How can I use strncat without buffer overflow concerns?

user_789840 0
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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

buffer-overflow c string
6 Answers

I'd use memccpy instead of strncat in this case - it's safer and much faster. (It's also faster than the approach with snprintf mentioned by Dave):

/**
 * Returns the number of bytes copied (not including terminating '\0').
 * Always terminates @buf with '\0'.
 */ 
int add_strings(char *buf, int len)
{
    char *p = buf;

    if (len <= 0)
        return 0;

    p[len - 1] = '\0'; /* always terminate */

    p = memccpy(buf, "String 1", '\0', len - 1);
    if (p == NULL)
        return len - 1;

    p = memccpy(p - 1, "String 2", '\0', len - 1 - (p - buf));
    if (p == NULL)
        return len - 1;

    p = memccpy(p - 1, "String 3", '\0', len - 1 - (p - buf));

    return (p == NULL ? len : p - buf) - 1;
}
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