Why you need symbol concat in macro

gistfile1.c

/* If you had this it would break */

#define SWAP(x, y, T) do { T temp = x; x = y; y = temp; } while (0)


int main(void) {
  int temp;
  int a = 1;
  int b = 2;

  SWAP(a, b, int);

}

/* Would be: */


int main(void) {
  int temp = 3;
  int a = 1;
  int b = 2;

  do { int temp = x; x = y; y = temp; } while (0);

}

/* Which would override the temp value */



/* If you had this it would break */

#define SWAP(x, y, T) do { T temp = x; x = y; y = temp; } while (0)


int main(void) {
  int temp;
  int a = 1;
  int b = 2;

  SWAP(a, b, int);

}

/* Would be: */


int main(void) {
  int temp = 3;
  int a = 1;
  int b = 2;

  do { int temp = x; x = y; y = temp; } while (0);

}

/* Which would override the temp value */



/* If you had this it would break */

#define SWAP(x, y, T) do { T temp = x; x = y; y = temp; } while (0)


int main(void) {
  int temp;
  int a = 1;
  int b = 2;

  SWAP(a, b, int);

}

/* Would be: */


int main(void) {
  int temp = 3;
  int a = 1;
  int b = 2;

  do { int temp = x; x = y; y = temp; } while (0);

}

/* Which would override the temp value */



/* If you had this it would break */

#define SWAP(x, y, T) do { T temp = x; x = y; y = temp; } while (0)


int main(void) {
  int temp;
  int a = 1;
  int b = 2;

  SWAP(a, b, int);

}

/* Would be: */


int main(void) {
  int temp = 3;
  int a = 1;
  int b = 2;

  do { int temp = x; x = y; y = temp; } while (0);

}

/* Which would override the temp value */



/* If you had this it would break */

#define SWAP(x, y, T) do { T temp = x; x = y; y = temp; } while (0)


int main(void) {
  int temp;
  int a = 1;
  int b = 2;

  SWAP(a, b, int);

}

/* Would be: */


int main(void) {
  int temp = 3;
  int a = 1;
  int b = 2;

  do { int temp = x; x = y; y = temp; } while (0);

}

/* Which would override the temp value */



/* If you have this it would not break */

#define SWAP(x, y, T) do { T temp##x##y = x; x = y; y = temp##x##y; } while (0)


int main(void) {
  int temp;
  int a = 1;
  int b = 2;

  SWAP(a, b, int);

}

/* Would be: */


int main(void) {
  int temp = 3;
  int a = 1;
  int b = 2;

  do { int temp12 = x; x = y; y = temp12; } while (0);

}

/* Which is ok */