Compatibility between the GCC or Glibc versions on Alibaba Cloud Linux 3 and CentOS 8 - Alibaba Cloud Linux

GNU Compiler Collection (GCC) 10 and Glibc 2.28 incorporate important changes compared with GCC 8 and Glibc 2.32. The changes affect the forward or backward compatibility of GCC 10 and Glibc 2.28. This topic describes the compatibility between the GCC versions and between the Glibc versions. This topic also describes how to mitigate the impacts.

Background information

The compatibility between the GCC or Glibc versions can be classified into Application Binary Interface (ABI) compatibility and API compatibility. The ABI compatibility between the GCC or Glibc versions refers to the compatibility in which binaries can run without the need for code recompilation. The API compatibility between the GCC or Glibc versions refers to the compatibility in which application code can run unchanged without the need for code recompilation.

ABI compatibility between the GCC or Glibc versions

Note

In most cases, the GCC compiler does not involve ABI compatibility. However, operating system distributions, such as Alibaba Cloud Linux and CentOS, include the standard C++ library (libstdc++) in GCC packages. The impacts of GCC on the ABI compatibility with applications refer to the impacts of the libstdc++ library on the ABI compatibility with applications.

Forward compatibility: libstdc++ and Glibc are highly forward-compatible. Most of the components that are compiled on GCC 8 and Glibc 2.28 on CentOS can run in GCC 10 and Glibc 2.32 environments, respectively, on Alibaba Cloud Linux 3 without the need for code compilation.
Backward compatibility: libstdc++ and Glibc are not backward-compatible. The components that are compiled on GCC 10 and Glibc 2.32 on Alibaba Cloud Linux 3 cannot run unchanged in GCC 8 and Glibc 2.32 environments, respectively, on CentOS 8.

API compatibility between the GCC or Glibc versions

GCC and Glibc are core toolchain components of operating systems and are highly backward-compatible and forward-compatible with APIs. However, during version updates, changes that affect API compatibility may be made to GCC and Glibc.

GCC
- Example of changes that affect forward API compatibility: The compilation parameters that are used in early GCC versions are deprecated in later GCC versions. As a result, you cannot compile applications that use the compilation parameters by using later GCC versions. To use later GCC versions to compile the applications, you must modify the compilation parameters for the applications.
- Example of changes that affect backward API compatibility: New compilation parameters are introduced in later GCC versions. As a result, you cannot compile applications that use the new compilation parameters by using early GCC versions. To use early GCC versions to compile the applications, you must modify the compilation parameters for the applications.
Glibc
- Example of changes that affect forward API compatibility: Functions or header files that are used in early Glibc versions are deprecated in later Glibc versions. As a result, you cannot compile applications that use the functions or header files by using later Glibc versions. To use later Glibc versions to compile the applications, you must modify the application code.
- Example of changes that affect backward API compatibility: New functions and implementations are introduced in later Glibc versions. As a result, you cannot compile applications that use the new functions by using early Glibc versions. To use early Glibc versions to compile the applications, you must modify the application code.

Changes in Glibc 2.32 and impacts of the changes

The following table describes the important changes in Glibc 2.28 compared with Glibc 2.32, the impacts of the changes, and how to mitigate the impacts.

Type	Change	Compatibility type	Impact	How to mitigate the impacts
`time`	The `stime` function is no longer declared in the `time.h` header file.	API	If the `stime` function is used in the source code, a compilation error occurs.	Use the `clock_settime` function instead of the `stime` function.
	The `ftime` function is deprecated.	API	If the `ftime` function is used in the source code, a compilation error occurs.	Use the `clock_gettime` function instead of the `ftime` function.
	The `gettimeofday` function no longer reports information about the time zone.	API	The `gettimeofday` function cannot obtain information about the time zone.	Use the `clock_gettime` function instead of the `gettimeofday` function.
		API
	The `settimeofday` function is labeled by `POSIX` as obsolete.	API	The `settimeofday` function cannot configure the time and the time drift at the same time.	Use the `clock_settime and adjtime` functions instead of the `settimeofday` function.
	The `nanosleep` user-mode interface uses the `clock_nanosleep` system call instead of the `nanosleep` system call.	API	If an application runs in `seccomp` mode and the `clock_nanosleep` function is not added to the seccomp whitelist, the application cannot call the nanosleep function.	Add the `clock_nanosleep` to the seccomp whitelist.
`sysctl`	The `sys/sysctl.h` header file is removed.	API	The code that references the header file cannot be compiled.	Delete the reference to the `sys/sysctl.h` header file from the code.
`sysctl`	The `sysctl` function is removed and continues to exist as a compatibility symbol.	API	The `sysctl` system call always fails with an `ENOSYS` error.	Access the `/proc/sys` virtual process file system instead of using the `sysctl` interface. Use the `getentropy` function to obtain random bytes.
`sstk`	The `sstk` function is no longer declared in header files.	ABI	Applications that are complied on `Glibc` 2.28 are ABI-compatible with Glibc 2.32. If a program calls this function, the call returns a failure, which may cause unexpected behaviors in the program.	The kernel and `Glibc` no longer support the `sstk` function. Remove calls to the `sstk` function.
`sstk`	The `sstk` function is no longer declared in header files.	API	An error occurs if you use Glibc 2.32 to compile applications that use the sstk function.
`signal`	The `sigmask` macro and the following legacy signal handling functions are deprecated: `siginterrupt`, `sigpause`, `sighold`, `sigrelse`, `sigignore`, and `sigset`.	ABI	Binaries that are compiled on Glibc 2.28 remain ABI-compatible with `libc.so`.	No solution is required.
		API	When you use Glibc 2.32 to compile applications that use the functions or macro, a `warning` or an `error` is returned.	Modify the source code and use the `sigsuspend, sigprocmask, and sigaction` functions instead.
	The `sys_siglist`, `_sys_siglist`, and `sys_sigabbrev` functions are no longer declared in the `<string.h>` header file.	ABI	Binaries that are compiled on Glibc 2.28 remain ABI-compatible with `libc.so`.	No solution is required.
		API	When you use Glibc 2.32 to compile applications that use the functions, a `warning` or an `error` is returned.	Modify the source code and use the `strsignal` function instead.
`sys error`	The `sys_errlist`, `_sys_errlist`, `sys_nerr`, and `_sys_nerr` functions are no longer declared in the `stdio.h` header file.	ABI	Binaries that are compiled on Glibc 2.28 remain ABI-compatible with `libc.so`.	No solution is required.
`sys error`		API	When you use Glibc 2.32 to compile applications that use the functions, a `warning` or an `error` is returned.	Use the `strerror and strerror_r` functions instead.
`copy_file_range`	The simulated implementation of the `copy_file_range` function is removed.	API	If the kernel does not support the `copy_file_range` function and an application calls the function, an ENOSYS error occurs.	If the kernel does not support the `copy_file_range` function, the applications must provide the functionality of the `copy_file_range` function. The default kernel version of Alibaba Cloud Linux 3 is 5.10, which supports the `copy_file_range` function.
`thread`	The `<sys/single_threaded.h>` header file, which declares the `_libc_single_threaded` variable, is added.	API	Using weak references to `libpthread` functions, such as `pthread_create and pthread_key_create`, to detect the singled-threaded nature of an application is an obsolete feature. Future versions of `Glibc` will define `pthread_create` in `libc.so.6`. As a result, such detections will always label the program as multi-threaded.	Use the `_libc_single_threaded` variable instead to check the singled-threaded nature of applications.
`Sun RPC`	`Sun RPC` is no longer supported.	API	Dependencies must be changed for all code that references `Sun RPC` header files.	Use the `rpc/rpc.h` header file provided by the `libtorpc-devel` package. Use the `rpcsvc/*.h` header file provided by the `rpcsvc-proto-devel` package. Use the `rpcgen` program provided by the `rpcgen` package.

Changes in GCC 10 and impacts of the changes

The following table describes the important changes in GCC 10 compared with GCC 8, the impacts of the changes, and how to mitigate the impacts.

Type	Change	Compatibility type	Impact	How to mitigate the impacts
`-fno-common`	`GCC 10` defaults to the `-fno-common` option.	API	Multiple uninitialized duplicate global variables can be defined in the `common` segment. The variables are merged and stored in the `bss` segment during the linking stage. Only initialized variables that have a unique name can exist the linking stage. If the `-fno-common` option is enabled, a linker error occurs in a program when the same global variable is defined in multiple source files of the program.	Make sure that the global variable is defined only in a single source file and add the `extern` keyword to the declaration of the variable in the other source files. Enable the `-fcommon` option to ensure forward compatibility.
`fortran`	If a parameter does not match an argument, `Fortran` in `GCC 10` reports a compilation `error`.	API	A compilation error occurs if parameters do not match arguments.	Make sure that parameters match arguments in code. Degrade `-fallow-argument-mismatch` to `warnings`.
`C++ header`	`<stdexcept>` and `<string>` are no longer included in specific standard `C++` `headers`.	API	Errors occur while compiling applications.	Add required header files to the applications based on the error messages.
Enumeration check	`GCC 10` enforces stricter checks on enumerations and prohibits the use of enumeration elements to assign values to other enumeration elements.	API	Errors occur while compiling applications.	Use enumerations in the correct manner.
Large System Extensions (LSE)	The `-moutline-atomics` option is added to `GCC 10` and is automatically enabled for `-march=armv8-a`.	API	This option is used to detect the support of `AArch64` machines for LSE instructions at runtime and uses the instructions for standard atomic operations, which improves atomic concurrency.	Use this option in the correct manner. For information about other `Arm64` instruction sets supported by `GCC 10`, see 3.19.1 AArch64 Options.

Changes to the warning options of GCC 10

The following table describes the changes to the warning options of GCC 10 compared with GCC 8.

Note

Compilation warnings do not directly affect API compatibility. If compilation warnings appear when you migrate applications to Alibaba Cloud Linux 3, we recommend that you modify the application code to suppress the warnings.

Option	Compatibility type	Change type	Description
`-Waddress-of-packed-member`	API	Addition	Warns that taking the address of a `packed` member of a struct or `union` results in an unaligned pointer value. By default, this option is enabled.
`-Wzero-length-bounds`	API	Addition	Warns about accesses to elements of zero-length array members that may overlap other members of the same object. This option is enabled by `-Warray-bounds`.
`-Warray-bounds`	API	Enhancement	Warns about out-of-bounds accesses to member arrays and accesses to elements of zero-length arrays.
`-Wattribute-alias`	API	Enhancement	Warns about declarations that use the alias and similar attributes whose target is incompatible with the type of the alias.
`-Wformat-overflow`	API	Enhancement	Warns about calls to formatted input/output functions that may overflow the destination buffer. This warning is triggered when certain or likely buffer overflow in calls to formatted output functions is detected.
`-Wmissing-attributes`	API	Addition	Warns when particular function attributes are missing in the aliases but not in their target.
`-Wformat-truncation`	API	Enhancement	Warns about calls to formatted input/output functions, such as `snprintf`, that may result in output truncation. This warning option works together with the -Wformat-overflow option.
`-Wstringop-overflow`	API	Enhancement	Warns about out-of-bounds stores to member arrays, including zero-length arrays, dynamically allocated objects and variable-length arrays, and instances of reads of unterminated character arrays by the built-in string functions. Warns about out-of-bounds accesses by calls to user-defined functions declared by using the new attribute access.
`-Wreturn-local-addr`	API	Enhancement	Warns about instances of return statements returning addresses of automatic variables.
`-Wrestrict`	API	Enhancement	Warns about overlapping accesses to dynamically allocated objects.
`-Warith-conversion`	API	Enhancement	`-Warith-conversion` re-enables warnings from `-Wconversion`, `-Wfloat-conversion`, and `-Wsign-conversion`, which are automatically `disabled`.

References

For information about other changes in the GCC and Glibc versions, see the Release notes for Glibc and GCC.