Test case:
#include <cstdlib>
#include <iostream>
#include <unistd.h>
#include <string.h>
int
main(int argc, char * argv[])
{
if (argc != 2)
{
std::cerr << "Usage: ./test writer | ./test reader" << std::endl;
return 2;
}
if (strcmp("reader", argv[1]) == 0)
{
// note, no input/output is allowed to happen before this call.
std::cin.sync_with_stdio(false);
int length = 0;
while (std::cin.good())
{
char buffer[BUFSIZ];
(void)std::cin.read(buffer, sizeof(buffer));
length += std::cin.gcount();
}
if (length == 82)
{
std::cerr << "Success" << std::endl;
return EXIT_SUCCESS;
}
else
{
std::cerr << "Failure, length is " << length << std::endl;
return EXIT_FAILURE;
}
}
else if (strcmp("writer", argv[1]) == 0)
{
std::cout << "1234567890123456789012345678901234567890" << std::endl;
sleep(1);
std::cout << "1234567890123456789012345678901234567890" << std::endl;
return EXIT_SUCCESS;
}
else
{
std::cerr << "Bad argument" << std::endl;
return 3;
}
}
Analysis:
When sync_with_stdio(false), cin uses basic_filebuf which is based on
__basic_file, and its xsgetn function:
streamsize
__basic_file<char>::xsgetn(char* __s, streamsize __n)
{
streamsize __ret;
do
__ret = read(this->fd(), __s, __n);
while (__ret == -1L && errno == EINTR);
return __ret;
}
When fd() refers to a pipe, this function may return before __n characters are
read, but also before an end-of-file is detected (say if the pipe supplier just
isn't finished yet). For the Posix read function this is a normal operating
mode. However for one client of this function (basic_filebuf::xsgetn, called
by istream::read), if less than __n characters are returned, this represents an
error, subsequently setting failbit|eofbit in the istream.
The fix is to change __basic_file<char>::xsgetn to hang in there until either
EOF is detected, or until there is a non-recoverable error. I believe the
following rewrite will do the job:
streamsize
__basic_file<char>::xsgetn(char* __s, streamsize __n)
{
streamsize __ret = 0;
for (streamsize __i; __n > 0; __ret += __i, __s += __i, __n -= __i)
{
__i = read(this->fd(), __s, __n);
if (__i == 0)
break;
if (__i == -1)
{
if (!(errno == EINTR || errno == EAGAIN))
break;
__i = 0;
}
}
return __ret;
}
--
Summary: __basic_file<char>::xsgetn does not deal well with pipes
Product: gcc
Version: 4.0.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: hhinnant at apple dot com
GCC host triplet: Mac OS 10.4.5
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27168
