How do I portably parse a JSON file using libjq?

Question

Consider the following code snippet based on libjq.

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

int main(void) {
    jq_state *jq = jq_init();
    if (!jq) {
        fprintf(stderr, "Failed to initialize jq\n");
        return 1;
    }

    const char *json_text = "{\"data\":\"abc\\u0000def\"}";

    jv parsed = jv_parse(json_text);
    if (!jv_is_valid(parsed)) {
        fprintf(stderr, "Invalid JSON\n");
        exit(0);
    }

    if (!jq_compile(jq, ".data")) {
        fprintf(stderr, "Failed to compile jq filter\n");
        exit(1);
    }

    jq_start(jq, parsed, 0);

    jv data = jq_next(jq);
    printf("jv_kind = %s\n", jv_kind_name(jv_get_kind(data)));
    const char *str = jv_string_value(data);
    int length = jv_string_length_bytes(data);
    for (int i = 0; i < length; i++) {
      printf("%c", str[i]);
    }
    return 0;
}

On MacOS, this code outputs a bunch of null bytes:

./hello_jq | xxd
00000000: 6a76 5f6b 696e 6420 3d20 7374 7269 6e67  jv_kind = string
00000010: 0a00 0000 0000 0000                      ........

On Linux, this code outputs the expected value:

./hello_jq | xxd
00000000: 6a76 5f6b 696e 6420 3d20 7374 7269 6e67  jv_kind = string
00000010: 0a61 6263 0064 6566                      .abc.def

What is causing this discrepancy, and how do I get the proper behavior on MacOS?

On both systems, the jq cli tool produces the expected output in raw mode.

Answer 1

You’re hitting jv ownership/consumption rules.
jv_string_value(jv) consumes its jv argument. After you call it, data is invalid. Your next call (jv_string_length_bytes(data)) uses a freed value—UB that “works” on Linux but fails on macOS.

Use jv_copy(data) (or compute length first), then print by length:

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

int main(void) {
    jq_state *jq = jq_init();
    if (!jq) return 1;

    const char *json_text = "{\"data\":\"abc\\u0000def\"}";
    jv parsed = jv_parse(json_text);
    if (!jv_is_valid(parsed)) return 1;

    if (!jq_compile(jq, ".data")) return 1;
    jq_start(jq, parsed, 0);

    jv data = jq_next(jq);                 // data is a valid jv string
    printf("jv_kind = %s\n", jv_kind_name(jv_get_kind(data)));

    int len = jv_string_length_bytes(jv_copy(data));     // don't consume data
    const char *str = jv_string_value(jv_copy(data));    // also don't consume data
    fwrite(str, 1, len, stdout);
    fputc('\n', stdout);

    jv_free(data);
    jq_teardown(&jq);
    return 0;
}