| Summary: | memcheck warnings when using C++ | ||
|---|---|---|---|
| Product: | [Developer tools] valgrind | Reporter: | leonardopsantos |
| Component: | memcheck | Assignee: | Julian Seward <jseward> |
| Status: | RESOLVED NOT A BUG | ||
| Severity: | normal | CC: | tom |
| Priority: | NOR | ||
| Version First Reported In: | 3.19.0 | ||
| Target Milestone: | --- | ||
| Platform: | Other | ||
| OS: | Linux | ||
| Latest Commit: | Version Fixed/Implemented In: | ||
| Sentry Crash Report: | |||
Notice that the size of the struct is 0 in the C version and 1 in the C++ version because C++ has added a single padding byte to the structure. That padding byte is not initialised, but you are trying to print it, hence the warning. |
A very simple application produces a non-clean valgrind memcheck report for no obvious reason: `main.cpp` ``` #include <cstdio> struct s {}; int main (void) { struct s value = s(); const unsigned char* obj_bytes = (const unsigned char*) &value; size_t count = sizeof(value); printf("[%d %s] count : %ld, obj_bytes : %p\n", __LINE__, __FUNCTION__, count, obj_bytes); for (size_t i = 0; i < count; i++) { printf("%ld: %02x", i, obj_bytes[i]); } printf("\n"); return 0; } ``` Compiled with: ```sh g++ -O0 -g -std=c++11 -Wall -o main main.cpp ``` Generates a very lengthy report and I don't understand why: ``` VALGRIND_LIB=~/work/googletest/bug/valgrind/src/valgrind-3.19.0/.in_place ~/work/googletest/bug/valgrind/src/valgrind-3.19.0/coregrind/valgrind --track-origins=yes --show-reachable=yes --leak-check=full ./main ==2544006== Memcheck, a memory error detector ==2544006== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al. ==2544006== Using Valgrind-3.19.0 and LibVEX; rerun with -h for copyright info ==2544006== Command: ./main ==2544006== [12 main] count : 1, obj_bytes : 0x1ffefffbaf ==2544006== Use of uninitialised value of size 8 ==2544006== at 0x48C410A: _itoa_word (_itoa.c:180) ==2544006== by 0x48DF964: __vfprintf_internal (vfprintf-internal.c:1646) ==2544006== by 0x48CA58E: printf (printf.c:33) ==2544006== by 0x10921A: main (main.cpp:14) ==2544006== Uninitialised value was created by a stack allocation ==2544006== at 0x109189: main (main.cpp:5) ==2544006== ==2544006== Conditional jump or move depends on uninitialised value(s) ==2544006== at 0x48C411C: _itoa_word (_itoa.c:180) ==2544006== by 0x48DF964: __vfprintf_internal (vfprintf-internal.c:1646) ==2544006== by 0x48CA58E: printf (printf.c:33) ==2544006== by 0x10921A: main (main.cpp:14) ==2544006== Uninitialised value was created by a stack allocation ==2544006== at 0x109189: main (main.cpp:5) ==2544006== ==2544006== Conditional jump or move depends on uninitialised value(s) ==2544006== at 0x48E05E3: __vfprintf_internal (vfprintf-internal.c:1646) ==2544006== by 0x48CA58E: printf (printf.c:33) ==2544006== by 0x10921A: main (main.cpp:14) ==2544006== Uninitialised value was created by a stack allocation ==2544006== at 0x109189: main (main.cpp:5) ==2544006== ==2544006== Conditional jump or move depends on uninitialised value(s) ==2544006== at 0x48DFA87: __vfprintf_internal (vfprintf-internal.c:1646) ==2544006== by 0x48CA58E: printf (printf.c:33) ==2544006== by 0x10921A: main (main.cpp:14) ==2544006== Uninitialised value was created by a stack allocation ==2544006== at 0x109189: main (main.cpp:5) ==2544006== 0: 00 ==2544006== ==2544006== HEAP SUMMARY: ==2544006== in use at exit: 0 bytes in 0 blocks ==2544006== total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated ==2544006== ==2544006== All heap blocks were freed -- no leaks are possible ==2544006== ==2544006== For lists of detected and suppressed errors, rerun with: -s ==2544006== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0) ``` I first caught this issue when using Google Test: https://github.com/google/googletest/issues/3805 The C version of pretty much the same code works file: `main.c` ``` #include <stdio.h> struct s {}; int main (void) { struct s value; const unsigned char* obj_bytes = (const unsigned char*) &value; size_t count = sizeof(value); printf("[%d %s] count : %ld, obj_bytes : %p\n", __LINE__, __FUNCTION__, count, obj_bytes); for (size_t i = 0; i < count; i++) { printf("%ld: %02x", i, obj_bytes[i]); } printf("\n"); return 0; } ``` ```sh $ gcc -O0 -g -Wall -o main main.c ``` ``` $ VALGRIND_LIB=~/work/googletest/bug/valgrind/src/valgrind-3.19.0/.in_place ~/work/googletest/bug/valgrind/src/valgrind-3.19.0/coregrind/valgrind --track-origins=yes --show-reachable=yes --leak-check=full ./main ==2544226== Memcheck, a memory error detector ==2544226== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al. ==2544226== Using Valgrind-3.19.0 and LibVEX; rerun with -h for copyright info ==2544226== Command: ./main ==2544226== [12 main] count : 0, obj_bytes : 0x1ffefffbaf ==2544226== ==2544226== HEAP SUMMARY: ==2544226== in use at exit: 0 bytes in 0 blocks ==2544226== total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated ==2544226== ==2544226== All heap blocks were freed -- no leaks are possible ==2544226== ==2544226== For lists of detected and suppressed errors, rerun with: -s ==2544226== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) ``` STEPS TO REPRODUCE 1. Compile the code example above 2. Run valgrind on the binary OBSERVED RESULT Bunch of warnings EXPECTED RESULT No warnings SOFTWARE/OS VERSIONS valgrind-3.17.0 Operating System: Kubuntu 21.10 KDE Plasma Version: 5.22.5 KDE Frameworks Version: 5.86.0 Qt Version: 5.15.2