variations in fread() buffering

Hi all,

I was reading Vijaya Mukhi's The 'C' Odyssey UNIX - The Open-Boundless C (1st ed.). This is in reference to program 43 in the first chapter when file buffering is introduced.

Code:

int main() {
 FILE *fp;
 char buff[11]; 
 int pid;
 fp=fopen("baby1","r");
 pid=fork();
 if(!pid) {
  printf("initial file handle :: %d\n",ftell(fp));
  fread(buff,sizeof(buff),1,fp);
  buff[10]='\0';
  printf("child read :: %s\n",buff);
  printf("child file handle :: %d\n",ftell(fp));
 }
 else {
  wait((int *)0);
  printf("parent file handle :: %d\n",ftell(fp));
  fread(buff,sizeof(buff),1,fp);
  buff[10]='\0';
  printf("parent read :: %s\n",buff);
  printf("end file handle :: %d\n",ftell(fp));
 }
 return 0;
}

The contents of baby1 are:

As per the book, the output should be:

The book explains this by saying that fread() reads 1024 characters (whatever the block size is) by default into a buffer. Hence the file pointer also moves by 1024 bytes which is reflected because the process is different.

I tried the same code on Solaris and RHEL and got interesting results.

Output on Solaris:

This indicates that in Solaris, block size is the same as specified in the fread() arguments. There is no such thing called default block size.

Output on RHEL:

This indicates that in RHEL, block size is a number greater than 26 but the file pointer never exceeds the size of file.

From what I understood, across various OS'es system calls are different but library calls behave in the same way. Library calls should be system independent which is not the case here. I would like some clarification on this point.

I would also like to have some idea of how the file buffering takes place internally in case of system calls and library calls.

Thanks in advance.