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How to parse a small JSON file with jsmn on the embedded system

I need to parse a small JSON file in the embedded system (only 10K RAM / flash), and JSON looks like this:

{ "data1":[1,2,3,4,5,6,7,8,9], "data2":[ [3,4,5,6,1], [8,4,5,6,1], [10,4,5,3,61], [3,4,5,6,1], [3,4,5,6,1], [3,4,5,6,1] ]}

jsmn is great for this requirement, but it doesn't look like most JSON parsers, as it only gives you tokens. I tried, but could not understand.

Can someone share an example to analyze it using jsmn?

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jsmn will provide you with a set of tokens that reference tokens in your JSON reading from left to right.

In your case:

 token[0]: (outer) object, 2 children token[1]: string token ("data1") token[2]: array, 9 children token[3]: primitive token (1) etc...

The main code for parsing:

 int resultCode; jsmn_parser p; jsmntok_t tokens[128]; // a number >= total number of tokens jsmn_init(&p); resultCode = jsmn_parse(&p, yourJson, tokens, 256);

Another trick gets the value of the token. The token contains the start and end points of the data in the source string.

 jsmntok_t key = tokens[1]; unsigned int length = key.end - key.start; char keyString[length + 1]; memcpy(keyString, &yourJson[key.start], length); keyString[length] = '\0'; printf("Key: %s\n", keyString);

With this, you can understand how to iterate over the data.

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You can use tiny-json . It gives you more than tokens. I used 16 bits and 32 bits with microcontrollers, and it works great.

 char str[] = "{" "\"data1\":[1,2,3,4,5,6,7,8,9]," "\"data2\":[" "[3,4,5,6,1]," "[8,4,5,6,1]," "[10,4,5,3,61]," "[3,4,5,6,1]," "[3,4,5,6,1]," "[3,4,5,6,1]" "]" "}"; puts( str ); json_t pool[64]; json_t const* root = json_create( str, pool, 64 ); json_t const* data1 = json_getProperty( root, "data1" ); if ( data1 && JSON_ARRAY == json_getType( data1 ) ) { json_t const* field = json_getChild( data1 ); while( field ) { if ( JSON_INTEGER == json_getType( field ) ) { long long int data = json_getInteger( field ); printf("Integer from data1: %lld\n", data ); } field = json_getSibling( field ); } } json_t const* data2 = json_getProperty( root, "data2" ); if ( data2 && JSON_ARRAY == json_getType( data2 ) ) { json_t const* array = json_getChild( data2 ); while( array ) { if ( JSON_ARRAY == json_getType( array ) ) { puts("Array in data2"); json_t const* field = json_getChild( array ); while( field ) { if ( JSON_INTEGER == json_getType( field ) ) { long long int data = json_getInteger( field ); printf("Integer from array of data2: %lld\n", data ); } field = json_getSibling( field ); } } array = json_getSibling( array ); } }
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I was curious about this too, so I made a small program to show the structure of the token array:

This program:

 #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <string.h> #include "jsmn.h" jsmntok_t t[512]; char jsonStr[512*1024]; int main() { size_t pos = 0; int c; while ((c = getchar()) != EOF) { jsonStr[pos] = c; ++pos; if (pos == sizeof(jsonStr)) break; } jsonStr[pos] = 0; jsmn_parser p; jsmn_init(&p); int tCount = jsmn_parse(&p, jsonStr, strlen(jsonStr), t, sizeof(t)/sizeof(*t)); for (int i = 0; i != tCount; ++i) { jsmntok_t *token = t + i; char * type = 0; switch (token->type) { case JSMN_PRIMITIVE: type = "PRIMITIVE"; break; case JSMN_OBJECT: type = "OBJECT"; break; case JSMN_ARRAY: type = "ARRAY"; break; case JSMN_STRING: type = "STRING"; break; default: type = "UNDEF"; } #ifdef JSMN_PARENT_LINKS printf("node: %4d, %9s, parent: %4d, children: %5d, data:\n%.*s, \n", i, type, token->parent, token->size, token->end-token->start, jsonStr+token->start); #else printf("node: %4d, %9s, children: %5d, data:\n%.*s, \n", i, type, token->size, token->end-token->start, jsonStr+token->start); #endif } }

prints this json (taken from https://en.wikipedia.org/wiki/JSON ):

 { "firstName": "John", "lastName": "Smith", "isAlive": true, "age": 25, "address": { "streetAddress": "21 2nd Street", "city": "New York", "state": "NY", "postalCode": "10021-3100" }, "phoneNumbers": [ { "type": "home", "number": "212 555-1234" }, { "type": "office", "number": "646 555-4567" }, { "type": "mobile", "number": "123 456-7890" } ], "children": [], "spouse": null }

as:

 node: 0, OBJECT, parent: -1, children: 8, data: { "firstName": "John", "lastName": "Smith", "isAlive": true, "age": 25, "address": { "streetAddress": "21 2nd Street", "city": "New York", "state": "NY", "postalCode": "10021-3100" }, "phoneNumbers": [ { "type": "home", "number": "212 555-1234" }, { "type": "office", "number": "646 555-4567" }, { "type": "mobile", "number": "123 456-7890" } ], "children": [], "spouse": null }, node: 1, STRING, parent: 0, children: 1, data: firstName, node: 2, STRING, parent: 1, children: 0, data: John, node: 3, STRING, parent: 0, children: 1, data: lastName, node: 4, STRING, parent: 3, children: 0, data: Smith, node: 5, STRING, parent: 0, children: 1, data: isAlive, node: 6, PRIMITIVE, parent: 5, children: 0, data: true, node: 7, STRING, parent: 0, children: 1, data: age, node: 8, PRIMITIVE, parent: 7, children: 0, data: 25, node: 9, STRING, parent: 0, children: 1, data: address, node: 10, OBJECT, parent: 9, children: 4, data: { "streetAddress": "21 2nd Street", "city": "New York", "state": "NY", "postalCode": "10021-3100" }, node: 11, STRING, parent: 10, children: 1, data: streetAddress, node: 12, STRING, parent: 11, children: 0, data: 21 2nd Street, node: 13, STRING, parent: 10, children: 1, data: city, node: 14, STRING, parent: 13, children: 0, data: New York, node: 15, STRING, parent: 10, children: 1, data: state, node: 16, STRING, parent: 15, children: 0, data: NY, node: 17, STRING, parent: 10, children: 1, data: postalCode, node: 18, STRING, parent: 17, children: 0, data: 10021-3100, node: 19, STRING, parent: 0, children: 1, data: phoneNumbers, node: 20, ARRAY, parent: 19, children: 3, data: [ { "type": "home", "number": "212 555-1234" }, { "type": "office", "number": "646 555-4567" }, { "type": "mobile", "number": "123 456-7890" } ], node: 21, OBJECT, parent: 20, children: 2, data: { "type": "home", "number": "212 555-1234" }, node: 22, STRING, parent: 21, children: 1, data: type, node: 23, STRING, parent: 22, children: 0, data: home, node: 24, STRING, parent: 21, children: 1, data: number, node: 25, STRING, parent: 24, children: 0, data: 212 555-1234, node: 26, OBJECT, parent: 20, children: 2, data: { "type": "office", "number": "646 555-4567" }, node: 27, STRING, parent: 26, children: 1, data: type, node: 28, STRING, parent: 27, children: 0, data: office, node: 29, STRING, parent: 26, children: 1, data: number, node: 30, STRING, parent: 29, children: 0, data: 646 555-4567, node: 31, OBJECT, parent: 20, children: 2, data: { "type": "mobile", "number": "123 456-7890" }, node: 32, STRING, parent: 31, children: 1, data: type, node: 33, STRING, parent: 32, children: 0, data: mobile, node: 34, STRING, parent: 31, children: 1, data: number, node: 35, STRING, parent: 34, children: 0, data: 123 456-7890, node: 36, STRING, parent: 0, children: 1, data: children, node: 37, ARRAY, parent: 36, children: 0, data: [], node: 38, STRING, parent: 0, children: 1, data: spouse, node: 39, PRIMITIVE, parent: 38, children: 0, data: null,
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