ANSI C - Clearing a string

Question

I've got a string declared like this:

str=malloc(sizeof(char)*128);

I want to clear it completely so that whenI do strncat() operation, the new characters will be written to the beginning of str. The reason I need to clear it is that I'm writing over it with a simplified version of itself (deleting excess whitespace).

score 13 · Accepted Answer · answered Apr 17 '12 at 15:08

13

I suggest you simply do this:

*str = '\0';

You don't need to clear the entire contents of the buffer. You can just set the first char to zero and then you have the empty string.

answered Apr 17 '12 at 15:08

David Heffernan

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You beat me to this answer... He doesn't need to set the **entire** array to 0, so this is much faster. +1 – lnafziger Apr 17 '12 at 15:09

Sergey Kalinichenko · Answer 2 · 2012-04-17T15:26:58.207

4

Use memset:

memset(str, 0, sizeof(char)*128);

Regardless of this, if you are writing the string over itself, you shouldn't use strcat - the objects that you copy must not overlap:

If copying takes place between objects that overlap, the behavior is undefined.

and a string definitely overlaps with itself.

Removing whitespace from a string can be easily achieved with a simple function:

void remove_space(char* r) {
    char *w = r;
    do { *w = *r++; w += *w && !isspace(*w); } while (*w);
}

edited Apr 17 '12 at 15:26

answered Apr 17 '12 at 15:05

Sergey Kalinichenko

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how do I call `remove_space` function? I tried `char * a = "kk a"; remove_space(a); printf("[%s]\n", a);` but it returns `Segment fault`. – Jack Apr 17 '12 at 18:07
1

@Jack You need to make the string writable: `char a[] = "kk a"; remove_space(a); printf("[%s]\n", a);` – Sergey Kalinichenko Apr 17 '12 at 18:41

score 3 · Answer 3 · edited May 23 '17 at 10:34

You would like to use calloc instead of plain malloc?

Something like

calloc(128, sizeof(char));

EDIT Also, strncat doesn't require you having a null terminated string at the destination. Just make sure that the destination is large enough to hold the concatenated resulting string, including the additional null-character.

And as dasblinkenlight notes, do not copy overlappingly with strncat. Consider using normal dst[i++] = src[j++] way of copying.

ANSI C - Clearing a string

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