Both GCC and CLang support a C extension attribute((cleanup)) which allows you to define a function that is invoked when a stack variable exits scope. This typically used to free the memory allocated to it, though you're not restricted to this. For example it could be used to unlock a mutex.
We could use that functionality now, but the syntax is a bit ugly in plain C. Since version 2.44 of GLib, there have been a few macros to make it more friendly to use - g_autofree, g_autoptr and G_DEFINE_AUTOPTR_CLEANUP_FUNC. https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html https://blogs.gnome.org/desrt/2015/01/30/g_autoptr/ The key selling point is that it simplifies the cleanup code paths, often eliminating the need to goto cleanup labels. This improves the readability of the code and makes it less likely that you'll leak memory accidentally. Inspired by seeing it added to glib, and used in systemd, Libvirt finally got around to adopting this in Feb 2019. Overall our experience with it has been favourable/positive, with the code simplification being very nice. The main caveats with it are - Only works with GCC or CLang. We don't care as those are the only two compilers we declare support for. - You must always initialize the variables when declared to ensure predictable behaviour when they leave scope. Chances are most methods with goto jumps for cleanup are doing this already - You must not directly return the value that's assigned to a auto-cleaned variable. You must steal the pointer in some way. ie BAD: g_autofree char *wibble = g_strdup("wibble") .... return wibble; GOOD: g_autofree char *wibble = g_strdup("wibble") ... return g_steal_pointer(wibble); g_steal_pointer is an inline function which simply copies the pointer to a new variable, and sets the original variable to NULL, thus avoiding cleanup. I've illustrated the usage by converting a bunch of the crypto code in QEMU to use auto cleanup. Daniel P. Berrangé (3): glib: bump min required glib library version to 2.48 crypto: define cleanup functions for use with g_autoptr crypto: use auto cleanup for many stack variables configure | 2 +- crypto/afsplit.c | 28 ++++---------- crypto/block-luks.c | 74 +++++++++++-------------------------- crypto/block.c | 15 +++----- crypto/hmac-glib.c | 5 --- crypto/pbkdf.c | 5 +-- crypto/secret.c | 9 ++--- crypto/tlscredsanon.c | 16 +++----- crypto/tlscredspsk.c | 5 +-- crypto/tlscredsx509.c | 16 +++----- include/crypto/block.h | 2 + include/crypto/cipher.h | 2 + include/crypto/hmac.h | 2 + include/crypto/ivgen.h | 2 + include/crypto/tlssession.h | 2 + include/glib-compat.h | 42 +-------------------- 16 files changed, 66 insertions(+), 161 deletions(-) -- 2.21.0
