C Programming/POSIX Reference/unistd.h/close

For most file systems, a program terminates access to a file in a filesystem using the close system call. This flushes buffers, updates file metadata (which may include and end of file indicator in the data), de-allocates resources associated with the file (including the file descriptor) and updates the system wide table of files in use. Some languages maintain a structure of files opened by its run-time library and may close when the program terminates. Some operating systems will invoke the close if the program terminates. Some operating systems will invoke the close as part of an operating system recovery as a result of a system failure.

C library POSIX definition
The close call is standardized by the POSIX specification

int close (int filedes); int fclose(FILE *stream);

The function returns zero to indicate the file was closed successfully. If any error occurs, a value of -1 is returned and errno is appropriately set.

The errors that can occur include:

Reference
POSIX close