I am trying to compile a library using clang. The library makes calls to 'unlink', which is not defined by clang:
libmv/src/third_party/OpenExif/src/ExifImageFileWrite.cpp:162:17: error: use of undeclared identifier 'unlink'; did you mean 'inline'?
unlink( mTmpImageFile.c_str() ) ;
My question is, what is the clang equivalent of unlink? As I see it, the path forward would be to #define unlink somewhere with an equivalent routine.
There is no "Clang equivalent". Neither GCC nor Clang have ever been responsible for defining unlink, though they do probably distribute the POSIX headers which do (I don't recall specifically where POSIX headers come from).
Unfortunately, this appears to be a bug with the library you're using; the OpenExif developers failed to include the correct headers. Different C++ implementations may internally #include various headers for their own purposes, which has apparently masked this bug on your previous toolchain.
You can hack your copy and/or submit a patch to add:
#include <unistd.h>
Related
When I'm building POSIX C programs, I want to be portable and use only POSIX or standard C library functions. So, for example, with gcc or clang, I build like this:
gcc -std=c99 -D_XOPEN_SOURCE=600
Setting the standard to C99 removes all extensions, then _XOPEN_SOURCE exposes POSIX interfaces. I no longer have the environment polluted with extensions from GNU, BSD, etc.
However, the waters seem murkier with C++. I want to do this:
g++ -std=c++14 -D_XOPEN_SOURCE=600
This has worked fine for me on various operating systems: Linux/glibc, Haiku, MinGW, macOS, at least. But apparently, there are problems with POSIX feature test macros and C++. Oracle docs have this to say:
C++ bindings are not defined for POSIX or SUSv4, so specifying feature test macros such as _POSIX_SOURCE, _POSIX_C_SOURCE, and _XOPEN_SOURCE can result in compilation errors due to conflicting requirements of standard C++ and these specifications.
While I don't have a copy of Oracle Solaris, I am seeing issues with FreeBSD and OpenBSD.
On FreeBSD:
#include <iostream>
int main() { }
$ clang++ -std=c++14 -D_POSIX_C_SOURCE=200112L t.cpp
In file included from t.cpp:1:
In file included from /usr/include/c++/v1/iostream:37:
In file included from /usr/include/c++/v1/ios:215:
/usr/include/c++/v1/__locale:631:16: error: use of undeclared identifier 'isascii'
return isascii(__c) ? (__tab_[static_cast<int>(__c)] & __m) !=0 : false;
...
(This builds fine with _XOPEN_SOURCE=600). C++ headers on FreeBSD use isacii, a non-standard function, so it's not exposed when _POSIX_C_SOURCE is set.
Or on OpenBSD:
#include <fstream>
int main() { }
$ clang++ -std=c++14 -D_XOPEN_SOURCE=600 t.cpp
In file included from t.cpp:1:
In file included from /usr/include/c++/v1/fstream:183:
In file included from /usr/include/c++/v1/ostream:138:
In file included from /usr/include/c++/v1/ios:215:
In file included from /usr/include/c++/v1/__locale:32:
In file included from /usr/include/c++/v1/support/newlib/xlocale.h:25:
/usr/include/c++/v1/support/xlocale/__strtonum_fallback.h:23:64: error: unknown type name 'locale_t'
char **endptr, locale_t) {
Presumably <locale.h> isn't getting included somewhere it “should” be.
The worrisome conclusion I'm drawing is that you can't portably have a POSIX C++ environment that is free of non-POSIX extensions. These examples work fine on OpenBSD and FreeBSD if the feature test macros are removed. That looks to be because the BSD headers expose BSD functions unless in standard C mode, but they do not care about standard C++ mode (they explicitly check whether macros corresponding to C89, C99, or C11 are set). glibc looks to be the same: it still exposes non-standard C functions in standard C++ mode, so perhaps it's only a matter of time before I run into a build error there.
So the actual question is this: can you write portable POSIX C++ code which does not expose platform-specific functionality? Or if I'm targeting POSIX with C++ should I just not set any feature test macros and hope for the best?
EDIT:
I got to thinking about the implications of this (as in, why do I care?), and the following program, I think, illustrates it. This is Linux/glibc:
#include <ctime>
int dysize;
$ g++ -c -std=c++14 t.cpp
t.cpp:2:5: error: ‘int dysize’ redeclared as different kind of entity
2 | int dysize;
| ^~~~~~
In file included from t.cpp:1:
/usr/include/time.h:262:12: note: previous declaration ‘int dysize(int)’
262 | extern int dysize (int __year) __THROW __attribute__ ((__const__));
This is the standard <ctime> header, which is does not include anything called dysize. That's an old SunOS function that glibc includes for compatibility. A C program built with -std=c99 won't expose it, but C++ always does. And there's no real way of knowing which non-reserved identifiers will be used by various implementations. If -std=c++14 caused non-standard identifiers to be hidden, that would avoid this problem, but it doesn't, and I can't see a way around that.
Which might imply that the feature test macro is a red herring: the source of the problem is that, on at least some real-world implementations, C++ compilers are exposing symbols they're not supposed to.
My suggestion is to build a toolchain, and work from that with the libraries, includes, the correct compiler (perhaps a stripped version that can only use POSIX libraries, includes, etc).
To make it portable, generally you would build the application using static linkers. Other linker options may be necessary that point specifically or include your toolchain environment paths.
And if you're using POSIX threads, you may need -pthread.
I see that you are using system-wide headers and libraries, when really, you probably want a specific to your POSIX application toolchain, to avoid contamination.
I am running Kali-Linux (debian+gnome). When I compile I get compilation error:
cc -Wall -g -c -o frag.o frag.c
frag.c:7:10: fatal error: stropts.h: No such file or directory
7 | #include <stropts.h>
| ^~~~~~~~~~~
compilation terminated.
make: *** [<builtin>: frag.o] Error 1
Can anyone tell me what am I missing and what should I install?
I tried installing glibc-sources but still it didn't work out.
As mentioned by the other answer, this library is not used on Linux. Since this came up when trying to compile an application on Linux, it's possible an #if was not set correctly.
As a workaround, look at the source code to see what the #if surrounding the #include is, and set that to false when compiling.
For example, if the code looked like:
#if HAVE_STROPTS_H
#include <stropts.h>
#endif
And if you are using cmake or gcc, run them with -DHAVE_STROPTS_H=0.
stropts.h is part of the Posix STREAMS extension, which Linux never supported. (Since 2008, it has also been marked as obsolescent by Posix, so it may be removed in some future standard revision.)
There have been third-party implementations of STREAMS for Linux, but I don't kniw if any of them are still supported. One which is used by Linux-based telephony apps is contained in openss7.
Another reason not yet mentioned might be the confusion manual introduces. If you need to use ioctl() call, the quickest way to find out the header to include might be pulling up a man ioctl. Well, it turns out there are 2 different manual pages for ioctl(), and having both on the system will result in this misleading and wrong suggestion (accessible directly as man 3 ioctl) of including the stropts.h.
More likely you actually need a sys/ioctl.h, described in man 2 ioctl:
#include <sys/ioctl.h>
I've just updated MinGW using mingw-get-setup and i'm unable to build anyting that contains <cmath> header if I use anything larger than -O0 with -std=c++1y. (I also tried c++11 and c++98) I'm getting errors like this one:
g++.exe -pedantic-errors -pedantic -Wextra -Wall -std=c++1y -O3 -c Z:\Projects\C++\L6\src\events.cpp -o obj\src\events.o
In file included from z:\lander\mingw\lib\gcc\mingw32\4.8.1\include\c++\cmath:44:0,
from Z:\Projects\C++\L6\src\utils.h:4,
from Z:\Projects\C++\L6\src\events.cpp:10:
z:\lander\mingw\include\math.h: In function 'float hypotf(float, float)':
z:\lander\mingw\include\math.h:635:30: error: '_hypot' was not declared in this scope
{ return (float)(_hypot (x, y)); }
Is something wrong on my side?
Or version at mingw repo is bugged? And if so, is there any quick fix for this header?
To avoid any further speculation, and downright bad suggestions such as using #if 0, let me give an authoritative answer, from the perspective of a MinGW project contributor.
Yes, the MinGW.org implementation of include/math.h does have a bug in its inline implementation of hypotf (float, float); the bug is triggered when compiling C++, with the affected header included (as it is when cmath is included), and any compiler option which causes __STRICT_ANSI__ to become defined is specified, (as is the case for those -std=c... options noted by the OP). The appropriate solution is not to occlude part of the math.h file, with #if 0 or otherwise, but to correct the broken inline implementation of hypotf (float, float); simply removing the spurious leading underscore from the inline reference to _hypot (float, float), where its return value is cast to the float return type should suffice.
Alternatively, substituting an equivalent -std=gnu... for -std=c... in the compiler options should circumvent the bug, and may offer a suitable workaround.
FWIW, I'm not entirely happy with MinGW.org's current implementation of hypotl (long double, long double) either; correcting both issues is on my punch list for the next release of the MinGW runtime, but ATM, I have little time to devote to preparing this.
Update
This bug is no longer present in the current release of the MinGW.org runtime library (currently mingwrt-3.22.4, but fixed since release 3.22). If you are using anything older than this, (including any of the critically broken 4.x releases), you should upgrade.
As noted by Keith, this is a bug in the MinGW.org header.
As an alternative to editing the MinGW.org header, you can use MinGW-w64, which provides everything MinGW.org provides, and a whole lot more.
For a list of differences between the runtimes, see this wiki page.
MinGW uses gcc and the Microsoft runtime library. Microsoft's implementation support C90, but its support for later versions of the C standard (C99 and C11) is very poor.
The hypot function (along with hypotf and hypotl) was added in C99.
If you're getting this error with a program that calls hypot, such as:
#include <cmath>
int main() {
std::cout << std::hypot(3.0, 4.0)) << '\n';
}
then it's just a limitation of the Microsoft runtime library, and therefore of MinGW. If it occurs with any program that has #include <cmath>, then it's a bug, perhaps a configuration error, in MinGW.
I am using QT Creator to make a C++ program on Ubuntu. The program I had written was compiling fine, until I decided to start using C++11 rather than C++98 (which is the default in QT Creator). I am using my own cmake file, rather than qmake, and so to do this, I included the following line in my CMakeLists.txt file:
set(CMAKE_CXX_FLAGS "-std=c++0x")
Now, part of my code has the following (which was not written by me):
#if (linux && (i386 || __x86_64__))
# include "Linux-x86/OniPlatformLinux-x86.h"
#elif (linux && __arm__)
# include "Linux-Arm/OniPlatformLinux-Arm.h"
#else
# error Unsupported Platform!
#endif
After transferring to C++11, I get an error at the line error Unsupported Platform!. This is because, from what I can see, the variable linux is not defined anywhere, although the variable __x86_64__ is defined.
Therefore, I have two questions:
1) Why is the variable linux not defined, even though I am using Linux?
2) How can I tell C++11 to ignore this error?
Thanks.
The identifier linux is not reserved. A conforming compiler may not predefine it as a macro. For example, this program:
int main() {
int linux = 0;
return linux;
}
is perfectly valid, and a conforming compiler must accept it. Predefining linux causes the declaration to be a syntax error.
Some older compilers (including the compiler you were using, with the options you were giving it) predefine certain symbols to provide information about the target platform -- including linux to indicate a Linux system. This convention goes back to early C compilers, written before there was a distinction between reserved and unreserved identifiers.
The identifier __linux__, since it starts with two underscores, is reserved for use by the implementation, so compilers are allowed to predefine it -- and compilers for Linux systems typically do predefine it as a macro expanding to 1.
Confirm that your compiler predefines __linux__, and then change your code so it tests __linux__ rather than linux. You should also find out what reserved symbol is used instead of i386 (likely __i386__).
Related: Why does the C preprocessor interpret the word "linux" as the constant "1"?
Change your standard-selection flag to -std=gnu++0x instead of c++0x. The gnu flavors provide some non-standard extensions, apparently including predefining the macro linux. Alternatively, check for __linux__ instead.
I am trying to implement backtrace functionality for a large framework, which is used for different platforms and OS'es. In some of them, it is linked against glibc, while in the other, something different (eg. uclibc) is used. backtrace() function exists only in the former.
Is there any way to tell whether glibc is used? Any #define? I was unable to find an answer in glibc manual. I know I can't have linking-time information during compilation, but I guess include files have to differ. At least backtrace have to be declared somewhere.
I would like to check it without being forced to pass explicit flags to the compiler.
Include features.h, it contains the macros you need, e.g.
#define __GNU_LIBRARY__ 6
/* Major and minor version number of the GNU C library package. Use
these macros to test for features in specific releases. */
#define __GLIBC__ 2
#define __GLIBC_MINOR__ 4
There are the #defines __GNU_LIBRARY__, __GLIBC__ and __GLIBC_MINOR__ (6, 2 and 11 on my system with glibc-2.11) in features.h.
Checking for preprocessor macros is not a good solution. uClibc and possibly other libc implementations define macros to mimic glibc (without providing all of its bloated functionality) for much the same reasons that all browsers include "Mozilla" in their User-Agent strings: broken programs that expect to see glibc and turn off lots of features if they don't see it.
Instead you should write a configure script to probe for backtrace and use it only if it's available.
Empirically, both of the following compile and run fine on GCC 6.4:
#include <stdio.h>
int main(void) {
#ifdef __GLIBC__
puts("__GLIBC__");
#endif
return 0;
}
and:
int main(void) {
#ifdef __GLIBC__
puts("__GLIBC__");
#endif
return 0;
}
but only the first produces output of course.
This must mean that __GLIBC__ comes from stdio.h which must include features.h, see also: What is the purpose of features.h header?
Therefore, strictly speaking, __GLIBC__ by itself is not a clear indication that glibc is used, since even without headers, GCC already embeds runtime objects such as crt1.o in the finale executable, and those come from glibc.
So the main missing question is: does glibc guarantee that features.h gets included by every header? I could not find a clear documentation quote. TODO.
#if defined(__GLIBC__) && !defined(__UCLIBC__) && !defined(__MUSL__)
This is getting a bit ugly and syntactically ambiguous, but useful.