Multiple fdopen() on a same filedescriptor fd? - c

Question

I am trying to understand the standard I/O. I met a problem of calling fdopen().
What's the behavior if I call fdopen() on the same file descriptor as follows? Why do I get an ouput of '\377' (-1) ?
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
int main()
{
int fd1, fd2;
char c;
FILE *fp1, *fp2;
fd1 = open("foo.txt", O_RDONLY, 0);
fp1 = fdopen(fd1, "r");
fp2 = fdopen(fd1, "r");
if (fp2 == NULL)
printf("NULL\n");
if (errno)
printf("ERROR\n");
c = fgetc(fp1);
c = fgetc(fp2);
printf("c = %c\n", c);
exit(0);
}

Answer 1

Let's say your stdio buffer size is 4K. The first fgetc reads 4K into the buffer and returns the first byte. The fd is now advanced 4K into the file. The second fgetc reads from there. Your file is smaller than the buffer size, so you're at EOF. You print the EOF with %c and get a funny character.
Multiple fdopen on a single fd gets a vote of don't try it; it will hurt from me. With an exception for creating stdin, stdout, and stderr from a single tty descriptor if you're writing getty.

Answer 2

Multiple problems:
char is not the right type for storing the return value of fgetc. Use int.
You're accessing the same open file description via two different handles without performing the steps necessary to switch between them legally. This invokes undefined behavior.
Checking errno and inferring from it that there was an error is not valid. If you already know there was an error, errno tells you which one. It does not tell you whether or not an error occurred, and in case one did not, any nonzero value may have been written to errno.
We don't know your file contents so we can't know what you expect to be read.