[GH-ISSUE #96] error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction] #72

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opened 2026-03-04 12:18:42 +03:00 by kerem · 3 comments

kerem commented

2026-03-04 12:18:42 +03:00

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Originally created by @jaygooby on GitHub (Jun 16, 2022).
Original GitHub issue: https://github.com/ADD-SP/ngx_waf/issues/96

How to trigger this bug?

Building nginx on MacOS 12.3.1 with Xcode gcc Apple clang version 13.1.6 (clang-1316.0.21.2.5), nginx 1.20.1 and ngx_waf current branch.

I downloaded the nginx and ngx_waf sources. Using the current branch of ngx_waf I added cjson and uthash:

cd ngx_waf_current
git clone -b v2.3.0 https://github.com/troydhanson/uthash.git lib/uthash
git clone -b v1.7.15 https://github.com/DaveGamble/cJSON.git lib/cjson

nginx configures OK, but make generates these errors (and many more that I haven't pasted here for sake of clarity):

-o objs/addon/src/ngx_http_waf_module_config.o \
/tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c
/tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c:92:25: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]
p = (ngx_str_t*)utarray_next(array, NULL);
        ^~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/nw/ngx_waf-current/lib/uthash/include/utarray.h:228:73: note: expanded from macro 'utarray_next'
#define utarray_next(a,e) (((e)==NULL) ? utarray_front(a) : (((a)->i != utarray_eltidx(a,e)+1) ? _utarray_eltptr(a,utarray_eltidx(a,e)+1) : NULL))
                                                        ^~~~~~~~~~~~~~~~~~~
/tmp/nw/ngx_waf-current/lib/uthash/include/utarray.h:231:42: note: expanded from macro 'utarray_eltidx'
#define utarray_eltidx(a,e) (((char*)(e) - (a)->d) / (a)->icd.sz)
              ~~~~~~~~~~ ^
/tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c:92:25: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]
p = (ngx_str_t*)utarray_next(array, NULL);
        ^~~~~~~~~~~~~~~~~~~~~~~~~

Which I guess are coming from the uthash library?

The version or branch of ngx_waf.
current

Output of nginx -V.

It doesn't compile, but this is how it was configured:

LIB_MODSECURITY=/opt/modsecurity ./configure --add-module=/tmp/nw/ngx_waf-current

Optional Info

MacOS 12.3.1

Whether nginx is running in a virtual environment such as Docker. If it is running in Docker, please provide the image name.

Not running under Docker

Originally created by @jaygooby on GitHub (Jun 16, 2022). Original GitHub issue: https://github.com/ADD-SP/ngx_waf/issues/96 * How to trigger this bug? Building nginx on MacOS 12.3.1 with Xcode gcc `Apple clang version 13.1.6 (clang-1316.0.21.2.5)`, `nginx 1.20.1` and ngx_waf `current` branch. I downloaded the nginx and ngx_waf sources. Using the `current` branch of ngx_waf I added `cjson` and `uthash`: cd ngx_waf_current git clone -b v2.3.0 https://github.com/troydhanson/uthash.git lib/uthash git clone -b v1.7.15 https://github.com/DaveGamble/cJSON.git lib/cjson nginx configures OK, but `make` generates these errors (and many more that I haven't pasted here for sake of clarity): -o objs/addon/src/ngx_http_waf_module_config.o \ /tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c /tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c:92:25: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction] p = (ngx_str_t*)utarray_next(array, NULL); ^~~~~~~~~~~~~~~~~~~~~~~~~ /tmp/nw/ngx_waf-current/lib/uthash/include/utarray.h:228:73: note: expanded from macro 'utarray_next' #define utarray_next(a,e) (((e)==NULL) ? utarray_front(a) : (((a)->i != utarray_eltidx(a,e)+1) ? _utarray_eltptr(a,utarray_eltidx(a,e)+1) : NULL)) ^~~~~~~~~~~~~~~~~~~ /tmp/nw/ngx_waf-current/lib/uthash/include/utarray.h:231:42: note: expanded from macro 'utarray_eltidx' #define utarray_eltidx(a,e) (((char*)(e) - (a)->d) / (a)->icd.sz) ~~~~~~~~~~ ^ /tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c:92:25: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction] p = (ngx_str_t*)utarray_next(array, NULL); ^~~~~~~~~~~~~~~~~~~~~~~~~ Which I guess are coming from the `uthash` library? * The version or branch of `ngx_waf`. `current` * Output of `nginx -V`. It doesn't compile, but this is how it was configured: LIB_MODSECURITY=/opt/modsecurity ./configure --add-module=/tmp/nw/ngx_waf-current ## Optional Info MacOS 12.3.1 * Whether `nginx` is running in a virtual environment such as `Docker`. If it is running in `Docker`, please provide the image name. Not running under Docker

kerem

2026-03-04 12:18:42 +03:00

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kerem commented

2026-03-04 12:18:43 +03:00

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@jaygooby commented on GitHub (Jun 16, 2022):

If I remove -Werror from the nginx objs/Makefile then it will compile

@jaygooby commented on GitHub (Jun 16, 2022): If I remove `-Werror` from the nginx `objs/Makefile` then it will compile

kerem commented

2026-03-04 12:18:43 +03:00

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@ADD-SP commented on GitHub (Jun 27, 2022):

It looks like the compiler's dataflow analysis is behaving differently, not a bug.

@ADD-SP commented on GitHub (Jun 27, 2022): It looks like the compiler's dataflow analysis is behaving differently, not a bug.

kerem commented

2026-03-04 12:18:43 +03:00

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@stale[bot] commented on GitHub (Jul 4, 2022):

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
此 issue 因为最近没有任何活动已经被标记，如果在此之后的一段时间内仍没有任何活动则会被关闭。感谢您对项目的支持。

@stale[bot] commented on GitHub (Jul 4, 2022): This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. 此 issue 因为最近没有任何活动已经被标记，如果在此之后的一段时间内仍没有任何活动则会被关闭。感谢您对项目的支持。

kerem referenced this issue

2026-03-13 16:49:00 +03:00

[GH-ISSUE #72] Error when install modsec and libmaxmind #187