c linux中但不是在Windows中的分段错误

Question

I am trying to write a code to split given Path by the character ":" and there is my code :

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

void parser()
{
    char ** res  = NULL;
    char *  p    = strtok (getenv("PATH"), ":");
    int n_spaces = 0, i;

    /* split string and append tokens to 'res' */

     while (p)
     {
         res = realloc (res, sizeof (char*) * ++n_spaces);

         if (res == NULL)
             exit (-1); /* memory allocation failed */

         res[n_spaces-1] = p;

         p = strtok (NULL, ":");
     }

     /* realloc one extra element for the last NULL */

      res = realloc (res, sizeof (char*) * (n_spaces+1));
      res[n_spaces] = 0;

    /* print the result */

      for (i = 0; i < (n_spaces+1); ++i)
          printf ("res[%d] = %s\n", i, res[i]);

    /* free the memory allocated */

     free (res);
}

int main(int argc , char* argv[])
{
    parser();
    return 0;
}

this code gives me segmentation fault in linux but when a tried to run it on windows , it worked fine !!

Answer 1

You are missing an include, namely #include <string.h> which is responsible for providing the prototype for the strtok function you are using. Missing the prototype for this is undefined behaviour, and should not surprise you to not work.

Additionally (credit to @milevyo for pointing this out):

You are not supposed to modify the pointer beeing returned by getenv() .

C Standard, Sec. 7.20.4.5, The getenv function

Using getenv()

The return value might be aimed at

 a read-only section of memory a single buffer whose contents are modified on each call getenv() returns the same value on each call a dynamically-allocated buffer that might be reallocated on the next call a tightly packed set of character strings with no room for expansion 

Use the returned string before calling getenv() again. Do not modify the returned string.

So by calling strtok to a variable assigned a pointer that has been returned from getenv() you are invoking additional undefined behaviour.

To correct this, copy the string that the pointer which getenv() returned is pointing to into an auxiliary variable with strdup()