Thread: null termination with str(n)cpy and strlen

#1) Just because you don't get an error doesn't mean the code is bug-free; the errors those bugs could cause may show up later, if you add to the program or use a different compiler/different computer.

But you are right about using the +1 to include the \0.

Originally Posted by jeanluca

are you sure you need +1 for strncpy ?

In this case I think so:

Originally Posted by GNU Reference

char * strncpy (char *restrict to, const char *restrict from, size_t size)

If the length of from is more than size, then strncpy copies just the first size characters. Note that in this case there is no null terminator written into to.

If the length of from is less than size, then strncpy copies all of from, followed by enough null characters to add up to size characters in all. This behavior is rarely useful, but it is specified by the ISO C standard.

It doesn't actually say what would happen if they are the same size, but I would presume the former.

By the way, sizeof(char) is always equal to 1. If you feel that the sizeof is necessary, it would be better to write:

Code:

char* dest = malloc( len * sizeof(*dest) );

on account that the result of sizeof would be appropriate to the type of dest, and an error is more likely should the name of the pointer be changed.

And also check the return value of malloc. Better to do some error handling.

-ssharish

Originally Posted by bithub

If you are dynamically allocating the memory to ensure the destination buffer has enough space to fit the string, there is no need to use strncpy(). In this case (as in your example), you can just use strcpy().

I want to echo this good advice, I feel like it's being lost in the thread.

Originally Posted by slingerland3g

You are correct in using:

Code:

strncpy(dest, src, len + 1) ; // so it copies the null terminator too

Make sure you have room in the buffer. But why not just call strcpy()? (or, since you know the length, memcpy()?) If you just want to duplicate the string:

Code:

const char *src_string;
char *dst_string

Code:

// Quite possibly the easiest way...
// If your system doesn't have strdup(), write one.
dst_string = strdup(src_string);

or

Code:

dst_string = malloc(strlen(src_string) + 1);
if(!dst_string)
	// No memory.
strcpy(dst_string, src_string);

or

Code:

size_t len = strlen(src_string);
dst_string = malloc(len + 1);
if(!dst_string)
	// No memory.
// If strncpy() actually copies n chars, this will null terminate.
dst_string[len] = 0;
strncpy(dst_string, src_string, len);