Index into a char array to append a string

Question

I am doing this to append some text:

char text[100];
strcpy(text, "Hello");

char input[] = "random";
strncpy(text + strlen(text), input, sizeof(text) - strlen(text));

I did this and it seems to work fine for ASCII text. But I fear I am not being safe doing pointer arithmetic. What if the input is UTF-8?

FYI, when I do text + strlen(text) I am getting a pointer to the end of the sentence and then appending to the end of the sentence.

i.e.

text => |h|e|l|l|o|NUL||||||....

text + strlen(text) => |NUL|||||.....

Actually I was confused. I'm concerned of situations where text+1 is not equal to text[1]. — alex murphy
– alex murphy, Commented Jan 20, 2016 at 8:35
In general, you should avoid strncpy, it is a dangerous and unsafe function only ever intended to be used for anything but ancient string formats in Unix. Better and safer alternatives are memcpy and strcpy (with buffer size known/checked). — Lundin
– Lundin, Commented Jan 20, 2016 at 10:04

Spade · Accepted Answer · 2016-01-20 06:42:52Z

2

This is exactly why strcat exists:

char text[100];
strcpy(text, "Hello");

char input[] = "random";
strcat(text, input);

To ensure memory sensitive concatenation preventing overflow, please use the following amendment:

   char *text;

   //allocate memory
   text = (char *) malloc(15);
   strcpy(text, "Hello");


   char input[] = "random";

   //reallocate memory
   text = (char *) realloc(text, strlen(text)+strlen(input) +1);
   strcat(text, input);
   free(text);

edited Jan 20, 2016 at 6:42

answered Jan 20, 2016 at 5:59

Spade

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3 Comments

alex murphy Over a year ago

What if the text buffer isn't large enough. And if I use strncat I still need to calculate how much empty space is left over in text right?

CiaPan Over a year ago

Please add a note to your solution that a caller function is then responsible for freeing the memory allocated by concat after use.

CiaPan Over a year ago

Yes, strcat simplifies the task – but it does no size testing and can overflow the destination array, as opposite to the OP's approach.

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