My previous post covered one way of "how to do it". This is a separate post because the focus is on "why are you doing that?" and a re-coding of your original code.

I did some bit of "mind reading" and re-coded your C function. It looks like you wanted 2 different ways for your function to return the result? If you gave it a "ret string", you wanted that string modified "in place"? Otherwise you wanted a new result to be passed to $output? Very confusing to me. Please un-confuse me.

Anyway, I don't see the need for this "ret" input parm at all. Take in a string, do something that generates another string and return that string. What is the need for this second input string? It is possible to modify a string "in place", but that would require the caller to provide enough memory so that when the string becomes longer, it does not overflow allocated memory. I have no idea about the details of your string transformation equation - I leave that detail to you. Here is my version below. There is one way to get the input and one way to get the output.

use v5.10;

use Inline 'C';

say "At least we have compiled a bit of C code!";

my $output = enctypex_msname("HD2");

say "Output from enctypex_msname() call: $output\n";

=OUTPUT:
At least we have compiled a bit of C code!
Output from enctypex_msname() call: HD2.ms14.host.com
=cut

__END__
__C__
unsigned char *enctypex_msname (unsigned char *name) {
    unsigned char temp[256];
    unsigned int c;
    unsigned char* mover;

    if(!*name) return(NULL); /* empty string "" */
                             /* not sure if this will result in undef?
+? */

    unsigned int server_num = 0;
    for( mover = name; *mover; mover++) {
        c = tolower(*mover);
        server_num = c - (server_num * 0x63306ce7); /*some comment app
+ropriate here!!*/
    }
    server_num %= 20;

    snprintf(temp, sizeof(temp), "%s.ms%d.host.com", name, server_num)
+;

    int length_of_temp_string = strlen(temp)+1;
    char* ret_buf = (char*)malloc(length_of_temp_string); //could just
+ allocate 256, so what?
    if (!ret_buf) {
        perror("malloc");
        exit(1);
    }

    memcpy(ret_buf, temp, length_of_temp_string);

    return(ret_buf);
}
[download]

It is apparent that the malloc'ed memory gets passed as an SV to $output. At that point, I don't know for sure how this works, but Perl would have to be responsible for calling "free()" on that pointer when the memory for $output is not needed anymore.

I didn't allocate more memory than needed, but that detail doesn't matter much. What does matter is not "re-using memory in a way that is opaque to the user". That is what your version with "static" would do. I recommend against that.

Update: Geez, I am updating a lot tonight. My brain is working on a different project at the moment...But the thought does occur that I don't see anything in this code that would warrant a C subroutine (there is no performance issue). If you explain more about the string transformation algorithm, I am confident that this could be written in native Perl code. In my testing, it does take some extra time to invoke the C compiler and any generated messages are a bit harder to understand than it would be with a native C program. A "pure Perl" implementation would start faster and run time would make no difference. But I get it that you are playing with various options of passing parameters and getting results.

Well as I continue to learn...Update...it is not necessary to do this malloc() stuff...it is possible to create/allocate memory for a new SV string object in one C statement as shown below. This stuff is complex. To say the least!

use v5.10;
use Inline 'C';

say "At least we have compiled a bit of C code!";

my $output = enctypex_msname("HD2");

say "Output from enctypex_msname() call: $output\n";

=OUTPUT:
At least we have compiled a bit of C code!
Output from enctypex_msname() call: HD2.ms14.host.com
=cut

__END__
__C__
SV *enctypex_msname (unsigned char *name) {
    unsigned int c;
    unsigned char* mover;

    if(!*name) return(NULL); /* empty string "" */
                             /* not sure if this will result in undef?
+? */

    unsigned int server_num = 0;
    for( mover = name; *mover; mover++) {
        c = tolower(*mover);
        server_num = c - (server_num * 0x63306ce7); /*some comment app
+ropriate here!!*/
    }
    server_num %= 20;
    
    return (newSVpvf("%s.ms%d.host.com", name, server_num));   // mall
+oc is here...
                                                               // temp
+ allocation here too
}
[download]

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