https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79283
Bug ID: 79283
Summary: read_symlink fails with /proc symlinks
Product: gcc
Version: 6.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: achille.fouilleul+gcc at gadz dot org
Target Milestone: ---
Consider the following program:
#include <cstdio>
#include <exception>
#include <experimental/filesystem>
//#include <boost/filesystem.hpp>
namespace fs = std::experimental::filesystem;
//namespace fs = boost::filesystem;
int main()
{
try {
fs::path symlinkPath("/proc/self");
auto targetPath = fs::read_symlink(symlinkPath);
auto s = targetPath.native();
printf("%s\n", s.c_str());
return 0;
}
catch (const std::exception& e)
{
fprintf(stderr, "%s\n", e.what());
return 1;
}
}
On Linux it fails with the following message:
filesystem error: read_symlink: Invalid argument [/proc/self]
When run with strace I get this:
...
lstat("/proc/self", {st_mode=S_IFLNK|0777, st_size=0, ...}) = 0
readlink("/proc/self", 0x7ffe21d33700, 0) = -1 EINVAL (Invalid argument)
...
Indeed in libstdc++-v3/src/filesystem/ops.cc the readlink call is preceeded by
a lstat call to determine the size of the buffer. This does not work with
Linux's /proc (or /sys for that matter), since the st_size of pseudo-filesystem
links is always zero.
Boost's read_symlink follows a different approach: it starts with a 64-byte
buffer and double its size until readlink succeeds.
