char) * strlen("string2") + 1);
    strcpy(s2, "string2");

    s1_s2 = malloc(sizeof(char) * (strlen(s1) + strlen(s2)) + 2); // shouldn't it be only 1 null char ?
    strcpy(s1_s2, s1);
    strcat(s1_s2, s2);
}

in this question, they use 2 null bytes for each string. Can some one shed some light? Thanks

All strings only needs one extra byte, since there is only one string terminator. The same goes for `s1_s2` in your example. — Some programmer dude, Jan 24 '12 at 11:54

score 3 · Answer 1 · answered Jan 24 '12 at 11:55

3

No, you don't need two extra null bytes.

In memory, your strings will look like:

s1 -> 's' 't' 'r' 'i' 'n' 'g' '1' '\0'

s2 -> 's' 't' 'r' 'i' 'n' 'g' '2' '\0'

s1_s2 -> 's' 't' 'r' 'i' 'n' 'g' '1' 's' 't' 'r' 'i' 'n' 'g' '2' '\0'

answered Jan 24 '12 at 11:55

Renan Greinert

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score 2 · Accepted Answer · answered Jan 24 '12 at 11:54

2

Only one is needed.

In the question you linked, they are actually also adding an extra space character which requires an extra byte.

answered Jan 24 '12 at 11:54

Vadim

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score 1 · Answer 3 · answered Jan 24 '12 at 11:54

1

The final string should end with a null byte so one is enough.

Edit: The question you sent, there's a space between the two strings.

strcpy(both, first);
strcat(both, " ");
strcat(both, second);

answered Jan 24 '12 at 11:54

iabdalkader

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score 1 · Answer 4 · answered Jan 24 '12 at 11:54

1

Only one is needed. You always need just one null character at the end of the string, which here happens to be a concatenation of two strings.

answered Jan 24 '12 at 11:54

leif

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score 1 · Answer 5 · answered Jan 24 '12 at 11:56

1

In the question that you linked to, a space is put between the strings that are concatenated together. An extra char must be allocated for this space.

In your example, you do not put a space between the strings, so only one additional char is needed.

answered Jan 24 '12 at 11:56

Mankarse

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how many null bytes, concatenating strings in C

5 Answers5