test_append.c

#include<assert.h>
#include<stdio.h>

int main (int argc, char *argv[])
{
   FILE *fp;
   int res;
   long len;
   
   fp = fopen("testappend", "wb+");
   res = fwrite("1234567890", 10, 1, fp);
   fclose(fp);

   fp = fopen("testappend", "ab+");
   res = fwrite("1234567890", 10, 1, fp);

   fseek(fp, -7, SEEK_END);
   len = ftell(fp);
   assert(len == 13);

   fclose(fp);
   return 0;
}