The difference between r+ and w+ in fopen

r+: Open for reading and writing. The stream is positioned at the beginning of the file.

W+:open for reading and writing.  The file is created if it does isn't exist, otherwise it is truncated. The stream is positioned at the beginning of the file.

R+ has read and write properties, start writing from the file header, preserving content that is not covered in the original file;

The w+ has read and write properties, and when written, if the file exists, it is emptied and written from the beginning.

R to open a read-only file, the file must exist.
r+ open a writable file, the file must exist.
W Open Write-only file, if the file exists then the file length is clear to 0, that is, the contents of the file will disappear. If the file does not exist, the file is created.
w+ Open a read-write file, if the file exists then the file length is clear to zero, that is, the contents of the file will disappear. If the file does not exist, the file is created.
A write-only file opens in an additional way. If the file does not exist, the file will be created, and if the file exists, the data written will be added to the end of the file, that is, the original content of the file will be retained.
A + opens readable and writable files in an additional way. If the file does not exist, the file will be created, and if the file exists, the data written will be added to the end of the file, that is, the original content of the file will be retained.
These morphological strings can be added with a B character, such as RB, w+b, or ab+ combinations, and a B character is used to tell the library that the file opened is a binary file, not a plain text file. However, in a POSIX system, including Linux ignores the character.

The difference between r+ and w+ in fopen