Using libproc.h

proc.c

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

// Uses proc_pidinfo from libproc.h to find the parent of given pid.
// Call this repeatedly until ppid(pid) == pid to get ancestors.
int ppid(pid_t pid) {
    struct proc_bsdinfo info;
    proc_pidinfo(pid, PROC_PIDTBSDINFO, 0, &info, sizeof(info));
    return info.pbi_ppid;
}

void print_info(pid_t pid) {
    // proc_pidinfo(pid, flavor, arg, buffer, buffersize)
    // flavor = PROC_PIDLISTFDS, buffer = &proc_fdinfo
    // flavor = PROC_PIDTASKALLINFO, buffer = &proc_taskallinfo
    // flavor = PROC_PIDTBSDINFO, buffer = &proc_bsdinfo
    // flavor = PROC_PIDTASKINFO, buffer = &proc_taskinfo
    // flavor = PROC_PIDTHREADINFO, buffer = &proc_threadinfo
    // flavor = PROC_PIDVNODEPATHINFO, buffer = &proc_vnodepathinfo
    // flavor = PROC_PIDPATHINFO, buffer = char *
    // see more in sys/proc_info.h
    struct proc_bsdinfo info;    
    proc_pidinfo(pid, PROC_PIDTBSDINFO, 0, &info, sizeof(info));
    
    // proc_name(pid, buffer, buffersize)
    // Copies the contents of info.pbi_name or info.pbi_comm into buffer.
    // Since pbi_comm is limited to first 15 chars, pbi_name is preferred.
    char name[2*MAXCOMLEN];
    proc_name(pid, name, sizeof(name));
    
    // proc_pidpath(pid, buffer, buffersize)
    char path[PROC_PIDPATHINFO_MAXSIZE];
    proc_pidpath(pid, path, sizeof(path));

    printf("%d\t%d\t%-15s\t%s\tproc_name=%s\tpath=%s\n",
           info.pbi_pid, info.pbi_ppid, info.pbi_comm, info.pbi_name,
           name, path);
}

int main(void) {
    // proc_listpids(type, typeinfo, buffer, buffersize)
    // type = PROC_ALL_PIDS, typeinfo = 0 (use proc_listallpids)
    // type = PROC_PGRP_ONLY, typeinfo = process group id (use proc_listpgrppids)
    // type = PROC_TTY_ONLY, typeinfo = tty
    // type = PROC_UID_ONLY, typeinfo = uid
    // type = PROC_RUID_ONLY, typeinfo = ruid 
    // type = PROC_PPID_ONLY, typeinfo = ppid (use proc_listchildpids)
    // Call with buffer = NULL to return number of pids.
    int n = proc_listpids(PROC_ALL_PIDS, 0, NULL, 0);
    
    int *buffer = (int *)malloc(sizeof(int)*n);
    
    // Then call again with int *buffer and nonzero buffersize.
    // The return value is number of pids placed into buffer (<= buffersize).
    int k = proc_listpids(PROC_ALL_PIDS, 0, buffer, n*sizeof(int));

    printf("%d processes:\n", k);
    for (int i = 0; i < k; i++) {
        int pid = buffer[i];
        if (pid == 0) continue; // ignore loginwindow spam
        print_info(pid);
    }
    return 0;
}

@mrexodia, I believe you are mixing proc_listpids() and proc_listallpids(). The last returns the number of processes, but the first returns a buffer to store an array of the PIDs. That's exactly is what we can see in the kernel sources (https://opensource.apple.com/source/xnu/xnu-7195.141.2/libsyscall/wrappers/libproc/libproc.c.auto.html):

int
proc_listallpids(void * buffer, int buffersize)
{
	int numpids;
	numpids = proc_listpids(PROC_ALL_PIDS, (uint32_t)0, buffer, buffersize);

	if (numpids == -1) {
		return -1;
	} else {
		return numpids / sizeof(int);
	}
}

Ah yes, you are totally correct my bad!