d2hms.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

double d2hms(const double d, double *h, double *m, double *s)
{ double foo;
  foo=modf(d,h);
  foo*=60;
  foo=modf(foo,m);
  foo*=60;
  foo=modf(foo,s);
  return foo;
}

int main(int argc, char* argv[], char* envp[])
{ 
  double d,h,m,s,c;

  if(argc==2) {
    d=atof(argv[1]); /* better use strtod */
  } else {
    printf("Usage:\n%s hora.decimal\n",argv[0]);
    return -1;
  }

  c=d2hms(d,&h,&m,&s);
  printf("%f=%.0f:%02.0f:%06.3f\n", d, h, m, s+c);
  return 0;
}

---
$ gcc d2hms.c -lm
$ ./a.out 12.345678
12.345678=12:20:44.441