homepage

open() and friends (like temporaryfile) does not document 'e' letter in second arguement. At least on Linux, it opens file with O_CLOEXEC.
Not sure under Windows.
Also, there are other interesting letters (man -a fopen), like:
c (since glibc 2.3.3)
 Do not make the open operation, or subsequent read and write operations, thread cancellation points.
e (since glibc 2.7)
 Open the file with the O_CLOEXEC flag. See open(2) for more information.
m (since glibc 2.3)
 Attempt to access the file using mmap(2), rather than I/O system calls (read(2), write(2)). Currently, use of mmap(2) is only attempted for a file opened for reading.
x Open the file exclusively (like the O_EXCL flag of open(2)). If the file already exists, fopen() fails, and sets errno to EEXIST. This flag is ignored for fdopen().