C ++ decimal integer delimiter

The original question titled A game company is as follows: write a thousand-bit separator algorithm, and the function prototype is char * format_thousands_separator (unsigned long val). The expected effect is 1. the user does not need to release the returned string pointer 2. A maximum of 16 calls are supported without returning the same pointer address. You can use the following methods to test
Printf ("num1 (% s), num2 (% s), num3 (% s)", format_thousands_separator (0), format_thousands_separator (123456), format_thousands_separator (23456789 ));
Note: Compile the code that can be compiled and run.

After modification, the simplest C code is described as follows:
1char * format_thousands_separator (unsigned long val)
2 {
3 static char buf [16] [16];
4 static int c = 0;
5
6 long m, n = 0;
7 char * p = & buf [c ++ % 16] [15];
8 * p =;
9
10 do
11 {
12 m = val % 10;
13 val = val/10;
14 * -- p = 0 + m;
15
16 if (val &&! (++ N % 3 ))
17 * -- p = ,;
18
19} while (val );
20
21 return p;
22}
Here we will make a slight extension to support negative numbers. The code description is as follows:
1char * format_thousands_separator (long val)
2 {
3 static char buf [16] [16];
4 static int c = 0;
5
6 long m, n = 0;
7 char * p = & buf [c ++ % 16] [15];
8 * p =;
9
10 do
11 {
12 m = val % 10;
13 val = val/10;
14 * -- p = 0 + (m <0? -M: m );
15
16 if (! Val & m <0)
17 * -- p = -;
18
19 if (val &&! (++ N % 3 ))
20 * -- p = ,;
21
22} while (val );
23
24 return p;
25}