parsing YAML to values with libyaml in C

Question

I'm trying to parse a YAML file using C on a linux server (it's a modification to an existing app, so changing the language is not an option).

I have read through the tutorial at http://wpsoftware.net/andrew/pages/libyaml.html and on the libyaml wiki.

What I want to do is move the database configuration of this app out of a header file and into YAML so that I can compile and configure as separate steps, allowing me to use Chef to manage the configuration.

Here is the yaml:

---
db_server: "localhost"
db_password: "wibble"
db_username: "test"
national_rail_username: test
national_rail_password: wibble

And what I want to do is iterate over the file and set variables based on the keyname.

The psuedo-code is as follows:

config = YAML::load("file.yaml")
DBUSER = config['db_username']
DBPASS = config['db_password']
DBSERVER = config['db_server']
NATIONAL_RAIL_USERNAME = config['national_rail_username']
NATIONAL_RAIL_PASSWORD = config['national_rail_password']

(if the above looks a bit like ruby/python, that's because I'm used to using those languages!)

I managed to get a test setup working using YAML-cpp, then I realized that I'd been barking up the wrong tree for three hours because the primary app is written in C, not C++.

EDIT: Here is the code I have so far. It is a cut and paste from the tutorial website above, however I do not believe it is the correct approach and it does not appear provide me with a way to allocate YAML "keys" to variables in the C code.

#include <stdio.h>
#include <yaml.h>

int main(void)
{
  FILE *fh = fopen("config.yaml", "r");
  yaml_parser_t parser;
  yaml_token_t  token;   /* new variable */

  /* Initialize parser */
  if(!yaml_parser_initialize(&parser))
    fputs("Failed to initialize parser!\n", stderr);
  if(fh == NULL)
    fputs("Failed to open file!\n", stderr);

  /* Set input file */
  yaml_parser_set_input_file(&parser, fh);

  /* BEGIN new code */
  do {
    yaml_parser_scan(&parser, &token);
    switch(token.type)
    {
    /* Stream start/end */
    case YAML_STREAM_START_TOKEN: puts("STREAM START"); break;
    case YAML_STREAM_END_TOKEN:   puts("STREAM END");   break;
    /* Token types (read before actual token) */
    case YAML_KEY_TOKEN:   printf("(Key token)   "); break;
    case YAML_VALUE_TOKEN: printf("(Value token) "); break;
    /* Block delimeters */
    case YAML_BLOCK_SEQUENCE_START_TOKEN: puts("<b>Start Block (Sequence)</b>"); break;
    case YAML_BLOCK_ENTRY_TOKEN:          puts("<b>Start Block (Entry)</b>");    break;
    case YAML_BLOCK_END_TOKEN:            puts("<b>End block</b>");              break;
    /* Data */
    case YAML_BLOCK_MAPPING_START_TOKEN:  puts("[Block mapping]");            break;
    case YAML_SCALAR_TOKEN:  printf("scalar %s \n", token.data.scalar.value); break;
    /* Others */
    default:
      printf("Got token of type %d\n", token.type);
    }
    if(token.type != YAML_STREAM_END_TOKEN)
      yaml_token_delete(&token);
  } while(token.type != YAML_STREAM_END_TOKEN);
  yaml_token_delete(&token);
  /* END new code */

  /* Cleanup */
  yaml_parser_delete(&parser);
  fclose(fh);
  return 0;
}

Show us the relevant code you have written so far, and explain what is the actual problem with it. — user694733, Dec 17 '13 at 07:26

score 5 · Answer 1 · answered Dec 17 '13 at 08:24

I took a chunk of example code from that tutorial and hacked on it. Caveat emptor, it's some time since I wrote C last!

I used the token API, the event API actually looks simpler.

#include <stdio.h>
#include <yaml.h>

typedef struct Conf {
  char* db_server;
  char* db_pass;
  char* db_user;
  char* rail_user;
  char* rail_pass;
}

Conf* readConf(char* filename) {
  FILE* fh = fopen(filename, "r");
  yaml_parser_t parser;
  yaml_token_t token;
  Conf* conf = malloc(sizeof(Conf));

  if (!yaml_parser_initialize(&parser))
    fputs("Failed to initialize parser!\n", stderr);
  if (fh == NULL)
    fputs("Failed to open file!\n", stderr);
  yaml_parser_set_input_file(&parser, fh);

  do {
    /* As this is an example, I'll just use:
     *  state = 0 = expect key
     *  state = 1 = expect value
     */
    int state = 0;
    char** datap;
    char* tk;

    yaml_parser_scan(&parser, &token);
    switch(token.type)
    {
    case YAML_KEY_TOKEN:     state = 0; break;
    case YAML_VALUE_TOKEN:   state = 1; break;
    case YAML_SCALAR_TOKEN:
      tk = token.data.scalar.value;
      if (state == 0) {
        /* It's safe to not use strncmp as one string is a literal */
        if (!strcmp(tk, "db_server")) {
          datap = &conf.db_server;
        } else if (!strcmp(tk, "db_password")) {
          datap = &conf.db_pass;
        } else if (!strcmp(tk, "db_username")) {
          datap = &conf.db_user;
        } else if (!strcmp(tk, "national_rail_username")) {
          datap = &conf.rail_user;
        } else if (!strcmp(tk, "national_rail_password")) {
          datap = &conf.rail_pass;
        } else {
          printf("Unrecognised key: %s\n", tk);
        }
      } else {
        *datap = strdup(tk);
      }
      break;
    default: break;
    }
    if (token.type != YAML_STREAM_END_TOKEN)
      yaml_token_delete(&token);
  } while (token.type != YAML_STREAM_END_TOKEN);
  yaml_token_delete(&token);

  yaml_parser_delete(&parser);
  fclose(fh);
  return conf;
}

Would probably be better to use events rather than tokens - produces more straight-forward output - particularly when yaml files become more complex. — cirrusio, Jun 19 '18 at 05:11

mrzahrada · Answer 2 · 2015-10-29T09:39:00.710

I thing that in lxs answer should be

int state = 0;
char** datap;
char* tk;

before do-while cycle and &conf->key instead of &conf.key.

there is code, which works for me:

#include <stdio.h>
#include <string.h>
#include <yaml.h>

typedef struct Conf {
    char* db_server;
    char* db_pass;
    char* db_user;
    char* rail_user;
    char* rail_pass;
}

Conf* readConf(char* filename) {
    FILE* fh = fopen(filename, "r");
    yaml_parser_t parser;
    yaml_token_t token;
    Conf* conf = malloc(sizeof(Conf));

    if (!yaml_parser_initialize(&parser))
        fputs("Failed to initialize parser!\n", stderr);
    if (fh == NULL)
        fputs("Failed to open file!\n", stderr);
    yaml_parser_set_input_file(&parser, fh);


    /* As this is an example, I'll just use:
     *  state = 0 = expect key
     *  state = 1 = expect value
     */
    int state = 0;
    char** datap;
    char* tk;

    do {
        yaml_parser_scan(&parser, &token);
        switch(token.type) {
            case YAML_KEY_TOKEN:     state = 0; break;
            case YAML_VALUE_TOKEN:   state = 1; break;
            case YAML_SCALAR_TOKEN:
                tk = token.data.scalar.value;
                if (state == 0) {
                    /* It's safe to not use strncmp as 
                       one string is a literal */
                    if (!strcmp(tk, "db_server")) {
                        datap = &conf->db_server;
                    } else if (!strcmp(tk, "db_password")) {
                        datap = &conf->db_pass;
                    } else if (!strcmp(tk, "db_username")) {
                        datap = &conf->db_user;
                    } else if (!strcmp(tk, "national_rail_username")) {
                        datap = &conf->rail_user;
                    } else if (!strcmp(tk, "national_rail_password")) {
                        datap = &conf->rail_pass;
                    } else {
                        printf("Unrecognised key: %s\n", tk);
                    }
                } else {
                      *datap = strdup(tk);
                }
                break;
           default: break;
           }
       if (token.type != YAML_STREAM_END_TOKEN)
           yaml_token_delete(&token);
   } while (token.type != YAML_STREAM_END_TOKEN);

   yaml_token_delete(&token);
   yaml_parser_delete(&parser);
   fclose(fh);
   return conf;
 }

parsing YAML to values with libyaml in C

2 Answers2