C的小知识（一）Flexible array

Reference

flexible struct arrary as a memeber of another struct

Is using flexible array memebers in C bad practice?

My summary on Flexible array

What is a flexible array?

/**
 * struct ethtool_sfeatures - command to request change in device's features
 * @cmd: command number = %ETHTOOL_SFEATURES
 * @size: array size of the features[] array
 * @features: feature change masks
 */
struct ethtool_sfeatures {
	__u32	cmd;
	__u32	size;
	struct ethtool_set_features_block features[];
};

struct ethtool_set_features_block features[] is a flexible array.

is NOT a pointer, it is an in place array which is contiguous with the struct.

How to init a flexible array?

struct Header {
    size_t d;
    long v[];
};
typedef struct Header Header;
size_t n = 123; // can dynamically change during program execution
// ...
Header *h = malloc(sizeof(Header) + sizeof(long[n]));
h->n = n;

Or:

Header *h = malloc(sizeof *h + n * sizeof h->v[0]);

Note that sizeof(Header) includes eventual padding bytes, thus, the following allocation is incorrect and may yield a buffer overflow:

Header *h = malloc(sizeof(size_t) + sizeof(long[n])); // invalid!

My example code

#include <errno.h>
#include <linux/ethtool.h>
#include <linux/if.h>
#include <linux/sockios.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <unistd.h>

#define ETHTOOL_DEV_FEATURE_WORDS 2

static void ethnl_features_to_bitmap32(__u32 *dest, __u64 src) {
  unsigned int i;

  for (i = 0; i < ETHTOOL_DEV_FEATURE_WORDS; i++) {
    dest[i] = src >> (32 * i);
  }
}

static int init_struct() {
  int sockfd;
  struct ifreq ifr;
  // struct ethtool_sfeatures cmd;
  struct ethtool_sfeatures *cmd =
      malloc(sizeof *cmd + ETHTOOL_DEV_FEATURE_WORDS * sizeof cmd->features[0]);

  if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
    printf("socket error\n");
    return -1;
  }
  strcpy(ifr.ifr_name, "net0");
  printf("printf ifr_name: %s \n", ifr.ifr_name);
  cmd->cmd = ETHTOOL_SFEATURES;
  cmd->size = ETHTOOL_DEV_FEATURE_WORDS;
  __u32 valid[ETHTOOL_DEV_FEATURE_WORDS];
  __u64 features = (1UL << 9);
  ethnl_features_to_bitmap32(valid, features);
  for (int i = 0; i < ETHTOOL_DEV_FEATURE_WORDS; i++) {
    cmd->features[i].valid = valid[i];
    cmd->features[i].requested = 0;
  }
  printf("printf ifr_name: %s \n", ifr.ifr_name);
  ifr.ifr_data = (char *)cmd;
  printf("printf ifr_name: %s \n", ifr.ifr_name);
  if (ioctl(sockfd, SIOCETHTOOL, &ifr) < 0) {
    printf("ioctl error: %d\n", errno);
    close(sockfd);
    free(cmd);
    return -1;
  }
  close(sockfd);
  free(cmd);
  return -1;
}

static int get_features() {
  int sockfd;
  struct ifreq ifr;
  struct ethtool_gfeatures *cmd =
      malloc(sizeof *cmd + ETHTOOL_DEV_FEATURE_WORDS * sizeof cmd->features[0]);

  if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
    printf("socket error\n");
    return -1;
  }
  strcpy(ifr.ifr_name, "enp2s0");

  cmd->cmd = ETHTOOL_GFEATURES;
  cmd->size = ETHTOOL_DEV_FEATURE_WORDS;
  ifr.ifr_data = (char *)cmd;

  if (ioctl(sockfd, SIOCETHTOOL, &ifr) < 0) {
    printf("ioctl error: %d\n", errno);
    close(sockfd);
    free(cmd);
    return -1;
  }
  close(sockfd);
  free(cmd);
  return -1;
}
int main(int argc, char *argv[]) {
  init_struct();
  // get_features();
  return EXIT_SUCCESS;
}