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New in C and pretty confused about how to deal with several strings at the same time using strtok, for a simply example, I want to use strtok to extract the number and compare then.

#include <stdio.h>
#include <string.h>
int main()
{
   char s1[100]="11.54";
   char s2[100]="12.55";

   const char tok[2]=".";
   char* token1=strtok(s1,tok);
   char* token2=strtok(s2,tok);


while(token1 !=NULL && token2 !=NULL){
   int temp=strcmp(token1,token2);

   if(temp==0){
       token1=strtok(NULL,tok);
       token2=strtok(NULL,tok);

   }
   else if(temp<0){
     printf("%d\n",-1);
     return;
   } 
   else{
        printf("%d\n",1);
      return;
   }

}

if(token1 !=NULL){
  printf("%d\n",1);
  return; 
} 
if(token2 !=NULL){
   printf("%d\n",-1);
   return;
}

printf("%d\n",0);

return 0;
}

But when I use the strtok, the strtok(NULL,token)will point to the current string and will do like: 11->12>55->NULL and skip the 54

How could I deal with such situation? Thanks!!

ilyak
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  • `strsep` is also a good choice, if you have it. – user3386109 Nov 01 '15 at 18:50
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    As melpomene - `strtok()` can only handle one string at a time. Or, you could extract all the information you need from one string, then work on the other string. – Weather Vane Nov 01 '15 at 18:53
  • As mentioned above strtok() can handle one string at a time. It tokenizes based on "." . So at first, your tokens are token1:11 , token2:12 which are unequal. It prints -1 and returns from while. – Anusha HV Nov 01 '15 at 19:00

1 Answers1

7

Do not use strtok(). The documentation will tell you strtok() is not reentrant (i.e. should not be used across threads), but perhaps less obvious is the fact that the reason it is not reentrant is because it uses an internal save variable to remember where it's got to. That means you also can't use two instances at once. Instead use strtok_r() or failing that strsep() might work.

strtok_r() is just like strtok, save that you pass it a char ** (i.e. a pointer to char *) where it can save where it's got to.

The GNU libc manual page gives a good example of using a nested strtok_r which is what you are trying to do:

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

   int
   main(int argc, char *argv[])
   {
       char *str1, *str2, *token, *subtoken;
       char *saveptr1, *saveptr2;
       int j;

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

       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);
           }
       }

       exit(EXIT_SUCCESS);
   }
abligh
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