Converting Unicode codepoints to UTF-8 in C using iconv - c

Question

I want to convert a 32-bit value, which represents a Unicode codepoint, into a sequence of chars which is the utf-8 encoded string containing only the character corresponding to the codepoint.
For example, I want to turn the value 955 into the utf-8 encoded string "λ".
I tried to do this using iconv, but I could not get the desired result. Here is the code that I wrote:
#include <stdio.h>
#include <iconv.h>
#include <stdint.h>
int main(void)
{
uint32_t codepoint = U'λ';
char *input = (char *) &codepoint;
size_t in_size = 2; // lower-case lambda is a 16-bit character (0x3BB = 955)
char output_buffer[10];
char *output = output_buffer;
size_t out_size = 10;
iconv_t cd = iconv_open("UTF-8", "UTF-32");
iconv(cd, &input, &in_size, &output, &out_size);
puts(output_buffer);
return 0;
}
When I run it, only a newline is printed (puts automatically prints a newline,-- the first byte of outout_buffer is '\0').
What is wrong with my understanding or my implementation?

Answer 1

As said by minitech, you must use size = 4 for UTF32 in an uint32_t, and you must preset the buffer to null to have the terminating null after conversion.
This code works on Ubuntu :
#include <stdio.h>
#include <iconv.h>
#include <stdint.h>
#include <memory.h>
int main(void)
{
uint32_t codepoint = 955;
char *input = (char *) &codepoint;
size_t in_size = 4; // lower-case lambda is a 16-bit character (0x3BB = 955)
char output_buffer[10];
memset(output_buffer, 0, sizeof(output_buffer));
char *output = output_buffer;
size_t out_size = 10;
iconv_t cd = iconv_open("UTF-8", "UTF-32");
iconv(cd, &input, &in_size, &output, &out_size);
puts(output_buffer);
return 0;
}

Answer 2

Two problems:
Since you’re using UTF-32, you need to specify 4 bytes. The “lower-case lambda is a 16-bit character (0x3BB = 955)” comment isn’t true for a 4-byte fixed-width encoding; it’s 0x000003bb. Set size_t in_size = 4;.
iconv doesn’t add null terminators for you; it adjusts the pointers it’s given. You’ll want to add your own before calling puts.
*output = '\0';
puts(output_buffer);