linux libnftables export rules

Question

how to use :

# man 3 libnftables
const char *nft_ctx_get_output_buffer(struct nft_ctx *ctx);

I want save nftables rules (without print on console). but nft_run_cmd_from_buffer immediately print rules on console .

this is my example code :

#include <stdlib.h>
#include <nftables/libnftables.h>

int main(void)
{
    struct nft_ctx *ctx;
    int err;

    ctx = nft_ctx_new(0);
    if (!ctx) {
        perror("cannot allocate nft context");
        return EXIT_FAILURE;
    }

    nft_ctx_output_set_flags(ctx, NFT_CTX_OUTPUT_HANDLE | NFT_CTX_OUTPUT_JSON);

    const char* output = nft_ctx_get_output_buffer(ctx);
    err = nft_run_cmd_from_buffer(ctx, "list ruleset");
    if (err < 0)
        fprintf(stderr, "failed to run nftables command\n");

    nft_ctx_free(ctx);

    printf ("-----------------------------------------------------\n");
    printf(">> %s\n",output);
 
    return EXIT_SUCCESS;
}

Answer 1

Thank you , finally fixed the problem :
According to manual page :

At the very basic level, one has to allocate a new object of type struct nft_ctx using nft_ctx_new() function, then pass commands via nft_run_cmd_from_buffer() or nft_run_cmd_from_filename() functions. By default, any output is written to stdout (or stderr for error messages). These file pointers may be changed using nft_ctx_set_output() and nft_ctx_set_error() functions. On top of that, it is possible to have any output buffered by the library for later retrieval as a static buffer. See nft_ctx_buffer_output() and nft_ctx_buffer_error() functions for details.

so code is :

#include <stdio.h>
#include <stdlib.h>
#include <nftables/libnftables.h>

int main() {
  struct nft_ctx *ctx;
  int err;
  const char *output;

  ctx = nft_ctx_new(0);
  if (!ctx) {
      perror("cannot allocate nft context");
      return EXIT_FAILURE;
  }
 
  nft_ctx_output_set_flags(ctx, NFT_CTX_OUTPUT_HANDLE | NFT_CTX_OUTPUT_JSON);
  nft_ctx_buffer_output(ctx);

  err = nft_run_cmd_from_buffer(ctx, "list ruleset");
  if (err < 0)
     fprintf(stderr, "failed to run nftables command\n");

  output = nft_ctx_get_output_buffer(ctx); 
  if (output != NULL) {
     printf("Output: %s\n", output);
  }

  nft_ctx_free(ctx);
  return EXIT_SUCCESS;
}