How can I use strncat without buffer overflow concerns?

Question

I have a buffer, I am doing lot of strncat. I want to make sure I never overflow the buffer size.

char buff[64];

strcpy(buff, "String 1");

strncat(buff, "String 2", sizeof(buff));

strncat(buff, "String 3", sizeof(buff));

Instead of sizeof(buff), I want to say something buff - xxx. I want to make sure I never override the buffer

Answer 1

Take into consideration the size of the existing string and the null terminator

#define BUFFER_SIZE 64
char buff[BUFFER_SIZE];

//Use strncpy
strncpy(buff, "String 1", BUFFER_SIZE - 1);
buff[BUFFER_SIZE - 1] = '\0';

strncat(buff, "String 2", BUFFER_SIZE - strlen(buff) - 1);

strncat(buff, "String 3", BUFFER_SIZE - strlen(buff) - 1);