Valgrind error when using JSON_SORT_KEYS in json_dumpf

[hovis]

The following small c program when checked with valgrind shows an "Invalid read of size 4" when calling json_dumpf with JSON_SORT_KEYS. If JSON_SORT_KEYS is removed then the program is fine.

/*
    Valgrind error when using JSON_SORT_KEYS.

    Ubuntu 12.04 (64bit).  Jansson 2.7.

    Compile:-

        gcc -o test test.c -ljansson

    Runs with expected results.

    Check with valgrind:-

        valgrind ./test

    When JSON_SORT_KEYS is specified we get:-

        Invalid read of size 4
           at 0x4022CB: hashlittle
           by 0x403228: hashtable_get
           by 0x40457A: json_object_get
           by 0x401AA7: do_dump
           by 0x4016AA: do_dump
           by 0x402050: json_dump_callback
           by 0x401F74: json_dumpf
           by 0x400E50: main
         Address 0x51f339c is 44 bytes inside a block of size 47 alloc'd
           at 0x4C2CC4B: malloc
           by 0x403781: jsonp_malloc
           by 0x403141: hashtable_set
           by 0x40460D: json_object_set_new_nocheck
           by 0x4046AA: json_object_set_new
           by 0x400E22: main
*/

#include <stdio.h>
#include <jansson.h>

int main(int argc, char *argv[])
{
    /*
        [{"123456": []}]
    */

    json_t *a = json_array();

    json_t *b = json_object();

    json_t *c = json_array();
    json_object_set_new(b, "123456", c);

    json_array_append_new(a, b);

    /* Replace the JSON_SORT_KEYS with 0 and the valgrind error goes away */
    json_dumpf(a, stdout, JSON_SORT_KEYS);
    printf("\n");

    json_decref(a);

    return 0;
}

[akheron]

This is expected behavior. The hash function (hashlittle) has an optimization that Valgrind doesn't like. See the comment in https://github.com/akheron/jansson/blob/4c4f692bd6814e598c9aa538a3c9979cdf05d566/src/lookup3.h#L239-L247.

IIRC, there's currently no way to disable the optimization using the configure script. With CMake, you can disable it with the JANSSON_TEST_WITH_VALGRIND flag.