strcat（）vs sprintf（）

Question

What would be faster? This:

sprintf(&str[strlen(str)], "Something");

or

strcat(str, "Something");

Is there any performance difference?

Answer 1

strcat would be faster because sprintf has to first scan the string looking for format variables.

But the real win is the fact that everyone knows what strcat is doing - concatenating strings. Using sprintf for concatenation isn't a standard use. And it would cause people to do a double take.

Answer 2

Given the choice between the two, I'd choose strcat ; it's certainly more readable and makes your intentions clear. It might also be slightly faster than sprintf , but probably not by much.

But, regardless of which method you choose, you should definitely use snprintf or strncpy for buffer overrun protection.

You tagged this question as both c and c++ ; if you are using C++, it would be far better to use a std::string instead.

Answer 3

If there's a difference, that difference varies according to compiler flags, computer usage at the time the code is running, library implementation, ..., ...

For your specific case, measure .
Run both snippets a few billion times and time them.

I doubt very much you will find any significant difference.

I like strcat() better: it clearly conveys the meaning of the code.

Answer 4

strcat is far faster in this case. Remember that sprintf() parses the format string character by character looking for % escapes. Compare this to:

strcpy (str + strlen (str), "Something");

for example.

Answer 5

I agree with others: strcat() should be faster. But for "best-practice", if you really care about speed, you should use neither. You'd better have a struct like:

typedef struct { size_t len, max; char *str; } mystring_t;

to keep track of the end of the string. When you append a string, simply jump to the end and copy it. Double max if the allocated memory is not large enough. Alternatively, you may have something a bit fancy by packing len , max and str together.