cJson使用的一些问题
今天使用cJson，MCU出现周期性重启的情况，初步判定是内存泄漏，查找自己的代码，malloc的空间都是free掉的，一时间没有思路。最终找到是因为cJson中的问题。
//有格式打印：extern char *cJSON_Print(cJSON *item);
//无格式打印：extern char *cJSON_PrintUnformatted(cJSON *item);char *json_str;
json_str = cJSON_Print(root);以上语句会动态分配空间，因为cJson不知道用户何时需要释放，所以这种情况下，设计为用户自己free掉堆中的空间；
查看源代码如下：
extern char *cJSON_Print(cJSON *item);
/* Render a cJSON entity to text for transfer/storage without any formatting. Free the char* when finished. */
char *cJSON_Print(cJSON *item) {return print_value(item,0,1,0);}
/* Render a value to text. */
static char *print_value(cJSON *item,int depth,int fmt,printbuffer *p)
{
char *out=0;
if (!item) return 0;
if (p)
{
switch ((item->type)&255)
{
case cJSON_NULL: {out=ensure(p,5); if (out) strcpy(out,"null"); break;}
case cJSON_False: {out=ensure(p,6); if (out) strcpy(out,"false"); break;}
case cJSON_True: {out=ensure(p,5); if (out) strcpy(out,"true"); break;}
case cJSON_Number: out=print_number(item,p);break;
case cJSON_String: out=print_string(item,p);break;
case cJSON_Array: out=print_array(item,depth,fmt,p);break;
case cJSON_Object: out=print_object(item,depth,fmt,p);break;
}
}
else
{
switch ((item->type)&255)
{
case cJSON_NULL: out=cJSON_strdup("null"); break;
case cJSON_False: out=cJSON_strdup("false");break;
case cJSON_True: out=cJSON_strdup("true"); break;
case cJSON_Number: out=print_number(item,0);break;
case cJSON_String: out=print_string(item,0);break;
case cJSON_Array: out=print_array(item,depth,fmt,0);break;
case cJSON_Object: out=print_object(item,depth,fmt,0);break;
}
}
return out;
}static void *(*cJSON_malloc)(size_t sz) = malloc;
static void (*cJSON_free)(void *ptr) = free;
static char* cJSON_strdup(const char* str)
{
size_t len;
char* copy;
len = strlen(str) + 1;
if (!(copy = (char*)cJSON_malloc(len))) return 0;
memcpy(copy,str,len);
return copy;
}可以看出，确实与预想中一致；所以，添加下面的语句，解决内存泄漏的问题；
if(NULL != json_str)
free(json_str);下面是cJson的使用示例（来自官网）
#include
#include
#include "cJSON.h"
/* Parse text to JSON, then render back to text, and print! */
void doit(char *text)
{
char *out;cJSON *json;
json=cJSON_Parse(text);
if (!json) {printf("Error before: [%s]\n",cJSON_GetErrorPtr());}
else
{
out=cJSON_Print(json);
cJSON_Delete(json);
printf("%s\n",out);
free(out);
}
}
/* Read a file, parse, render back, etc. */
void dofile(char *filename)
{
FILE *f;long len;char *data;
f=fopen(filename,"rb");fseek(f,0,SEEK_END);len=ftell(f);fseek(f,0,SEEK_SET);
data=(char*)malloc(len+1);fread(data,1,len,f);fclose(f);
doit(data);
free(data);
}
/* Used by some code below as an example datatype. */
struct record {const char *precision;double lat,lon;const char *address,*city,*state,*zip,*country; };
/* Create a bunch of objects as demonstration. */
void create_objects()
{
cJSON *root,*fmt,*img,*thm,*fld;char *out;int i; /* declare a few. */
/* Our "days of the week" array: */
const char *strings[7]={"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
/* Our matrix: */
int numbers[3][3]={{0,-1,0},{1,0,0},{0,0,1}};
/* Our "gallery" item: */
int ids[4]={116,943,234,38793};
/* Our array of "records": */
struct record fields[2]={
{"zip",37.7668,-1.223959e+2,"","SAN FRANCISCO","CA","94107","US"},
{"zip",37.371991,-1.22026e+2,"","SUNNYVALE","CA","94085","US"}};
/* Here we construct some JSON standards, from the JSON site. */
/* Our "Video" datatype: */
root=cJSON_CreateObject();
cJSON_AddItemToObject(root, "name", cJSON_CreateString("Jack (\"Bee\") Nimble"));
cJSON_AddItemToObject(root, "format", fmt=cJSON_CreateObject());
cJSON_AddStringToObject(fmt,"type", "rect");
cJSON_AddNumberToObject(fmt,"width", 1920);
cJSON_AddNumberToObject(fmt,"height", 1080);
cJSON_AddFalseToObject (fmt,"interlace");
cJSON_AddNumberToObject(fmt,"frame rate", 24);
out=cJSON_Print(root); cJSON_Delete(root); printf("%s\n",out); free(out); /* Print to text, Delete the cJSON, print it, release the string. */
/* Our "days of the week" array: */
root=cJSON_CreateStringArray(strings,7);
out=cJSON_Print(root); cJSON_Delete(root); printf("%s\n",out); free(out);
/* Our matrix: */
root=cJSON_CreateArray();
for (i=0;i<3;i++) cJSON_AddItemToArray(root,cJSON_CreateIntArray(numbers[i],3));
/* cJSON_ReplaceItemInArray(root,1,cJSON_CreateString("Replacement")); */
out=cJSON_Print(root); cJSON_Delete(root); printf("%s\n",out); free(out);
/* Our "gallery" item: */
root=cJSON_CreateObject();
cJSON_AddItemToObject(root, "Image", img=cJSON_CreateObject());
cJSON_AddNumberToObject(img,"Width",800);
cJSON_AddNumberToObject(img,"Height",600);
cJSON_AddStringToObject(img,"Title","View from 15th Floor");
cJSON_AddItemToObject(img, "Thumbnail", thm=cJSON_CreateObject());
cJSON_AddStringToObject(thm, "Url", "http:/*www.example.com/image/481989943");
cJSON_AddNumberToObject(thm,"Height",125);
cJSON_AddStringToObject(thm,"Width","100");
cJSON_AddItemToObject(img,"IDs", cJSON_CreateIntArray(ids,4));
out=cJSON_Print(root); cJSON_Delete(root); printf("%s\n",out); free(out);
/* Our array of "records": */
root=cJSON_CreateArray();
for (i=0;i<2;i++)
{
cJSON_AddItemToArray(root,fld=cJSON_CreateObject());
cJSON_AddStringToObject(fld, "precision", fields[i].precision);
cJSON_AddNumberToObject(fld, "Latitude", fields[i].lat);
cJSON_AddNumberToObject(fld, "Longitude", fields[i].lon);
cJSON_AddStringToObject(fld, "Address", fields[i].address);
cJSON_AddStringToObject(fld, "City", fields[i].city);
cJSON_AddStringToObject(fld, "State", fields[i].state);
cJSON_AddStringToObject(fld, "Zip", fields[i].zip);
cJSON_AddStringToObject(fld, "Country", fields[i].country);
}
/* cJSON_ReplaceItemInObject(cJSON_GetArrayItem(root,1),"City",cJSON_CreateIntArray(ids,4)); */
out=cJSON_Print(root); cJSON_Delete(root); printf("%s\n",out); free(out);
}
int main (int argc, const char * argv[]) {
/* a bunch of json: */
char text1[]="{\n\"name\": \"Jack (\\\"Bee\\\") Nimble\", \n\"format\": {\"type\": \"rect\", \n\"width\": 1920, \n\"height\": 1080, \n\"interlace\": false,\"frame rate\": 24\n}\n}";
char text2[]="[\"Sunday\", \"Monday\", \"Tuesday\", \"Wednesday\", \"Thursday\", \"Friday\", \"Saturday\"]";
char text3[]="[\n [0, -1, 0],\n [1, 0, 0],\n [0, 0, 1]\n ]\n";
char text4[]="{\n \"Image\": {\n \"Width\": 800,\n \"Height\": 600,\n \"Title\": \"View from 15th Floor\",\n \"Thumbnail\": {\n \"Url\": \"http:/*www.example.com/image/481989943\",\n \"Height\": 125,\n \"Width\": \"100\"\n },\n \"IDs\": [116, 943, 234, 38793]\n }\n }";
char text5[]="[\n {\n \"precision\": \"zip\",\n \"Latitude\": 37.7668,\n \"Longitude\": -122.3959,\n \"Address\": \"\",\n \"City\": \"SAN FRANCISCO\",\n \"State\": \"CA\",\n \"Zip\": \"94107\",\n \"Country\": \"US\"\n },\n {\n \"precision\": \"zip\",\n \"Latitude\": 37.371991,\n \"Longitude\": -122.026020,\n \"Address\": \"\",\n \"City\": \"SUNNYVALE\",\n \"State\": \"CA\",\n \"Zip\": \"94085\",\n \"Country\": \"US\"\n }\n ]";
/* Process each json textblock by parsing, then rebuilding: */
doit(text1);
doit(text2);
doit(text3);
doit(text4);
doit(text5);
/* Parse standard testfiles: */
/* dofile("../../tests/test1"); */
/* dofile("../../tests/test2"); */
/* dofile("../../tests/test3"); */
/* dofile("../../tests/test4"); */
/* dofile("../../tests/test5"); */
/* Now some samplecode for building objects concisely: */
create_objects();
return 0;
} 最后，想起一个老师说过“勿在浮沙筑高台”，共勉