Many UNIX/POSIX functions put certain constraints on the values of their parameters. The behavior for some of those UNIX/POSIX functions is not
defined but instead, their behavior is implementation-defined, if one passes incorrectly constrained parameters. This may lead to undefined behavior
depending on a function’s concrete implementation. The constraints include the following:
- Allocation sizes of
calloc, malloc, realloc, reallocf, alloca and
valloc must be strictly positive
-
open and openat should be called with a flag that contains one of the access modes: O_RDONLY,
O_WRONLY, or O_RDWR
-
open and openat with flag O_CREAT should be called with a third argument
- The
O_EXCL flag should be used with O_CREAT
- The first argument of
pthread_once should not have automatic storage duration and should be initialized by the constant
PTHREAD_ONCE_INIT
Failing to pass correctly constrained parameters can result in undefined behavior.
// Depending on the implementation, either NULL is returned, or the behavior is
// as if the passed size parameter were a non-zero value, except that accesses
// of the returned pointer result in undefined behavior.
void *mem = alloca(0); // May result in undefined behavior.
int fd = open("example.txt", O_ APPEND); // Access mode is missing, may result in undefined behavior.
// Third argument should be used to specify the file's access permissions.
int fd_1 = open("another_example.txt", O_CREAT); // May result in undefined behavior.
// Since `O_EXCL` prevents file creation if it already exists, the flag for
// file creation `O_CREAT` should be used in combination with `O_EXCL`.
int fd_3 = open("further_example.txt", O_EXCL); // `O_CREAT` flag is missing, may result in undefined behavior.
int counter = 0;
void inc_counter() { ++counter; }
void bar() {
// May trigger undefined behavior, because `once_control`'s storage duration
// is automatic. `counter` might be incremented with each call to `bar`.
pthread_once_t once_control = PTHREAD_ONCE_INIT;
pthread_once(&once_control, &inc_counter); // May result in undefined behavior.
}
