while(true){ //infinite loop
 
if(fgets(line,size,fp)!=NULL)
    process line.
 
}

while ( 1 ) {
  while ( fgets(line,size,fp)!=NULL) {
    // process lines
  }
  clearerr( fp );  // reset end of file flag
  sleep( 1 );  // wait for more lines to arrive
}

#include<stdio.h>
#include<string.h>
 
int main(void)
{
    char buff[100];
    FILE* fp_1;
 
    fp_1 = fopen("./test.txt","r");
    memset(buff,0,sizeof(buff));
 
    while(1)
    {
        if(fgets(buff,100,fp_1))
              ;
        else
        {
            printf("\n %s \n",buff);  //process the last appended line here
            fclose(fp_1);
            fp_1 = fopen("./test.txt","r");
        }
 
    }
 
    return 0;
}