》Nowhere But the first thing that came to me upon "mode argument" was the fopen() function because it has an argument named 'mode'
Yes, fopen() and many other functions have an argument called "mode" but only some do truncate a file. FYI
int open(const char *pathname, int mode);
FILE *fopen(const char *path, const char *mode);
int creat(const char *pathname, mode_t mode);
int chmod(const char *path, mode_t mode);
int mkdir(const char *pathname, mode_t mode);
int faccessat(int dirfd, const char *pathname, int mode, int flags);
int access(const char *pathname, int mode);
An argument, can't exsist without a function, and it is missing in the question.Also there's not a clue that q. is about files.
》and the word "truncate" sounded like - open file for read/write and reset the file to zero bytes.
That is what you think, almost all data structures and types can be truncated...
Whoever can answer this question as it is, should be able to code a solution for an unknown problem.