Assuming we're talking C here:
I would opt for fread() with the following notes:
- there are no guarantees that the whole file counts as one string.
- you need to know the length of the file to allocate enough memory.
- you need to append a trailing 0 in memory to terminate the string.
fgets() does return a 0-terminated string but is not exactly what is asked since it only reads until the next newline character which may be in the middle of the file.