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3.6.5 Uncommon Options

These options apply to all tools, as they affect certain obscure workings of the Valgrind core. Most people won't need to use these.

• --run-libc-freeres=<yes|no> [default: yes] The GNU C library (libc.so), which is used by all programs, may allocate memory for its own uses. Usually it doesn't bother to free that memory when the program ends--there would be no point, since the Linux kernel reclaims all process resources when a process exits anyway, so it would just slow things down. The glibc authors realised that this behaviour causes leak checkers, such as Valgrind, to falsely report leaks in glibc, when a leak check is done at exit. In order to avoid this, they provided a routine called __libc_freeres specifically to make glibc release all memory it has allocated. Memcheck therefore tries to run __libc_freeres at exit. Unfortunately, in some very old versions of glibc, __libc_freeres is sufficiently buggy to cause segmentation faults. This was particularly noticeable on Red Hat 7.1. So this flag is provided in order to inhibit the run of __libc_freeres. If your program seems to run fine on Valgrind, but segfaults at exit, you may find that --run-libc-freeres=no fixes that, although at the cost of possibly falsely reporting space leaks in ‘libc.so’.
• --sim-hints=hint1,hint2,... Pass miscellaneous hints to Valgrind which slightly modify the simulated behaviour in nonstandard or dangerous ways, possibly to help the simulation of strange features. By default no hints are enabled. Use with caution! Currently known hints are:
  • lax-ioctls: Be very lax about ioctl handling; the only assumption is that the size is correct. Doesn't require the full buffer to be initialized when writing. Without this, using some device drivers with a large number of strange ioctl commands becomes very tiresome.
  • enable-inner: Enable some special magic needed when the program being run is itself Valgrind.
• --kernel-variant=variant1,variant2,... Handle system calls and ioctls arising from minor variants of the default kernel for this platform. This is useful for running on hacked kernels or with kernel modules which support nonstandard ioctls, for example. Use with caution. If you don't understand what this option does then you almost certainly don't need it. Currently known variants are:
  • bproc: Support the sys_broc system call on x86. This is for running on BProc, which is a minor variant of standard Linux which is sometimes used for building clusters.
• --show-emwarns=<yes|no> [default: no] When enabled, Valgrind will emit warnings about its CPU emulation in certain cases. These are usually not interesting.
• --smc-check=<none|stack|all> [default: stack] This option controls Valgrind's detection of self-modifying code. Valgrind can do no detection, detect self-modifying code on the stack, or detect self-modifying code anywhere. Note that the default option will catch the vast majority of cases, as far as we know. Running with all will slow Valgrind down greatly. Running with none will rarely speed things up, since very little code gets put on the stack for most programs. Some architectures (including ppc32 and ppc64) require programs which create code at runtime to flush the instruction cache in between code generation and first use. Valgrind observes and honours such instructions. Hence, on ppc32/Linux and ppc64/Linux, Valgrind always provides complete, transparent support for self-modifying code. It is only on x86/Linux and amd64/Linux that you need to use this flag.