How to parse json using sscanf in C?

Question

I have the following string in the form of json:

{
   "num":1,
   "data":{
      "city":"delhi"
   }
}

I need to get the value of "num" key using sscanf. Here is my attempt. I know it's incorrect. But I don't know how to do it.

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    
    char *str = "{\"num\":1,\"data\":{\"city\":\"delhi\"}}";
    char *ret = malloc(sizeof(char) * 10);
    
    sscanf(str, "{\"num\":%s, %s" , ret);
    
    printf("%s", ret);
    return 0;
}

Any suggestions?

Don't use sscanf. JSON is too complex for that. Use a proper JSON parser instead. — Oliver Mason
– Oliver Mason, Commented Apr 24, 2020 at 10:06
There are two possibilities: use a dirty hack, if you are sure, that the string is exactly formatted this way int num = atoi(strchr(str, ':')+1); or use a json parser, if the string can vary. — Ctx
– Ctx, Commented Apr 24, 2020 at 10:08
@OliverMason, I have no other option than sscanf and regex. My entire implementation is in C. I can't even use third party plugins, libraries etc. — JessePinkman
– JessePinkman, Commented Apr 24, 2020 at 10:10
@Ctx, I think your approach would work, since my string has a fixed format. Thanks a lot. — JessePinkman
– JessePinkman, Commented Apr 24, 2020 at 10:12
Using sscanf for this is a terrible idea. Here's a very lightweight JSON parser written entirely in ANSI (portable) C. You can just drop it into your project: github.com/udp/json-parser — Alex Reynolds
– Alex Reynolds, Commented Apr 24, 2020 at 10:14

bruno · Accepted Answer · 2020-04-24 10:19:02Z

3

sscanf(str, "{\"num\":%s, %s" , ret);

is wrong, first you have two "%s" but you give only one location to save string (ret), and it does not extract as you expect

you want

#include <stdio.h>
#include <stdlib.h>

int main(void) {

    char *str = "{\"num\":1,\"data\":{\"city\":\"delhi\"}}";
    char *ret = malloc(10); /* by definition sizeof(char) is 1 */

    if (sscanf(str, "{\"num\":%9[^,]" , ret) == 1)
        printf("value is '%s'\n", ret);

    free(ret);
    return 0;
}

Compilation and execution

/tmp % gcc -Wall p.c
/tmp % ./a.out
value is '1'
/tmp %

but to use scanf to parse is limited

edited Apr 24, 2020 at 10:19

answered Apr 24, 2020 at 10:11

bruno

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2 Comments

JessePinkman Over a year ago

Just what I wanted. Thank you so much. I am using sscanf because my json is of fixed length and fixed format. So, this would work.

bruno Over a year ago

@no_one so fine. When you read/extract using one of the scanf function I recommend you to always check the return value to know how much elements was got, and when it is a string to limit the length where scanf will write (here 9 even the string is sized 10 because the ending null character is not count even written)

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