How to get MAC address of your machine using a C program?

Expanding on the answer given by @user175104 ...

std::vector<std::string> GetAllFiles(const std::string& folder, bool recursive = false)
{
  // uses opendir, readdir, and struct dirent.
  // left as an exercise to the reader, as it isn't the point of this OP and answer.
}

bool ReadFileContents(const std::string& folder, const std::string& fname, std::string& contents)
{
  // uses ifstream to read entire contents
  // left as an exercise to the reader, as it isn't the point of this OP and answer.
}

std::vector<std::string> GetAllMacAddresses()
{
  std::vector<std::string> macs;
  std::string address;

  // from: https://stackoverflow.com/questions/9034575/c-c-linux-mac-address-of-all-interfaces
  //  ... just read /sys/class/net/eth0/address

  // NOTE: there may be more than one: /sys/class/net/*/address
  //  (1) so walk /sys/class/net/* to find the names to read the address of.

  std::vector<std::string> nets = GetAllFiles("/sys/class/net/", false);
  for (auto it = nets.begin(); it != nets.end(); ++it)
  {
    // we don't care about the local loopback interface
    if (0 == strcmp((*it).substr(-3).c_str(), "/lo"))
      continue;
    address.clear();
    if (ReadFileContents(*it, "address", address))
    {
      if (!address.empty())
      {
        macs.push_back(address);
      }
    }
  }
  return macs;
}

netlink socket is possible

man netlink(7) netlink(3) rtnetlink(7) rtnetlink(3)

#include <assert.h>
#include <stdio.h>
#include <linux/if.h>
#include <linux/rtnetlink.h>
#include <unistd.h>

#define SZ 8192

int main(){

  // Send
  typedef struct {
    struct nlmsghdr nh;
    struct ifinfomsg ifi;
  } Req_getlink;
  assert(NLMSG_LENGTH(sizeof(struct ifinfomsg))==sizeof(Req_getlink));
  int fd=-1;
  fd=socket(AF_NETLINK,SOCK_RAW,NETLINK_ROUTE);
  assert(0==bind(fd,(struct sockaddr*)(&(struct sockaddr_nl){
    .nl_family=AF_NETLINK,
    .nl_pad=0,
    .nl_pid=getpid(),
    .nl_groups=0
  }),sizeof(struct sockaddr_nl)));
  assert(sizeof(Req_getlink)==send(fd,&(Req_getlink){
    .nh={
      .nlmsg_len=NLMSG_LENGTH(sizeof(struct ifinfomsg)),
      .nlmsg_type=RTM_GETLINK,
      .nlmsg_flags=NLM_F_REQUEST|NLM_F_ROOT,
      .nlmsg_seq=0,
      .nlmsg_pid=0
    },
    .ifi={
      .ifi_family=AF_UNSPEC,
      // .ifi_family=AF_INET,
      .ifi_type=0,
      .ifi_index=0,
      .ifi_flags=0,
      .ifi_change=0,
    }
  },sizeof(Req_getlink),0));

  // Receive
  char recvbuf[SZ]={};
  int len=0;
  for(char *p=recvbuf;;){
    const int seglen=recv(fd,p,sizeof(recvbuf)-len,0);
    assert(seglen>=1);
    len += seglen;
    if(((struct nlmsghdr*)p)->nlmsg_type==NLMSG_DONE||((struct nlmsghdr*)p)->nlmsg_type==NLMSG_ERROR)
      break;
    p += seglen;
  }

  struct nlmsghdr *nh=(struct nlmsghdr*)recvbuf;
  for(;NLMSG_OK(nh,len);nh=NLMSG_NEXT(nh,len)){
    if(nh->nlmsg_type==NLMSG_DONE)
      break;

    struct ifinfomsg *ifm=(struct ifinfomsg*)NLMSG_DATA(nh);

    printf("#%d ",ifm->ifi_index);
    #ifdef _NET_IF_H
    #pragma GCC error "include <linux/if.h> instead of <net/if.h>"
    #endif

    // Part 3 rtattr
    struct rtattr *rta=IFLA_RTA(ifm); // /usr/include/linux/if_link.h
    int rtl=RTM_PAYLOAD(nh);

    for(;RTA_OK(rta,rtl);rta=RTA_NEXT(rta,rtl))switch(rta->rta_type){
    case IFLA_IFNAME:printf("%s ",(const char*)RTA_DATA(rta));break;
    case IFLA_ADDRESS:
      printf("hwaddr ");
      for(int i=0;i<5;++i)
        printf("%02X:",*((unsigned char*)RTA_DATA(rta)+i));
      printf("%02X ",*((unsigned char*)RTA_DATA(rta)+5));
      break;
    case IFLA_BROADCAST:
      printf("bcast ");
      for(int i=0;i<5;++i)
        printf("%02X:",*((unsigned char*)RTA_DATA(rta)+i));
      printf("%02X ",*((unsigned char*)RTA_DATA(rta)+5));
      break;
    case IFLA_PERM_ADDRESS:
      printf("perm ");
      for(int i=0;i<5;++i)
        printf("%02X:",*((unsigned char*)RTA_DATA(rta)+i));
      printf("%02X ",*((unsigned char*)RTA_DATA(rta)+5));
      break;
    }

    printf("\n");

  }

  close(fd);
  fd=-1;
  return 0;

}

Example

#1 lo hwaddr 00:00:00:00:00:00 bcast 00:00:00:00:00:00
#2 eth0 hwaddr 57:da:52:45:5b:1a bcast ff:ff:ff:ff:ff:ff perm 57:da:52:45:5b:1a
#3 wlan0 hwaddr 3c:7f:46:47:58:c2 bcast ff:ff:ff:ff:ff:ff perm 3c:7f:46:47:58:c2

This is a Bash line that prints all available mac addresses, except the loopback:

for x in `ls /sys/class/net |grep -v lo`; do cat /sys/class/net/$x/address; done

Can be executed from a C program.