strncpy and using sizeof to copy maximum characters

Question

Hello,

I am using the code below

char call[64] = {'\0'} /* clean buffer */
strncpy(call, info.called, sizeof(call));

I always use the sizeof for the destination for protecting a overflow, incase source is greater than the destination. This way I can prevent a buffer overflow as it will only copy as much as the destination can handle.

But I am now wondering if it will null terminate the destination.

A couple of cases.

1) If the source is greater. I could do this:

call[strlen(call) - 1] = '\0'; /* insert a null at the last element.*/

2) If the source is less than the destination. call is 64 bytes, and I copy 50 bytes as that is the size of the source. Will it automatically put the null in the 51 element?

Many thanks for any information,

Answer 1

Your idea:

call[strlen(call) - 1] = '\0';

would not work, as you would be calling strlen() on a non-terminated string

Answer 2

If the source's length is less than the max number passed as third parameter strncpy will null-terminate the destination, otherwise - not.

If the source is equal or greater in length than the destination - it's your problem to deal with it. Doing like you suggest - calling strlen() - will not work since the buffer will be not null-terminated and you'll run into undefined behaviour.

You could allocate a bigger buffer:

char buffer[bufferSize + 1];
strncpy( buffer, source, bufferSize );
*(buffer + bufferSize ) = 0;

Answer 3

strncpy will not null-terminate the destination if it truncates the string. If you must use strncpy, you need to ensure that the result is terminated, something like:

strncpy(call, info.called, sizeof(call) - 1);
call[sizeof(call) - 1] = '\0';

BSD's strlcpy(), among others, is generally considered superior:

http://www.openbsd.org/cgi-bin/man.cgi?query=strlcpy

Answer 4

But I am now wondering if it will null terminate the destination.

No, strncpy does not promise that target string would be null terminated.

char tar[2]={0,0};
char bar="AB";
strncpy(tar,bar,2);
// result tar[0]=='A'; tar[1]=='B'

It order to make it correct you should use:

strncpy(traget,string,sizeof(target)-1);
target[sizeof(target)-1]=0

Answer 5

1) From cplusplus.com: "No null-character is implicitly appended to the end of destination, so destination will only be null-terminated if the length of the C string in source is less than num." So you if you need your string to be null-terminated, you need to do this:

call[sizeof(call) - 1] = '\0';

A good way to do this would be to write a wrapper function for strncpy that always makes sure the string is terminated.

2) If the source is shorter than the destination, the destination will be null-terminated.