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Using and understanding the Valgrind core
The error-checking tools detect numerous problems in the system libraries, such as the C library, which come pre-
installed with your OS. You can’t easily fix these, but you don’t want to see these errors (and yes, there are many!)
So Valgrind reads a list of errors to suppress at startup. A default suppression file is created by the
./configure
script when the system is built.
You can modify and add to the suppressions file at your leisure, or, better, write your own. Multiple suppression files
are allowed. This is useful if part of your project contains errors you can’t or don’t want to fix, yet you don’t want to
continuously be reminded of them.
Note: By far the easiest way to add suppressions is to use the
--gen-suppressions=yes
option described in
Core Command-line Options . This generates suppressions automatically. For best results, though, you may want to
edit the output of
--gen-suppressions=yes
by hand, in which case it would be advisable to read through this
section.
Each error to be suppressed is described very specifically, to minimise the possibility that a suppression-directive
inadvertently suppresses a bunch of similar errors which you did want to see. The suppression mechanism is designed
to allow precise yet flexible specification of errors to suppress.
If you use the -v option, at the end of execution, Valgrind prints out one line for each used suppression, giving its name
and the number of times it got used. Here’s the suppressions used by a run of valgrind --tool=memcheck
ls -l :
--27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getgrgid_r
--27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getpwuid_r
--27579-- supp: 6 strrchr/_dl_map_object_from_fd/_dl_map_object
Multiple suppressions files are allowed. By default, Valgrind uses $PREFIX/lib/valgrind/default.supp .
You can ask to add suppressions from another file, by specifying --suppressions=/path/to/file.supp .
If you want to understand more about suppressions, look at an existing suppressions file whilst reading the following
documentation. The file
glibc-2.3.supp
, in the source distribution, provides some good examples.
Each suppression has the following components:
• First line: its name. This merely gives a handy name to the suppression, by which it is referred to in the summary
of used suppressions printed out when a program finishes. It’s not important what the name is; any identifying
string will do.
• Second line: name of the tool(s) that the suppression is for (if more than one, comma-separated), and the name of
the suppression itself, separated by a colon (n.b.: no spaces are allowed), eg:
tool_name1,tool_name2:suppression_name
Recall that Valgrind is a modular system, in which different instrumentation tools can observe your program whilst it
is running. Since different tools detect different kinds of errors, it is necessary to say which tool(s) the suppression
is meaningful to.
Tools will complain, at startup, if a tool does not understand any suppression directed to it. Tools ignore
suppressions which are not directed to them. As a result, it is quite practical to put suppressions for all tools
into the same suppression file.
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