Conversion of macros into String literals

The # preprocessor operator converts the argument that follows it into a string literal. The preprocessor operator # can be used only in a function-like macro definition.

#include <stdio.h>

#define PRINT(name) printf("The value of " #name " is %d\n", name)

main()
{
 int abc = 100;

 PRINT(abc);
}

Output: The value of abc is 100
Macro Expansion
--> printf("The value of " #name " is %d\n", name)
--> printf("The value of " "abc" " is %d\n", 100)
--> printf("The value of abc is %d\n", 100)
The unary # operator produces a string from its operand. Adjacent string literals are getting concatenated in above example. If the operand to # contains double quotes or escape sequences, they are also expanded.
Similarly ## operator concatenate two tokens passed in macro to a single token.

#define JOIN(a,b)  a ## b
main()
{
   int myname = 100;
   printf("%d", JOIN(my,name));
}
It will be converted into printf("%d", myname);

setting mode permission from command line

type following command in command prompt.
chmod {mode} filename, file2, file3

In C language its used as:
int chmod(const char *File_path, mode_t mode_value);

How to decide mode octal value:
mode is of 4 octal digits e.g. 0754, equivalent to (userID)(user)(group)(Other)

0755: u: full, GO: read and execute

0777: all : full permission

0555: all : read and execute


 symbolic representation of file permissions:

Representation	Class	Description
u	User	Owner
g	Group	Members of file group
o	Others	Neither owner nor group
a	All	Everyone

Octal notation:

Octal	System	Description
0	---	No permission
1	--x	execute
2	-w-	write
3	-wx	Write and execute
4	r--	Read
5	r-x	Read and execute
6	rw-	Read and write
7	rwx	Read, write and execute

These octal values are decided based on binary position of R W X.
R W X
0  0  0
0  0  1 : execute
0  1  0 : Write
1  0  0 : Read

Ex: changing permission for files in whole directory recursively as read, write, execute
chmod -R -v 777 ./*

reference: chmod