I have been chasing a memory problem in my program that links against -lcairo and -lX11. Finally, I decided to comment out all lines in my main() and make sure that valgrind is happy. To my surprise, it is not:
==7570== Memcheck, a memory error detector
==7570== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==7570== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==7570== Command: ./Test
==7570==
==7570==
==7570== HEAP SUMMARY:
==7570== in use at exit: 10,360 bytes in 5 blocks
==7570== total heap usage: 5 allocs, 0 frees, 10,360 bytes allocated
==7570==
==7570== 2,072 bytes in 1 blocks are still reachable in loss record 1 of 5
==7570== at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7570== by 0x5FCCE9A: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FCBACE: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FCD585: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5F7F508: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x4010139: call_init.part.0 (dl-init.c:78)
==7570== by 0x4010222: _dl_init (dl-init.c:36)
==7570== by 0x4001309: ??? (in /lib/x86_64-linux-gnu/ld-2.19.so)
==7570==
==7570== 2,072 bytes in 1 blocks are still reachable in loss record 2 of 5
==7570== at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7570== by 0x5FCCE9A: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FCA61F: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FCD5A0: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5F7F508: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x4010139: call_init.part.0 (dl-init.c:78)
==7570== by 0x4010222: _dl_init (dl-init.c:36)
==7570== by 0x4001309: ??? (in /lib/x86_64-linux-gnu/ld-2.19.so)
==7570==
==7570== 2,072 bytes in 1 blocks are still reachable in loss record 3 of 5
==7570== at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7570== by 0x5FCCE9A: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FE5A8F: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FBC1A5: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FCD5AB: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5F7F508: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x4010139: call_init.part.0 (dl-init.c:78)
==7570== by 0x4010222: _dl_init (dl-init.c:36)
==7570== by 0x4001309: ??? (in /lib/x86_64-linux-gnu/ld-2.19.so)
==7570==
==7570== 2,072 bytes in 1 blocks are still reachable in loss record 4 of 5
==7570== at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7570== by 0x5FCCE9A: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x60053CF: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FCD5AB: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5F7F508: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x4010139: call_init.part.0 (dl-init.c:78)
==7570== by 0x4010222: _dl_init (dl-init.c:36)
==7570== by 0x4001309: ??? (in /lib/x86_64-linux-gnu/ld-2.19.so)
==7570==
==7570== 2,072 bytes in 1 blocks are still reachable in loss record 5 of 5
==7570== at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7570== by 0x5FCCE9A: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5FCFCBF: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x5F7F508: ??? (in /usr/lib/x86_64-linux-gnu/libpixman-1.so.0.30.2)
==7570== by 0x4010139: call_init.part.0 (dl-init.c:78)
==7570== by 0x4010222: _dl_init (dl-init.c:36)
==7570== by 0x4001309: ??? (in /lib/x86_64-linux-gnu/ld-2.19.so)
==7570==
==7570== LEAK SUMMARY:
==7570== definitely lost: 0 bytes in 0 blocks
==7570== indirectly lost: 0 bytes in 0 blocks
==7570== possibly lost: 0 bytes in 0 blocks
==7570== still reachable: 10,360 bytes in 5 blocks
==7570== suppressed: 0 bytes in 0 blocks
==7570==
==7570== For counts of detected and suppressed errors, rerun with: -v
==7570== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
Can a program with an empty main() allocate memory and end up with some reachable blocks? Is there a way to get rid of this strange problem?
Yes. It is possible.
One way is that there might be some static objects in the libraries you are linking against. This has already been pointed out by @M.M
The other way is that if you are linking against shared libraries, with no static objects, and certain functions in such a library uses
__attribute__((constructor)). This attribute causes these functions to execute before main() (works only with gcc), and hence you see memory allocations. These functions are chiefly employed by libraries to declare certain attributes about their functions if they are used in your program. For more details about these attributes you can refer to :http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html