Re: "undefined behavior"?

On 13/06/2024 02:19, Janis Papanagnou wrote:

On 13.06.2024 00:22, Keith Thompson wrote:

>
This:
     char outliers[100] = "";
initializes all 100 elements to zero.  So does this:
     char outliers[100] = { '\0' };
Any elements or members not specified in an initializer are set to zero.

 Oops! This surprised me. (But you are right.) The overhead isn't
[syntactically] obvious, but I'm anyway always setting a single
'\0' character if I want to store strings in a 'char[]' and have
it initialized to an empty string (like below).
 

If you want to set an array's 0th element to 0 and not waste time
initializing the rest, you can assign it separately:
     char outliers[100];
     outliers[0] = '\0';
or
     char outliers[100];
     strcpy(outliers, "");
though the overhead of the function call is likely to outweigh the
cost of initializing the array.

 It wouldn't occur to me to use the strcpy() function, but is the
function call really that expensive in C ?
 

That depends on your toolchain.
If you are using a Windows-based compiler with an external DLL for the C library and the compiler doesn't handle the strcpy() directly, then it can be quite a lot of overhead.  You have the call to the DLL, which involves a few steps of indirection.  The library strcpy() may be optimised for handling large strings, and may save and restore a lot of registers (such as SIMD vector registers).
If you are using a compiler (whatever the platform) that optimises "strcpy", it will generate identical code to "outliers[0] = '\0';".