What does _r suffix mean?

• The suffix "_r" is historical, where the 'r' stood for "reentrant".
• A computer program or subroutine is called reentrant.
• if it can be interrupted in the middle of its execution and then safely be called again ("re-entered") before its previous invocations complete execution. 
• 
  
• Using global variable inside the function, ISR(interuppt call) may try to access our temporary changed global value in outside.
• Solution:
• Use the local variable instead of global variable will clear this issue
• _r reason:
• Using _r to make sure global variables are not affected.


• _r function will be thread safe.
• EG: strtok vs strtok_r
• strtok_r() is thread-safe.
• The strtok_r() function is a reentrant version of strtok().
• Strtok_r uses the 3arg as the local variable instead of global variable to record the progress through s1.
• https://en.wikipedia.org/wiki/Reentrancy_(computing)

Return:





The strtok() and strtok_r() functions return a pointer to the next token, or NULL if there
       are no more tokens.

Attributes:





For an explanation of the terms used in this section, see attributes(7).

       ┌───────────┬───────────────┬───────────────────────┐
       │Interface  │ Attribute     │ Value                 │
       ├───────────┼───────────────┼───────────────────────┤
       │strtok()   │ Thread safety │ MT-Unsafe race:strtok │
       ├───────────┼───────────────┼───────────────────────┤
       │strtok_r() │ Thread safety │ MT-Safe               │
       └───────────┴───────────────┴───────────────────────┘

Program:

/* strtok vs strtok_r by Velraj.K
   Check : http://velrajcoding.blogspot.in
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
    char *str1, *str2, *token, *subtoken;
    char *saveptr1, *saveptr2;
    char str_second[256] = {'\0'};
    int j;

    if (argc != 4) {
        fprintf(stderr, "Usage: %s string delim subdelim\n",
                argv[0]);
        exit(EXIT_FAILURE);
    }

    strncpy(str_second, argv[1], sizeof(str_second));

    printf("\n\n\t\t Do the token using strtok_r \n");

    /* use strtok_r */
    for (j = 1, str1 = argv[1]; ; j++, str1 = NULL) {
        token = strtok_r(str1, argv[2], &saveptr1);
        if (token == NULL)
            break;
        printf("%d: %s\n", j, token);

        for (str2 = token; ; str2 = NULL) {
            subtoken = strtok_r(str2, argv[3], &saveptr2);
            if (subtoken == NULL)
                break;
            printf(" --> %s\n", subtoken);
        }
    }
    printf("Original value = %s \n", argv[1]);

    printf("\n\n\t\tDo the token using strtok str_second = %s \n", str_second);
    /* Initialize the variable to NULL to start token again with strtok */
    str1 = NULL;
    str2 = NULL;
    token = NULL;
    subtoken = NULL;
    saveptr1 = NULL;
    saveptr2 = NULL;

    /* use strtok */
    for (j = 1, str1 = str_second; ; j++, str1 = NULL) {
        token = strtok(str1, argv[2]);
        if (token == NULL)
            break;
        printf("%d: %s\n", j, token);
#if 0 // Could not use the 2nd strtok with different delim because it uses the global pointer
        for (str2 = token; ; str2 = NULL) {
            subtoken = strtok(str2, argv[3]);
            if (subtoken == NULL)
                break;
            printf(" --> %s\n", subtoken);
        }
#endif
    }

    exit(EXIT_SUCCESS);
}

Output:

learn$ ./a.out this,isvelraj,check,strok,new , .

                 Do the token using strtok_r
1: this
 --> this
2: isvelraj
 --> isvelraj
3: check
 --> check
4: strok
 --> strok
5: new
 --> new
Original value = this

                Do the token using strtok str_second = this,isvelraj,check,strok,new
1: this
2: isvelraj
3: check
4: strok
5: new