Simple JSON parser in C. Parse until find the values you want. Allocate just the values you want.

json_parser.c

#ifndef _JSON_PARSER_H_
#define _JSON_PARSER_H_

#include <stdlib.h>
#include <string.h>
#include <ctype.h>

#ifndef JSON_ALLOC
#define JSON_ALLOC malloc
#endif

char *json_get(const char *json, const char *key) {
	unsigned int i, size, key_size;
	int start = 0, end = 0;
	const char *_key = NULL;
	
	if (json == NULL) return NULL;
	if (key == NULL) return NULL;
	
	size = strlen(json);
	key_size = strlen(key);
	
	for (i=0; i<size; i++) {
		char c = json[i];
		
		if (c == '\"') {
			start = i + 1;
			
			for (i=i+1; i<size; i++) {
				c = json[i];
				
				if (c == '\"' && json[i-1] != '\\') break;
			}
			
			_key = json + start;
		}
		else if (c == ':') {
			
			for (i=i+1; i<size; i++) {
				c = json[i];
				
				if (c == '\"') {
					start = i + 1;
					
					for (i=i+1; i<size; i++) {
						c = json[i];
						
						if (c == '\"' && json[i-1] != '\\') break;
					}
					
					end = i - start;
					break;
				}
				else if (c == '[') {
					int braces = 0;
					int brackets = 0;
					char instring = 0;
					start = i;
					
					for (i=i+1; i<size; i++) {
						c = json[i];
						
						if (c == '\"' && json[i-1] != '\\') instring = !instring;
						
						if (instring) continue;
						
						if (c == '[') brackets++;
						else if (c == ']') brackets--;
						else if (c == '{') braces++;
						else if (c == '}') braces--;
						
						if (braces > 0) continue;
						if (brackets > 0) continue;
						
						if (brackets < 0) break;
					}
					
					end = i - start + 1;
					break;
				}
				else if (c == '{') {
					int braces = 0;
					int brackets = 0;
					char instring = 0;
					start = i;
					
					for (i=i+1; i<size; i++) {
						c = json[i];
						
						if (c == '\"' && json[i-1] != '\\') instring = !instring;
						
						if (instring) continue;
						
						if (c == '[') brackets++;
						else if (c == ']') brackets--;
						else if (c == '{') braces++;
						else if (c == '}') braces--;
						
						if (braces > 0) continue;
						if (brackets > 0) continue;
						
						if (braces < 0) break;
					}
					
					end = i - start + 1;
					break;
				}
				else if (!isspace(c)) {
					start = i;
					
					for (i=i+1; i<size; i++) {
						c = json[i];
						
						if (isspace(c)) break;
						if (c == ',') break;
						if (c == '}') break;
					}
					
					end = i - start;
					break;
				}
			}
			
			if (_key && strncmp(_key, key, key_size) == 0) {
				char *value = JSON_ALLOC(end + 1);
				if (value == NULL) return NULL;
				
				strncpy(value, json + start, end);
				value[end] = '\0';
				
				return value;
			}
		}
	}
	
	return NULL;
}

int array_length(const char *array) {
	unsigned int i, size, count;
	int brackets, braces;
	char instring;
	
	if (array == NULL) return -1;
	
	size = strlen(array);
	count = 0;
	braces = 0;
	brackets = -1;
	instring = 0;
	
	for (i=0; i<size; i++) {
		char c = array[i];
		
		if (count == 0 && c != '[' && c != ']' && !isspace(c)) count++;
		if (c == '\"' && array[i-1] != '\\') instring = !instring;
		
		if (instring) continue;
		
		if (c == '[') brackets++;
		else if (c == ']') brackets--;
		else if (c == '{') braces++;
		else if (c == '}') braces--;
		
		if (brackets > 0) continue;
		if (braces > 0) continue;
		
		if (c == ',') count++;
	}
	
	return count;
}

char *array_get(const char *array, unsigned int index) {
	unsigned int i, size, count;
	int start = 0, end = 0;
	char *value;
	
	if (array == NULL) return NULL;
	
	size = strlen(array);
	count = 0;
	
	for (i=0; i<size; i++) {
		char c = array[i];
		
		if (c == '[') break;
	}
	
	for (i=i+1; i<size; i++) {
		char c = array[i];
		
		if (c == '\"') {
			start = i + 1;
			
			for (i=i+1; i<size; i++) {
				c = array[i];
				
				if (c == '\"' && array[i-1] != '\\') break;
			}
			
			end = i - start;
			if (count == index) break;
			count++;
		}
		else if (c == '[') {
			int braces = 0;
			int brackets = 0;
			char instring = 0;
			start = i;
			
			for (i=i+1; i<size; i++) {
				c = array[i];
				
				if (c == '\"' && array[i-1] != '\\') instring = !instring;
				
				if (instring) continue;
				
				if (c == '[') brackets++;
				else if (c == ']') brackets--;
				else if (c == '{') braces++;
				else if (c == '}') braces--;
				
				if (brackets < 0) break;
			}
			
			end = i - start + 1;
			if (count == index) break;
			count++;
		}
		else if (c == '{') {
			int braces = 0;
			int brackets = 0;
			char instring = 0;
			start = i;
			
			for (i=i+1; i<size; i++) {
				c = array[i];
				
				if (c == '\"' && array[i-1] != '\\') instring = !instring;
				
				if (instring) continue;
				
				if (c == '[') brackets++;
				else if (c == ']') brackets--;
				else if (c == '{') braces++;
				else if (c == '}') braces--;
				
				if (braces < 0) break;
			}
			
			end = i - start + 1;
			if (count == index) break;
			count++;
		}
		else if (c != ',' && c != ']' && !isspace(c)) {
			start = i;
			
			for (i=i+1; i<size; i++) {
				c = array[i];
				
				if (c == ']') break;
				if (c == ',') break;
				if (isspace(c)) break;
			}
			
			end = i - start;
			if (count == index) break;
			count++;
		}
	}
	
	if (i == size) count--;
	if (index != count) return NULL;
	
	value = JSON_ALLOC(end + 1);
	if (value == NULL) return NULL;
	
	strncpy(value, array + start, end);
	value[end] = '\0';
	
	return value;
}
#endif

/*///////////////////////////////////
	
		Testing:
		
///////////////////////////////////*/

#include <stdio.h>

int main(void) {
	const char *jsonString = "{"
							"  \"name\": \"John Doe\","
							"  \"age\": 25,"
							"  \"height\": 1.75,"
							"  \"single\": true,"
							"  \"colors\": [\"red\", \"green\", \"blue\"],"
							"  \"address\": {"
							"    \"street\": \"123 Main St\","
							"    \"city\": \"Example City\","
							"    \"state\": \"EX\""
							"  },"
							"  \"friends\": ["
							"    {\"name\": \"Friend 1\", \"age\": 28},"
							"    {\"name\": \"Friend 2\", \"age\": 30}"
							"  ]"
							"}";

	char *name = json_get(jsonString, "name");
	char *age = json_get(jsonString, "age");
	char *height = json_get(jsonString, "height");
	char *single = json_get(jsonString, "single");
	char *colors = json_get(jsonString, "colors");
	char *address = json_get(jsonString, "address");
	char *friends = json_get(jsonString, "friends");

	printf("name => %s\n", name);
	printf("age => %i\n", atoi(age));
	printf("height => %.2f\n", atof(height));
	printf("single => %s\n", single);
	printf("colors => %s\n", colors);
	printf("address => %s\n", address);
	printf("friends => %s\n", friends);
	printf("\n");

	int i, size;

	size = array_length(colors);
	for (i = 0; i < size; i++) {
		char *color = array_get(colors, i);
		printf("colors[%i] = %s\n", i, color);
		free(color);
	}
	printf("\n");

	char *street = json_get(address, "street");
	char *city = json_get(address, "city");
	char *state = json_get(address, "state");

	printf("street => %s\n", street);
	printf("city => %s\n", city);
	printf("state => %s\n", state);
	printf("\n");

	size = array_length(friends);
	for (i = 0; i < size; i++) {
		char *friend = array_get(friends, i);
		char *friendName = json_get(friend, "name");
		char *friendAge = json_get(friend, "age");

		printf("friends[%i].name = %s\n", i, friendName);
		printf("friends[%i].age = %i\n", i, atoi(friendAge));
		
		free(friend);
		free(friendName);
		free(friendAge);
	}
	
	free(name);
	free(age);
	free(height);
	free(single);
	free(colors);
	free(address);
	free(friends);
	free(street);
	free(city);
	free(state);
	
	return 0;
}