Small test program and library for time_t on 32bit platforms. Both program and library are compiled with 32bit as well as 64bit time_t and then combined. The ABI changes but the linker will happily load even invalid combinations. The program calls three library functions, one that returns a value, one that puts the value in a pointer and one that gets a time_t as parameter. A mismatch mix may lead to garbage passed around or crash.
$ uname -m
x86_64
$ make test
cc -o 32/libnow.so.1 now.c -m32 -fPIC -Wall -g -O2 -D_FORTIFY_SOURCE=2 -fstack-protector -shared -Wl,-soname,libnow.so.1
ln -sf libnow.so.1 32/libnow.so
cc -o main main.c -m32 -fPIC -Wall -g -O2 -D_FORTIFY_SOURCE=2 -fstack-protector -L32 -lnow
cc -o 64/libnow.so.1 now.c -m32 -fPIC -Wall -g -O2 -D_FORTIFY_SOURCE=2 -fstack-protector -D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64 -shared -Wl,-soname,libnow.so.1
ln -sf libnow.so.1 64/libnow.so
cc -o main64 main.c -m32 -fPIC -Wall -g -O2 -D_FORTIFY_SOURCE=2 -fstack-protector -D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64 -L64 -lnow
LD_LIBRARY_PATH=$PWD/32 ./main
32bit time_t program using a 32bit time_t library
ret: 32323232
ptr: 32323232
prm: 0 32323232 0
LD_LIBRARY_PATH=$PWD/64 ./main64
64bit time_t program using a 64bit time_t library
ret: 6464646432323232
ptr: 6464646432323232
prm: 0 6464646432323232 0
### this one gets garbage
LD_LIBRARY_PATH=$PWD/32 ./main64
64bit time_t program using a 32bit time_t library
ret: 0c0debad32323232
ptr: 0c0debad32323232
prm: 0 32323232 64646464
### the following should crash
LD_LIBRARY_PATH=$PWD/64 ./main
32bit time_t program using a 64bit time_t library
ret: 32323232
ptr: 32323232
prm: 0 0000000032323232 8049102
make: *** [Makefile:12: test] Segmentation fault (core dumped)
$ uname -m
armv7l
$ make test
LD_LIBRARY_PATH=$PWD/32 ./main
32bit time_t program using a 32bit time_t library
ret: 32323232
ptr: 32323232
prm: 0 32323232 0
LD_LIBRARY_PATH=$PWD/64 ./main64
64bit time_t program using a 64bit time_t library
ret: 6464646432323232
ptr: 6464646432323232
prm: 0 6464646432323232 0
### this one gets garbage
LD_LIBRARY_PATH=$PWD/32 ./main64
64bit time_t program using a 32bit time_t library
ret: 0000000032323232
ptr: 0000000032323232
prm: 0 00000000 32323232
### the following should crash
LD_LIBRARY_PATH=$PWD/64 ./main
32bit time_t program using a 64bit time_t library
ret: 32323232
ptr: 32323232
prm: 0 0000000000000000 40