Convert C character into Ruby string

Question

I am writing a sample C Ruby extension which needs a C character to Ruby string conversion.

I have created extconf.rb and a file called x.c.

extconf.rb:

require 'mkmf'
create_makefile 'x'

x.c:

#include "ruby.h"

void Init_x() {
    char c = 'b' ;

    VALUE b = rb_str_new_cstr(c) ;
    rb_gv_set("$a", b) ;
    rb_global_variable(&b) ;
}

rb_str_new_cstr() expects a C string, not a character. When I compile and run this code, I get segmentation fault, and Ruby crashes.

I can do this instead, which works just fine:

#include "ruby.h"

void Init_x() {
    char *c = "b" ;

    VALUE b = rb_str_new_cstr(c) ;
    rb_gv_set("$a", b) ;
    rb_global_variable(&b) ;
}

But the problem is if I have something like fscanf(file, "%c", &char) which sets char to a character, I have to convert it to a string first and change %c to %1s, which sounds like a bit slower approach.

Is there a way to directly convert a C character into a Ruby string?

cremno · Accepted Answer · 2021-04-12T17:16:22.967

The rb_str_new_cstr() function is only for null-terminated strings - so called C strings (thus the suffix). Besides various other variations the unsuffixed rb_str_new() function exists for use cases like yours in which you have characters and you know how many:

#include "ruby.h"

void Init_x() {
    char c = 'b' ;

    VALUE b = rb_str_new(&c, 1); // or sizeof(c)
    rb_gv_set("$a", b) ;
}

Btw. you don't need to call rb_global_variable() here. It has nothing to do with Ruby global variables. It is used to manually tell the garbage collector about Ruby objects not exposed to Ruby. The garbage collector obviously knows about the created object since the string is accessible via $a in Ruby code.

score 0 · Answer 2 · answered Apr 11 '21 at 22:59

I wouldn't worry much about the performance of reading 1 character, but char *c = "b"; is a string literal and it is not mutable.
There are many detailed answers if you follow the duplicates.

However, to read 1 byte you don't need fscanf(), because fgetc() does exactly that.

char c[2] = "b";    // 1 character + NULL

c[0] = fgetc(file); // returns the character read

And it's already C string that you can pass, without & because arrays decay to pointers to their first element

Convert C character into Ruby string

2 Answers2