sprintf() without trailing null space in C

Is there a way to use the C sprintf() function without it adding a '\0' character at the end of its output? I need to write formatted text in the middle of a fixed width string.

There is no way to tell sprintf() not to write a trailing null. What you can do is use sprintf() to write to a temporary string, and then something like strncpy() to copy only the bytes that you want.

sprintf returns the length of the string written (not including the null terminal), you could use that to know where the null terminal was, and change the null terminal character to something else (ie a space). That would be more efficient than using strncpy.

 unsigned int len = sprintf(str, ...);
 str[len] = '<your char here>';

You can't do this with sprintf(), but you may be able to with snprintf(), depending on your platform.

You need to know how many characters you are replacing (but as you're putting them into the middle of a string, you probably know that anyway).

This works because some implementations of snprintf() do NOT guarantee that a terminating character is written - presumably for compatibility with functions like stncpy().

char message[32] = "Hello 123, it's good to see you.";

snprintf(&message[6],3,"Joe");

After this, "123" is replaced with "Joe".

On implementations where snprintf() guarantees null termination even if the string is truncated, this won't work. So if code portability is a concern, you should avoid this.

Most Windows-based versions of snprintf() exhibit this behaviour.

But, MacOS and BSD (and maybe linux) appear to always null-terminate.

You could also use your fixed width string as a format string like this:

char my_fixed_width_string_format[] = "need 10 chars starting here: %10s";
char my_fixed_width_string[40];
char string_to_print[] = "abcdefghijklmnop";
sprintf(my_fixed_width_string, my_fixed_width_string_format, string_to_print;
printf(my_fixed_width_string);

should yield

need 10 chars starting here: abcdefghij