Can anything bad happen (like undefined behavior, file corruption, etc.) if several threads concurrently call
Streams in C1 are thread-safe2. Functions are required to lock the stream before accessing it3.
fflush function is thread-safe and may be called from any thread at any time, as long as the stream is an output stream or an update stream4.
1 As per the current standard, which is C11.
2 (Quoted from: ISO/IEC 9899:201x 7.21.3 Streams 7):
"Each stream has an associated lock that is used to prevent data races when multiple threads of execution access a stream, and to restrict the interleaving of stream operations performed by multiple threads. Only one thread may hold this lock at a time. The lock is reentrant: a single thread may hold the lock multiple times at a given time."
3 (Quoted from: ISO/IEC 9899:201x 7.21.3 Streams 8):
"All functions that read, write, position, or query the position of a stream lock the stream before accessing it. They release the lock associated with the stream when the access is complete. reentrant: a single thread may hold the lock multiple times at a given time."
4 (Quoted from: ISO/IEC 9899:201x 18.104.22.168 The fflush function 2):
"If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined."