Avoids undefined behavior.  Found by autofuzz.

If we want to override `strerror()` in libjson-c
to make tests consistent across platforms, we
need to do it build-wide as configure/build
option.

Apple linkers make it really hard to override functions
at link-time, and this seems to be locked down on travis-ci.org
[ for security reasons I assume ].
While I got it to work locally, it did not work
when running on travis.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

Issue originally found in the json-c 0.11 internal copy in GDAL but also found
in latest git version.

If doing things like
json_object* obj = json_object_new_double(1e300);
json_object_set_serializer(obj, json_object_double_to_json_string, "%f", NULL);
json_object_to_json_string(obj)

    size = snprintf(buf, sizeof(buf),
        format ? format :
          (modf(jso->o.c_double, &dummy) == 0) ? "%.17g.0" : "%.17g",
          jso->o.c_double);
will return a value greater than 128 since at least 300 characters are needed.
This value is then passed to printbuf_memappend(pb, buf, size); that tries to
read size bytes in buf.

So we should clamp size to sizeof(buf). And on Windows, _snprintf() returns -1
in that situation, so deal also with this case.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1682
Credit to OSS-Fuzz

Hawciz pointed out that the previous commit modifies the public
interface of printbuf. Per his suggestion, sprintbuf() was restored
and a new pair of macros was added that wraps printbuf_memappend().

Using a wrapper macro instead of modifying sprintbuf() also reduces
function call overhead, bringing total performance gains to
approximately 400%.

Removes variadic prints for ~3x performance improvement.

closes https://github.com/json-c/json-c/issues/278

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

This reverts commit aaba8c1080.

This commit breaks builddir != srcdir build, but, more importantly, it
also adds a dependency on a header, "config.h", which is not installed
and it's supposed to be private — since it's generated at configuration
time and it's not meant to be used by projects compiling against a
library.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

Json object set xxx

Also correct a couple of calls to _to_json_string to check the return value.

Also undo NULL check in json_tokener_set_flags(): it's not at all valid toi
 try to set flags on a NULL pointer, and doing so should not be silently
 ignored.

Minor changes in C source code

This new method allows to also
get the length of the generated string.

Fix #165

Change-Id: Iea91404027f143ca3d29a4c58d7c07ae53556110
Signed-off-by: José Bollo <jose.bollo@iot.bzh>

To better distinguish between entry->k and entry->v being const within linkhash, but non-const outside, add lh_entry_v() and lh_entry_k() accessors.
Make lh_entry->k const.

Also, json_object_set_serializer is changed to respect the userdata
and user_delete parameters when to_string_func is NULL.

The user might want to use the userdata for something different, so the
serializer should ignore it by default.

Explicitly setting the serializer to json_object_double_to_json_string will
still make it interpret the userdata as a format string.

Fix various potential null ptr deref and int32 overflows

Add utility function for comparing json_objects

Tests added.

This fix errors that can happen when ingesting very large JSON files
when hitting the maximum heap size of the process.