C example code to list all network interfaces and check whether they are wireless or not. Linux only. Tested on Ubuntu 10.04 and 12.04, but should be compatible with pretty much everything out there.

is_wireless.c

#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <ifaddrs.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <linux/wireless.h>


int check_wireless(const char* ifname, char* protocol) {
  int sock = -1;
  struct iwreq pwrq;
  memset(&pwrq, 0, sizeof(pwrq));
  strncpy(pwrq.ifr_name, ifname, IFNAMSIZ);

  if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
    perror("socket");
    return 0;
  }

  if (ioctl(sock, SIOCGIWNAME, &pwrq) != -1) {
    if (protocol) strncpy(protocol, pwrq.u.name, IFNAMSIZ);
    close(sock);
    return 1;
  }

  close(sock);
  return 0;
}


int main(int argc, char const *argv[]) {
  struct ifaddrs *ifaddr, *ifa;

  if (getifaddrs(&ifaddr) == -1) {
    perror("getifaddrs");
    return -1;
  }

  /* Walk through linked list, maintaining head pointer so we
     can free list later */
  for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
    char protocol[IFNAMSIZ]  = {0};

    if (ifa->ifa_addr == NULL ||
        ifa->ifa_addr->sa_family != AF_PACKET) continue;

    if (check_wireless(ifa->ifa_name, protocol)) {
      printf("interface %s is wireless: %s\n", ifa->ifa_name, protocol);
    } else {
      printf("interface %s is not wireless\n", ifa->ifa_name);
    }
  }

  freeifaddrs(ifaddr);
  return 0;
}

What's the problem with

int check_wireless(const char* ifname)
{
    char path[256] = {};
    snprintf(path, 255, "/sys/class/net/%s/wireless", ifname);
    return access(path, F_OK) == 0;
}

@32th-System this code is from 12 years ago. This didn't exist for Ubuntu 10.04 or 12.04, which what I has using at the time.