When zeroing a struct such as sockaddr_in, sockaddr_in6 and addrinfo before use, which is correct: memset, an initializer or either?

Question

Whenever I look at real code or example socket code in books, man pages and websites, I almost always see something like:

struct sockaddr_in foo;
memset(&         


        

Answer 1

One problem with the partial initializers approach (that is '{ 0 }') is that GCC will warn you that the initializer is incomplete (if the warning level is high enough; I usually use '-Wall' and often '-Wextra'). With the designated initializer approach, that warning should not be given, but C99 is still not widely used - though these parts are fairly widely available, except, perhaps, in the world of Microsoft.

I tend used to favour an approach:

static const struct sockaddr_in zero_sockaddr_in;

Followed by:

struct sockaddr_in foo = zero_sockaddr_in;

The omission of the initializer in the static constant means everything is zero - but the compiler won't witter (shouldn't witter). The assignment uses the compiler's innate memory copy which won't be slower than a function call unless the compiler is seriously deficient.

---

GCC has changed over time

GCC versions 4.4.2 to 4.6.0 generate different warnings from GCC 4.7.1. Specifically, GCC 4.7.1 recognizes the = { 0 } initializer as a 'special case' and doesn't complain, whereas GCC 4.6.0 etc did complain.

Consider file init.c:

struct xyz
{
    int x;
    int y;
    int z;
};

struct xyz xyz0;                // No explicit initializer; no warning
struct xyz xyz1 = { 0 };        // Shorthand, recognized by 4.7.1 but not 4.6.0
struct xyz xyz2 = { 0, 0 };     // Missing an initializer; always a warning
struct xyz xyz3 = { 0, 0, 0 };  // Fully initialized; no warning

When compiled with GCC 4.4.2 (on Mac OS X), the warnings are:

$ /usr/gcc/v4.4.2/bin/gcc -O3 -g -std=c99 -Wall -Wextra -c init.c
init.c:9: warning: missing initializer
init.c:9: warning: (near initialization for ‘xyz1.y’)
init.c:10: warning: missing initializer
init.c:10: warning: (near initialization for ‘xyz2.z’)
$

When compiled with GCC 4.5.1, the warnings are:

$ /usr/gcc/v4.5.1/bin/gcc -O3 -g -std=c99 -Wall -Wextra -c init.c
init.c:9:8: warning: missing initializer
init.c:9:8: warning: (near initialization for ‘xyz1.y’)
init.c:10:8: warning: missing initializer
init.c:10:8: warning: (near initialization for ‘xyz2.z’)
$

When compiled with GCC 4.6.0, the warnings are:

$ /usr/gcc/v4.6.0/bin/gcc -O3 -g -std=c99 -Wall -Wextra -c init.c
init.c:9:8: warning: missing initializer [-Wmissing-field-initializers]
init.c:9:8: warning: (near initialization for ‘xyz1.y’) [-Wmissing-field-initializers]
init.c:10:8: warning: missing initializer [-Wmissing-field-initializers]
init.c:10:8: warning: (near initialization for ‘xyz2.z’) [-Wmissing-field-initializers]
$

When compiled with GCC 4.7.1, the warnings are:

$ /usr/gcc/v4.7.1/bin/gcc -O3 -g -std=c99 -Wall -Wextra  -c init.c
init.c:10:8: warning: missing initializer [-Wmissing-field-initializers]
init.c:10:8: warning: (near initialization for ‘xyz2.z’) [-Wmissing-field-initializers]
$

The compilers above were compiled by me. The Apple-provided compilers are nominally GCC 4.2.1 and Clang:

$ /usr/bin/clang -O3 -g -std=c99 -Wall -Wextra -c init.c
init.c:9:23: warning: missing field 'y' initializer [-Wmissing-field-initializers]
struct xyz xyz1 = { 0 };
                      ^
init.c:10:26: warning: missing field 'z' initializer [-Wmissing-field-initializers]
struct xyz xyz2 = { 0, 0 };
                         ^
2 warnings generated.
$ clang --version
Apple clang version 4.1 (tags/Apple/clang-421.11.65) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
$ /usr/bin/gcc -O3 -g -std=c99 -Wall -Wextra -c init.c
init.c:9: warning: missing initializer
init.c:9: warning: (near initialization for ‘xyz1.y’)
init.c:10: warning: missing initializer
init.c:10: warning: (near initialization for ‘xyz2.z’)
$ /usr/bin/gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$

As noted by SecurityMatt in a comment below, the advantage of memset() over copying a structure from memory is that the copy from memory is more expensive, requiring access to two memory locations (source and destination) instead of just one. By comparison, setting the values to zeroes doesn't have to access the memory for source, and on modern systems, the memory is a bottleneck. So, memset() coding should be faster than copy for simple initializers (where the same value, normally all zero bytes, is being placed in the target memory). If the initializers are a complex mix of values (not all zero bytes), then the balance may be changed in favour of an initializer, for notational compactness and reliability if nothing else.

There isn't a single cut and dried answer...there probably never was, and there isn't now. I still tend to use initializers, but memset() is often a valid alternative.