c - Collatz and Printing with Spaces -

i have following code:

#include <stdlib.h> #include <stdio.h> #include <sys/mman.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h> #include <sys/types.h> #include <sys/wait.h>  int main(int argc, char** argv) {   const char* name = "collatz";   const int size = 4096;   int num = atoi(argv[1]);   int shm_fd;   void *ptr;   shm_fd = shm_open(name, o_creat | o_rdwr, 0666);   ftruncate(shm_fd, size);   ptr = mmap(0, size, prot_read, map_shared, shm_fd, 0);   pid_t id = fork(); if (id == 0) {     shm_fd = shm_open(name, o_rdwr, 0666);     ptr = mmap(0, size, prot_write, map_shared, shm_fd, 0);     while (num != 1) {         sprintf(ptr, "%d", num);         ptr++;         if (num % 2 == 0) num /= 2;         else num = 3 * num + 1;     }     sprintf(ptr, "%d", num);     ptr++; } else {     wait(null);     printf("parent: %s\n", (char*) ptr);     shm_unlink(name); }  return 0; } 

i print spaces between correct numbers haven't been able figure out when adding "%d " or " %d" , other formatting "things." when run program if type in 8 expect output be: 8 4 2 1 instead 8421. being said, when input number 19 long number "correct" not formatted correctly. love formatting assistance please!

also seems algorithm might incorrect - if use 3 should expect result: 3, 10, 5, 16, 8, 4, 2, 1 , 31518421

you can use:

sprintf(ptr, "%d ", num); 

this add space after number printing. can read more printf formatting @ printf doc

as per incorrect result, can see if take first character of each number in: 3, 10, 5, 16, 8, 4, 2, 1, 31518421. because shifting ptr 1 after printing in ptr++;. hence if printed 2 characters, second 1 gets overwritten on next step. need is:

int shift = sprintf(ptr, "%d ", num); ptr = ptr + shift; 

or, in more concise form:

ptr += sprintf(ptr, "%d ", num); 

here shift writer number of characters written, prevent being overwritten next call sprintf.