snprintf vs. strcpy (etc.) in C

Question

For doing string concatenation, I\'ve been doing basic strcpy, strncpy of char* buffers. Then I learned about the snprintf and friends

Answer 1

As others did point out already: Do not use strncpy.

• strncpy will not zero terminate in case of truncation.
• strncpy will zero-pad the whole buffer if string is shorter than buffer. If buffer is large, this may be a performance drain.

snprintf will (on POSIX platforms) zero-terminate. On Windows, there is only _snprintf, which will not zero-terminate, so take that into account.

Note: when using snprintf, use this form:

snprintf(buffer, sizeof(buffer), "%s", string);

instead of

snprintf(buffer, sizeof(buffer), string);

The latter is insecure and - if string depends on user input - can lead to stack smashes, etc.

Answer 2

snprintf is more robust if you want to format your string. If you only want to concatenate, use strncpy (don't use strcpy) since it's more efficient.

Answer 3

sprintf has an extremely useful return value that allows for efficient appending.

Here's the idiom:

char buffer[HUGE] = {0}; 
char *end_of_string = &buffer[0];
end_of_string += sprintf( /* whatever */ );
end_of_string += sprintf( /* whatever */ );
end_of_string += sprintf( /* whatever */ );

You get the idea. This works because sprintf returns the number of characters it wrote to the buffer, so advancing your buffer by that many positions will leave you pointing to the '\0' at the end of what's been written so far. So when you hand the updated position to the next sprintf, it can start writing new characters right there.

Constrast with strcpy, whose return value is required to be useless. It hands you back the same argument you passed it. So appending with strcpy implies traversing the entire first string looking for the end of it. And then appending again with another strcpy call implies traversing the entire first string, followed by the 2nd string that now lives after it, looking for the '\0'. A third strcpy will re-traverse the strings that have already been written yet again. And so forth.

So for many small appends to a very large buffer, strcpy approches (O^n) where n is the number of appends. Which is terrible.

Plus, as others mentioned, they do different things. sprintf can be used to format numbers, pointer values, etc, into your buffer.

Answer 4

For most purposes I doubt the difference between using strncpy and snprintf is measurable.

If there's any formatting involved I tend to stick to only snprintf rather than mixing in strncpy as well.

I find this helps code clarity, and means you can use the following idiom to keep track of where you are in the buffer (thus avoiding creating a Shlemiel the Painter algorithm):

char sBuffer[iBufferSize];
char* pCursor = sBuffer;

pCursor += snprintf(pCursor, sizeof(sBuffer) - (pCursor - sBuffer),  "some stuff\n");

for(int i = 0; i < 10; i++)
{
   pCursor += snprintf(pCursor, sizeof(sBuffer) - (pCursor - sBuffer),  " iter %d\n", i);
}

pCursor += snprintf(pCursor, sizeof(sBuffer) - (pCursor - sBuffer),  "into a string\n");

Answer 5

I think there is another difference between strncpy and snprintf.

Think about this:

const int N=1000000;
char arr[N];
strncpy(arr, "abce", N);

Usually, strncpy will set the rest of the destination buffer to '\0'. This will cost lots of CPU time. While when you call snprintf,

snprintf(a, N, "%s", "abce");

it will leave the buffer unchanged.

I don't know why strncpy will do that, but in this case, I will choose snprintf instead of strncpy.

Answer 6

All *printf functions check formatting and expand its corresponding argument, thus it is slower than a simple strcpy/strncpy, which only copy a given number of bytes from linear memory.

My rule of thumb is:

• Use snprintf whenever formatting is needed.
• Stick to strncpy/memcpy when only need to copy a block of linear memory.
• You can use strcpy whenever you know exatcly the size of buffers you're copying. Don't use that if you don't have full control over the buffers size.