Unless otherwise specified, all functions which allocate memory in GLib will abort the process if heap allocation fails. This is because it is too hard to recover from allocation failures in any non-trivial program and, in particular, to test all the allocation failure code paths.
All GLib, GObject and GIO functions accept and return strings in UTF-8 encoding unless otherwise specified.
Input strings to function calls are not checked to see if they are valid UTF-8: it is the application developer’s responsibility to validate input strings at the time of input, either at the program or library boundary, and to only use valid UTF-8 string constants in their application. If GLib were to UTF-8 validate all string inputs to all functions, there would be a significant drop in performance.
Similarly, output strings from functions are guaranteed to be in UTF-8, and this does not need to be validated by the calling function. If a function returns invalid UTF-8 (and is not documented as doing so), that’s a bug.
The general policy of GLib is that all functions are invisibly threadsafe with the exception of data structure manipulation functions, where, if you have two threads manipulating the same data structure, they must use a lock to synchronize their operation.
GLib creates a worker thread for its own purposes so GLib applications will always have at least 2 threads.
When writing code that runs with elevated privileges, it is important to follow some basic rules of secure programming. David Wheeler has an excellent book on this topic, Secure Programming for Linux and Unix HOWTO.
When it comes to GLib and its associated libraries, GLib and GObject are generally fine to use in code that runs with elevated privileges; they don’t load modules (executable code in shared objects) or run other programs ‘behind your back’. GIO, however, is not designed to be used in privileged programs, either ones which are spawned by a privileged process, or ones which are run with a setuid bit set.
setuid programs should always reset their environment to contain only known-safe values before calling into non-trivial libraries such as GIO. This reduces the risk of an attacker-controlled environment variable being used to get a privileged GIO process to run arbitrary code via loading a GIO module or similar.