our comment was:

    The following source code compiles into a util that will demonstrate the issue with the FIONREAD ioctl.  It will read the entire file, and you can see the ioctl's output overflowing every 4GB.

    --------------------------------------------------------------------------

    // This is a quick test to try to narrow down a possible bug in
    // ifstream::readsome()

    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <errno.h>
    #include <stdio.h>
    #include <stdlib.h>

    #include <sys/ioctl.h>

    int main( int argc, char **argv)
    {   
        if (argc != 2)
        {
            fprintf( stderr, "Usage: %s <test_file>\n", argv[0]);
            return 1;
        }

        int fd = open(argv[1], O_RDONLY);
        
        if ( fd == -1)
        {
            fprintf( stderr, "Failed to open %s\n", argv[1]);
            return 2;
        }
        
        const unsigned bufLen = 1024 * 1024;  // 1MB reads
        char *buf = (char *)malloc( bufLen);

        ssize_t bytesRead;
        unsigned long long bytesAvail;
        fprintf( stderr, "*** Size of 'bytesAvail': %d ***\n", sizeof(bytesAvail));
        do
        {
            if (ioctl( fd, FIONREAD, &bytesAvail) != 0)
            {
                perror( "IOCTL error");
            }
            else
                fprintf(stderr, "Bytes Available: %lu (%f MB)\n", bytesAvail, bytesAvail/(float)(1024*1024));

            bytesRead = read( fd, buf, bufLen);
        } while (bytesRead > 0);

        if (bytesRead == -1)
        {
            perror( "Error read from file");
        }
        else
        {
            fprintf( stderr, "Successfully read %s\n", argv[1]);
        }
        
        free( buf);
        return 0;
    }

gcc-libraries package doesn't contain libstdc++.  Moving to gcc component.

FIONREAD ioctl (at least on Linux) writes int, so truncates all the upper bits silently.
Therefore, the #c2 testcase is wrong, passing address of long long there is invalid.
If it is fine if the showmanyc() method will sometimes return smaller number of bytes than actually available, perhaps libstdc++ could keep using the FIONREAD ioctl, but just not trust it if it stored there 0 and sizeof(int) < sizeof(std::streamsize), because in that case it might return 0 even when there are bytes available.

(In reply to Jakub Jelinek from comment #6)
> If it is fine if the showmanyc() method will sometimes return smaller number
> of bytes than actually available,

That is fine. The value returned by showmanyc is a lower bound:

  Returns: An estimate of the number of characters available in the sequence,
  or -1. If it returns a positive value, then successive calls to underflow()
  will not return traits::eof() until at least that number of characters have
  been extracted from the stream. If showmanyc() returns -1, then calls to
  underflow() or uflow() will fail.

It's even conforming to return 0 when there are bytes available, although doing so is not very useful.

Closing this BZ as the support case is closed which means we need to focus on more critical customer BZs. If this RFE is still important please open a new case via the Red Hat Customer Portal, access.redhat.com and ask that this BZ be re-opened.