Sometimes I find in the source code files comments have quotation marks like these ,notice the ` :
For example, these are comments from GNU cat source code file, cat.c:
/* Plain cat. Copies the file behind `input_desc' to STDOUT_FILENO. */
/* Select which version of `cat' to use. If any options (more than -u, --version, or --help) were specified, use `cat', otherwise use `simple_cat'. */
/* Suppress `used before initialized' warning. */
While in other parts, "" is used :
/* Determines how many consecutive newlines there have been in the input. 0 newlines makes NEWLINES -1, 1 newline makes NEWLINES 1, etc. Initially 0 to indicate that we are at the beginning of a new line. The "state" of the procedure is determined by NEWLINES. */
What does ` mean? and what is it used for?