用C语言使用jansson解析JSON

Question

因此，我一直在用C语言编写一个程序，它可以访问各种加密货币网站的应用程序接口，比如www.cryptsy.com，我已经计算出了cURL部分，并给出了程序将JSON数据输出到一个char*变量中，如下所示：

{"success":"1","return":{"balances_available":{"ALF":"0.00000000","AMC":"0.00000000","ADT":"0.00000000","ANC":"0.00000000","ARG":"0.00000000","ASC":"0.00000000","BQC":"0.00000000","BTB":"0.00000000","BTC":"0.00535673","BTG":"0.00000000","CAP":"0.00000000","BTE":"0.00000000","CSC":"0.00000000","CNC":"0.00000000","COL":"0.00000000","CPR":"0.00000000","CLR":"0.00000000","CMC":"0.00000000","CRC":"0.00000000","BUK":"0.00000000","CGB":"0.00000000","Points":"0.00911000","DVC":"0.00000000","DMD":"0.00000000","DGC":"0.00000000","DBL":"0.00000000","ELC":"0.00000000","ELP":"0.00000000","EMD":"0.00000000","EZC":"0.00000000","FST":"0.00000000","FTC":"0.00000000","FLO":"0.00000000","FRK":"0.00000000","FRC":"0.00000000","GLX":"0.00000000","GME":"0.00000000","GLC":"0.00000000","GLD":"0.00000000","GDC":"0.00000000","HBN":"0.00000000","HYC":"0.00000000","IFC":"0.00000000","IXC":"0.00000000","XJO":"0.00000000","JKC":"0.00000000","KGC":"0.00000000","LTC":"0.00000000","LK7":"0.00000000","LKY":"0.00000000","MST":"0.00000000","MEC":"0.00000000","MEM":"0.00000000","MNC":"0.00000000","NMC":"0.00000000","NAN":"0.00000000","NEC":"0.00000000","NET":"0.00000000","NBL":"0.00000000","NRB":"0.00000000","NVC":"0.00000000","ORB":"0.00000000","PYC":"0.00000000","PPC":"0.00000000","CENT":"0.00000000","PHS":"0.00000000","PXC":"0.00000000","XPM":"0.00000000","PTS":"0.00000000","QRK":"0.00000000","RED":"0.00000000","RYC":"0.00000000","SRC":"0.00000000","SXC":"0.00000000","SPT":"0.00000000","SBC":"0.00000000","STR":"0.00000000","TAG":"0.00000000","TEK":"0.00000000","TRC":"0.00000000","TIX":"0.00000000","WDC":"0.00000000","XNC":"0.00000000","YAC":"0.00000000","ZET":"0.00000000"},"balances_hold":{"ALF":"0.00000000","AMC":"0.00000000","ADT":"0.00000000","ANC":"0.00000000","ARG":"0.00000000","ASC":"0.00000000","BQC":"0.00000000","BTB":"0.00000000","BTC":"0.00000000","BTG":"0.00000000","CAP":"0.00000000","BTE":"0.00000000","CSC":"0.00000000","CNC":"0.00000000","COL":"0.00000000","CPR":"0.00000000","CLR":"0.00000000","CMC":"0.00000000","CRC":"0.00000000","BUK":"0.00000000","CGB":"0.00000000","Points":"0.00000000","DVC":"0.00000000","DMD":"0.00000000","DGC":"0.00000000","DBL":"0.00000000","ELC":"0.00000000","ELP":"0.00000000","EMD":"0.00000000","EZC":"0.00000000","FST":"0.00000000","FTC":"0.00000000","FLO":"0.00000000","FRK":"0.00000000","FRC":"0.00000000","GLX":"0.00000000","GME":"0.00000000","GLC":"0.00000000","GLD":"0.00000000","GDC":"0.00000000","HBN":"0.00000000","HYC":"0.00000000","IFC":"0.00000000","IXC":"0.00000000","XJO":"0.00000000","JKC":"0.00000000","KGC":"0.00000000","LTC":"1.10231195","LK7":"0.00000000","LKY":"10.36912507","MST":"0.00000000","MEC":"0.00000000","MEM":"0.00000000","MNC":"0.00000000","NMC":"0.00000000","NAN":"0.00000000","NEC":"0.00000000","NET":"0.00000000","NBL":"0.00000000","NRB":"0.00000000","NVC":"0.00000000","ORB":"0.00000000","PYC":"0.00000000","PPC":"0.00000000","CENT":"0.00000000","PHS":"0.00000000","PXC":"0.00000000","XPM":"0.00000000","PTS":"0.00000000","QRK":"0.00000000","RED":"0.00000000","RYC":"0.00000000","SRC":"0.00000000","SXC":"0.00000000","SPT":"0.00000000","SBC":"0.00000000","STR":"0.00000000","TAG":"0.00000000","TEK":"0.00000000","TRC":"0.83964122","TIX":"0.00000000","WDC":"0.00000000","XNC":"0.00000000","YAC":"0.00000000","ZET":"0.00000000"},"servertimestamp":1387347714,"servertimezone":"EST","serverdatetime":"2013-12-18 01:21:54","openordercount":3}}

它看起来像是有效的JSON数据，所以我决定尝试使用jansson来进行JSON解析/解码，在我的代码中如下所示：

char *data;

if ( argc < 2 )
{
    fprintf( stderr, "Must provide URL to fetch.\n" );
    return 1;
}
//data = mmpool_api( argv[1] );
data = cryptsy_api( argv[1] );

json_t *root;
json_error_t error;

root = json_loads( data, 0, &error );
if ( !root )
{
    fprintf( stderr, "error: on line %d: %s\n", error.line, error.text );
    return 1;
}
if ( !json_is_array(root) )
{
    fprintf( stderr, "error: root is not an array\n" );
    json_decref(root);
    return 1;
}

for( int i = 0; i < json_array_size(root); i++ )
{
    json_t *data, *success, *returned, *bal_avail;
    const char *balance;

    data = json_array_get( root, i );
    if ( !json_is_object(data) )
    {
        fprintf( stderr, "error: data %d is not an object\n", i + 1 );
        json_decref(root);
        return 1;
    }

    success = json_object_get( data, "success" );
    if ( !json_is_number(success))
    {
        fprintf( stderr, "error: %d: success is not a number", i+1 );
        json_decref(root);
        return 1;
    }

    returned = json_object_get(data, "return" );
    if ( !json_is_object(returned) )
    {
        fprintf( stderr, "error: %d: return is not an object", i+1  );
        json_decref(root);
        return 1;
    }

    bal_avail = json_object_get( returned, "balances_available" );
    if ( !json_is_array( bal_avail ))
    {
        fprintf( stderr, "error: %d: bal_avail is not an array", i+1 );
        json_decref(root);
    }

    balance = json_string_value(bal_avail);
    printf( "%s %.*s\n",
        json_string_value(bal_avail),
        newline_offset(balance),
        balance);
}
json_decref(root);

但是程序在声明root不是一个数组时就失败了，因此它在JSON代码启动后的第二个if语句中终止。我注意到jansson有它自己的“请求(Url)”函数，你可以用它来向服务器请求JSON数据，但是要获得我得到的数据，你需要发送一个密钥/API密钥，并用SHA512对它们进行哈希处理，以获得有效的响应，这个函数似乎没有这个功能。

我知道还有其他用于解码JSON数据的C库，但在切换到JSON-C或其他库之前，我只是想确保这里没有做错什么。谢谢

编辑：：

好的，我尝试将代码更改为只解析“root”对象，但它似乎只通过"return“和"success”键。也许我仍然不完全理解，但这是我现在用来解析所有东西的代码，我读了jansson API指南，它告诉我，如果你想遍历对象中的每个键:值对，就使用迭代器：

char *data;

if ( argc < 2 )
{
    fprintf( stderr, "Must provide URL to fetch.\n" );
    return 1;
}
//data = mmpool_api( argv[1] );
data = cryptsy_api( argv[1] );

json_t *root;
json_error_t error;

root = json_loads( data, 0, &error );
if ( !root )
{
    fprintf( stderr, "error: on line %d: %s\n", error.line, error.text );
    return 1;
}

const char *key;
json_t *value;

void *iter = json_object_iter( root );
while( iter )
{
    key = json_object_iter_key(iter);
    value = json_object_iter_value(iter);

    printf("Key: %s, Value: %f\n", key, json_real_value(value) );

    iter = json_object_iter_next(root, iter);
}

json_decref(root);

但结果是这样的(只有前两项是地址，它们的顺序是相反的)：

Key:返回，取值: 0.000000 Key:成功，取值: 0.000000

Answer 1

JSON字符串的根元素是对象，而不是数组。JSON中的数组用方括号[和]表示，对象用花括号{和}表示。如果您已经知道要访问哪些键，则不需要遍历“object”根元素，只需使用json_object_get(root, "keyname");，直到您访问了所需的所有值。