What is the default C -std standard version for the current GCC (especially on Ubuntu)?

This is explained in depth in the gcc manual, available (if it's installed) by typing info gcc or online here. The relevant section of the 4.7.2 manual is here.

By default, gcc does not conform to any of the ANSI/ISO C standards. The current default is equivalent to -std=gnu90, which is the 1989/1990 standard with GNU-specific extensions. (Some diagnostics required by the language standard are not issued.) Version 5.1.0, released 2015-04-22, changed the default from -std=gnu90 to -std=gnu11, as documented here.

If you want standard conformance, you can use any of the following:

-std=c90 -pedantic
-std=c99 -pedantic
-std=c11 -pedantic

-std=c90 can also be spelled -ansi, -std=c89, or -std=iso9899:1990.

-std=iso9899:199409 supports the C90 standard plus the 1995 amendment, which added a few minor features (all of which are also in C99).

-std=c99 can also be spelled -std=c9x or -std=iso9899:1999 (the name c9x was used before the standard was published). C99 support is not quite complete, but it's close.

-std=c11 can also be spelled -std=c0x or -std=iso9899:2011 (the name c0x was used before the final standard was published; it was wrongly assumed that x would not exceed 9). C11 support is also incomplete; the current status is summarized here.

The -pedantic option causes gcc to print required diagnostics for violations of constraints and syntax rules. In some cases, those diagnostics are merely warnings -- and there's no easy way to distinguish between those warnings and other warnings that aren't required by the language. Replace -pedantic by -pedantic-errors to cause gcc to treat language violations as fatal errors.

A quick history of the standard:

• C89 was the first official C standard, published by ANSI in 1989.
• C90 was the ISO version of the standard, describing exactly the same language as C89. ANSI officially adopted ISO's version of the standard. There were two Technical Corrigenda, correcting some errors.
• C95 was an amendment to C90, adding a few features, mainly digraphs and wide character support. As far as I know, a merged version was never published.
• C99 was issued by ISO in 1999. There were three Technical Corrigenda.
• C11 was issued by ISO in 2011. There has been one Technical Corrigendum, fixing the definitions of __STDC_VERSION__ and __STDC_LIB_EXT1__.

ANSI did not issue its own versions of the 1999 or 2011 standards, adopting the ISO standards instead.

N1256 is a freely available draft of the C99 standard, with the 3 Technical Corrigenda merged into it.

N1570 is a freely available draft of the C11 standard. There are some minor differences between it and the published C11 standard, plus one Technical Corrigendum. For more details, see my answer to this question.

useful information from info gcc for gcc6 and https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gcc/Standards.html#Standards for gcc5

gcc version 6.3.1 - 10.1.0

2.1 C Language
==============
 The default, if no C language dialect options are given, is
'-std=gnu11'.

2.2 C++ Language
================
 The default, if no C++ language dialect options are given, is
'-std=gnu++14'.

gcc version 5.4.0

2.1 C Language
==============
The default, if no C language dialect options are given, is -std=gnu11

2.2 C++ Language
================
The default, if no C++ language dialect options are given, is -std=gnu++98

For C, default mode remains std=gnu11, but for C++ it has jumped from std=gnu++98 to std=gnu++14

Minimal test program

If you feel like finding it out empirically without reading any manuals.

c.c

#include <stdio.h>

int main(void) {
#ifdef __STDC_VERSION__
    printf("__STDC_VERSION__ = %ld \n", __STDC_VERSION__);
#endif
#ifdef __STRICT_ANSI__
    puts("__STRICT_ANSI__");
#endif
    return 0;
}

Test with:

#!/usr/bin/env bash
for std in c89 c99 c11 c17 gnu89 gnu99 gnu11 gnu17; do
  echo $std
  gcc -std=$std -o c.out c.c
  ./c.out
  echo
done
echo default
gcc -o c.out c.c
./c.out

Outcome:

c89
__STRICT_ANSI__

c99
__STDC_VERSION__ = 199901
__STRICT_ANSI__

c11
__STDC_VERSION__ = 201112
__STRICT_ANSI__

c17
__STDC_VERSION__ = 201710
__STRICT_ANSI__

gnu89

gnu99
__STDC_VERSION__ = 199901

gnu11
__STDC_VERSION__ = 201112

gnu17
__STDC_VERSION__ = 201710

default
__STDC_VERSION__ = 201710

Conclusion: gnu17 is used by default:

• __STRICT_ANSI__: GCC extension that is defined for -std=c but not for -std=gnu, see: https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html
• __STDC_VERSION__: C99+ ANSI C macro that set for each version. Not present in C89 where it was not yet defined by the standard.

For an explanation of -std=gnu* vs -std=c* see also: What are the differences between -std=c++11 and -std=gnu++11?

C++

main.cpp

#include <iostream>

int main(void) {
#ifdef __cplusplus
    std::cout << __cplusplus << std::endl;
#endif
#ifdef __STRICT_ANSI__
    std::cout << "__STRICT_ANSI__" << std::endl;
#endif
    return 0;
}

Test with:

#!/usr/bin/env bash
for std in c++98 c++11 c++14 c++17 gnu++98 gnu++11 gnu++14 gnu++17; do
  echo $std
  g++ -std=$std -o cpp.out cpp.cpp
  ./cpp.out
  echo
done
echo default
g++ -o cpp.out cpp.cpp
./cpp.out

Outcome:

c++98
199711
__STRICT_ANSI__

c++11
201103
__STRICT_ANSI__

c++14
201402
__STRICT_ANSI__

c++17
201703
__STRICT_ANSI__

gnu++98
199711

gnu++11
201103

gnu++14
201402

gnu++17
201703

default
201402

Conclusion: gnu++14 is the default:

• __cplusplus: macro defined by the C++ standard including on C++98 onwards

Tested on Ubuntu 18.10, GCC 8.2.0. GitHub upstream.

The first line will give your GCC version (4.7.2)

(Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2

When you compile your code, you can specify which C/C++ revision you want to use, by adding -std=c99 or -std=c99 ...

Note gnu89 is used by default.