Cannot compile on android due to undeclared functions

[lggcs]

I am having an issue compiling libsodium on Android 30, due to the following functions which are are not provided by libc: explicit_bzero, memset_s, memset_explicit. This causes multiple "error: call to undeclared function" in util.c

My proposed fix for the embedded libsodium in PyNaCl was to detect android in configure and manually set no to those so the libsodium uses it's native fallback method and the program compiles correctly, however the dev there suggested I upstream the issue as it occurs in the latest libsodium stable tar.gz as well.

[jedisct1]

Hi!

Does the build-aar.sh build script complete?

If not, can you post your config.log file? Usability of these functions should be detected by autoconf. If that is not the case, we have to find why rather than hack a band aid.

[jedisct1]

Android builds seem to work fine in CI https://github.com/jedisct1/libsodium/actions/runs/17679600451/job/50250355290 so this is a bit surprising.

[lggcs]

It is odd that CI is passing and it apparently works via cross-compile. Testing directly from a Termux shell produces this however, which was a problem when I was trying to pip install pynacl (and the same error confirmed by running make on latest stable directly)

Linux localhost 6.1.152-android14-11-g954c984f72d2 #1 SMP PREEMPT Tue Sep 16 22:24:30 UTC 2025 aarch64 Android
clang version 20.1.8
Target: aarch64-unknown-linux-android30
Thread model: posix
InstalledDir: /data/data/com.termux/files/usr/bin
Making all in builds
make[1]: Entering directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/builds'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/builds'
Making all in contrib
make[1]: Entering directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/contrib'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/contrib'
Making all in dist-build
make[1]: Entering directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/dist-build'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/dist-build'
Making all in src
make[1]: Entering directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/src'
Making all in libsodium
make[2]: Entering directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/src/libsodium'
Making all in include
make[3]: Entering directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/src/libsodium/include'
make[3]: Nothing to be done for 'all'.
make[3]: Leaving directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/src/libsodium/include'
make[3]: Entering directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/src/libsodium'
  CC       sodium/libsodium_la-utils.lo
sodium/utils.c:132:5: error: call to undeclared function 'explicit_bzero'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  132 |     explicit_bzero(pnt, len);
      |     ^
1 error generated.
make[3]: *** [Makefile:3045: sodium/libsodium_la-utils.lo] Error 1
make[3]: Leaving directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/src/libsodium'
make[2]: *** [Makefile:3400: all-recursive] Error 1
make[2]: Leaving directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/src/libsodium'
make[1]: *** [Makefile:417: all-recursive] Error 1
make[1]: Leaving directory '/data/data/com.termux/files/home/libsodium/libsodium-stable/src'
make: *** [Makefile:532: all-recursive] Error 1

[jedisct1]

So what does the config.log file look lke?

[lggcs]

[truncated paste]

[jedisct1]

You can just drag and drop the file :)

[lggcs]

Sorry about that, here: config.log

[jedisct1]

configure:22695: checking for explicit_bzero
configure:22695: gcc -o conftest  -O3 -pthread -fvisibility=hidden -fPIC -fPIE -fno-strict-aliasing -fno-strict-overflow -fstack-protecto...
configure:22695: $? = 0
configure:22695: result: yes

The test compiles a program that calls that function. And.... it compiles.

I honestly don't understand how the function can disappear later.

[lggcs]

That is a paradox. See if this makes sense since I asked AI and it came back with a working patch which may actually be correct.

…

---Why configure “finds” explicit_bzero but your build still fails At a glance this looks impossible—configure says “yes, your system has explicit_bzero,” yet when sodium/utils.c invokes it, the compiler complains it’s undeclared. In fact the culprit is a mismatch between how the configure checks for the symbol and how the actual build compiles your code. 1. How the configure test is mis-representing reality configure’s test only checks linkability, not header declarations. By default, Autoconf’s AC_CHECK_FUNCS([explicit_bzero]) (or equivalent) compiles a tiny C program without -std=c99 or “treat warnings as errors,” simply calling explicit_bzero() and seeing if it links against libc. If the linker finds a symbol—no matter whether there was actually a prototype in any header—configure reports “yes.” Your real build is stricter. libsodium’s Makefile invokes GCC/Clang with -std=c99 -Werror=implicit-function-declaration. Under C99, calling a function without including a matching prototype is a hard error, not just a warning. Since no header in your build actually exposes explicit_bzero() under those exact flags, the compiler errors out.

--- Given that, it proposed this more advanced configure check, all of which do properly fail. ```diff --git a/configure.ac b/configure.acindex 79d93174..c7bb5b18 100644--- a/configure.ac+++ b/configure.ac@@ -957,9 +957,136 @@ AS_IF([test "x$WASI" = "x"],[ AC_CHECK_FUNCS([posix_memalign nanosleep clock_gettime]) AS_IF([test "x$WASI" = "x"],[- AC_CHECK_FUNCS([memset_s explicit_bzero memset_explicit explicit_memset])++ AC_CHECK_HEADERS([string.h strings.h])++ # Test memset_s+ AC_MSG_CHECKING([for declaration of memset_s])+ AC_COMPILE_IFELSE(+ [AC_LANG_PROGRAM(+ [[#include <string.h>+ #include <strings.h>]],+ [[ if (memset_s((void*)0,0,(size_t)0) != 0) return 1;+ return 0; ]])],+ [ AC_MSG_RESULT([yes])+ AC_DEFINE([HAVE_DECL_MEMSET_S], [1],+ [Define to 1 if memset_s is declared]) ],+ [ AC_MSG_RESULT([no]) ]+ )++ AC_MSG_CHECKING([for linkable memset_s])+ if test "x$HAVE_DECL_MEMSET_S" = "x1"; then+ AC_LINK_IFELSE(+ [AC_LANG_PROGRAM(+ [[#include <string.h>+ #include <strings.h>]],+ [[ if (memset_s((void*)0,0,(size_t)0) != 0) return 1;+ return 0; ]])],+ [ AC_MSG_RESULT([yes])+ AC_DEFINE([HAVE_MEMSET_S], [1],+ [Define to 1 if memset_s is available]) ],+ [ AC_MSG_RESULT([no]) ]+ )+ else+ AC_MSG_RESULT([no])+ fi++ # Test explicit_bzero+ AC_MSG_CHECKING([for declaration of explicit_bzero])+ AC_COMPILE_IFELSE(+ [AC_LANG_PROGRAM(+ [[#include <string.h>+ #include <strings.h>]],+ [[ explicit_bzero((void*)0, (size_t)0);+ return 0; ]])],+ [ AC_MSG_RESULT([yes])+ AC_DEFINE([HAVE_DECL_EXPLICIT_BZERO], [1],+ [Define to 1 if explicit_bzero is declared]) ],+ [ AC_MSG_RESULT([no]) ]+ )++ AC_MSG_CHECKING([for linkable explicit_bzero])+ if test "x$HAVE_DECL_EXPLICIT_BZERO" = "x1"; then+ AC_LINK_IFELSE(+ [AC_LANG_PROGRAM(+ [[#include <string.h>+ #include <strings.h>]],+ [[ explicit_bzero((void*)0, (size_t)0);+ return 0; ]])],+ [ AC_MSG_RESULT([yes])+ AC_DEFINE([HAVE_EXPLICIT_BZERO], [1],+ [Define to 1 if explicit_bzero is available]) ],+ [ AC_MSG_RESULT([no]) ]+ )+ else+ AC_MSG_RESULT([no])+ fi++ # Test memset_explicit+ AC_MSG_CHECKING([for declaration of memset_explicit])+ AC_COMPILE_IFELSE(+ [AC_LANG_PROGRAM(+ [[#include <string.h>+ #include <strings.h>]],+ [[ memset_explicit((void*)0, 0, (size_t)0);+ return 0; ]])],+ [ AC_MSG_RESULT([yes])+ AC_DEFINE([HAVE_DECL_MEMSET_EXPLICIT], [1],+ [Define to 1 if memset_explicit is declared]) ],+ [ AC_MSG_RESULT([no]) ]+ )++ AC_MSG_CHECKING([for linkable memset_explicit])+ if test "x$HAVE_DECL_MEMSET_EXPLICIT" = "x1"; then+ AC_LINK_IFELSE(+ [AC_LANG_PROGRAM(+ [[#include <string.h>+ #include <strings.h>]],+ [[ memset_explicit((void*)0, 0, (size_t)0);+ return 0; ]])],+ [ AC_MSG_RESULT([yes])+ AC_DEFINE([HAVE_MEMSET_EXPLICIT], [1],+ [Define to 1 if memset_explicit is available]) ],+ [ AC_MSG_RESULT([no]) ]+ )+ else+ AC_MSG_RESULT([no])+ fi++ # Test explicit_memset+ AC_MSG_CHECKING([for declaration of explicit_memset])+ AC_COMPILE_IFELSE(+ [AC_LANG_PROGRAM(+ [[#include <string.h>+ #include <strings.h>]],+ [[ explicit_memset((void*)0, 0, (size_t)0);+ return 0; ]])],+ [ AC_MSG_RESULT([yes])+ AC_DEFINE([HAVE_DECL_EXPLICIT_MEMSET], [1],+ [Define to 1 if explicit_memset is declared]) ],+ [ AC_MSG_RESULT([no]) ]+ )++ AC_MSG_CHECKING([for linkable explicit_memset])+ if test "x$HAVE_DECL_EXPLICIT_MEMSET" = "x1"; then+ AC_LINK_IFELSE(+ [AC_LANG_PROGRAM(+ [[#include <string.h>+ #include <strings.h>]],+ [[ explicit_memset((void*)0, 0, (size_t)0);+ return 0; ]])],+ [ AC_MSG_RESULT([yes])+ AC_DEFINE([HAVE_EXPLICIT_MEMSET], [1],+ [Define to 1 if explicit_memset is available]) ],+ [ AC_MSG_RESULT([no]) ]+ )+ else+ AC_MSG_RESULT([no])+ fi ]) ++ AC_SUBST([LIBTOOL_EXTRA_FLAGS]) TEST_LDFLAGS='' ```

[lggcs]

Looks like my email butchered the patch file. I can send it tomorrow if interested. Patch does this. ... Patching configure.ac to correctly detect explicit_bzero Below is a recipe to make your configure script test both for the header declaration and for the linkable symbol before deciding that explicit_bzero exists. If either test fails, the fallback implementation will be used. What we’re changing Add a compile‐only check that includes <string.h>/<strings.h> and fails if explicit_bzero isn’t declared. Preserve the existing link test, but only honor it when the prototype check passed. Define HAVE_EXPLICIT_BZERO only when both tests succeed. How it works AC_CHECK_HEADERS([string.h strings.h]) Ensures we can include the two common headers where these functions might be declared. Prototype check (AC_COMPILE_IFELSE) Compiles a tiny snippet that calls the function. If there’s no declaration, compilation fails. On success it defines HAVE_DECL_<FUNC>. Linkage check (AC_LINK_IFELSE) Only runs if the prototype exists. It attempts to link the same snippet. On success it defines HAVE_<FUNC>. Fallback logic in your code In your C sources you already guard usage with #ifdef HAVE_<FUNC> … #else … to pick either the system call or the bundled implementation.

[jedisct1]

That doesn't make sense since the error you got is not a linking error.

[lggcs]

Well it has to be something with Termux having a broken build environment is the only other thing I can think of. If you have any other ideas or tests I can try  please let me know.