#include <stdio.h>
 
#define BUFMAX       256
#define CURLINE      0
#define NEXTLINE     1
#define TOGGLELINE   (CURLINE ^ NEXTLINE)
 
int main ()
{
   FILE* fp = stdin; // fopen("test-seeking.txt", "r");
 
   char lineBuffer[2][BUFMAX];
   char* curLine;
   char* nextLine;
   int lineSel;
 
   if (fp != NULL)
   {
      if ((curLine = fgets(lineBuffer[CURLINE], BUFMAX, fp)))
      {
         for (lineSel = NEXTLINE;
              (nextLine = fgets(lineBuffer[lineSel], BUFMAX, fp));
              lineSel ^= TOGGLELINE)
         {
            printf("Current line: \"%s\" / Next line \"%s\"\n",
                   curLine, nextLine);
 
            curLine = nextLine;
         }
      }
 
      fclose(fp);
   }
 
   return 0;
}

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line 1
line 2
line 3
line 4
line 5
line 6
line 7