What is the purpose of using -pedantic in GCC G compiler

Question

This note says      -ansi  tells the compiler to implement the ANSI language option  This turns   off certain  features  of GCC which   are incompatible with the ANSI   standard       -pedantic  used in conjunction with -ansi  this tells the compiler to be adhere strictly to the ANSI standard    rejecting any code which is not   compliant    First things first    What is the purpose of the -pedantic and -ansi options of the GCC G   compiler  I couldn t understand the above description   Can anyone tell me the right circumstances for using these two options   When should I use them  Are they important

User · Answer

Pedantic makes it so that the gcc compiler rejects all GNU C extensions not just the ones that make it ANSI compatible

User · Answer

lt -ansi is an obsolete switch that requests the compiler to compile according to the 30-year-old obsolete revision of C standard  ISO IEC 9899 1990  which is essentially a rebranding of the ANSI standard X3 159-1989  quot Programming Language C  Why obsolete  Because after C90 was published by ISO  ISO has been in charge of the C standardization  and any technical corrigenda to C90 have been standardized by ISO  Thus it is more apt to use the -std c90  Without this switch  the recent GCC C compilers will conform to the C language standardized in ISO IEC 9899 2011  or the newest 2018 revision  Unfortunately there are some lazy compiler vendors that believe it is acceptable to stick to an older obsolete standard revision  for which the standardization document is not even available from standard bodies  Using the switch helps ensuring that the code should compile in these obsolete compilers   The -pedantic is an interesting one  In absence of -pedantic  even when a specific standard is requested  GCC will still allow some extensions that are not acceptable in the C standard  Consider for example the program struct test       int zero size array 0       The C11 draft n1570 paragraph 6 7 6 2p1 says   In addition to optional type qualifiers and the keyword static  the   and   may delimit an expression or    If they delimit an expression  which specifies the size of an array   the expression shall have an integer type  If the expression is a constant expression  it shall have a value greater than zero        The C standard requires that the array length be greater than zero  and this paragraph is in the constraints  the standard says the following 5 1 1 3p1   A conforming implementation shall produce at least one diagnostic message  identified in an implementation-defined manner  if a preprocessing translation unit or translation unit contains a violation of any syntax rule or constraint  even if the behavior is also explicitly specified as undefined or implementation-defined  Diagnostic messages need not be produced in other circumstances 9   However  if you compile the program with gcc -c -std c90 pedantic test c  no warning is produced  -pedantic causes the compiler to actually comply to the C standard  so now it will produce a diagnostic message  as is required by the standard  gcc -c -pedantic -std c90 pedantic test c pedantic test c 2 9  warning  ISO C forbids zero-size array    zero size array     -Wpedantic       int zero size array 0                             Thus for maximal portability  specifying the standard revision is not enough  you must also use -pedantic  or -pedantic-errors  to ensure that GCC actually does comply to the letter of the standard   The last part of the question was about using -ansi with C    ANSI never standardized the C   language - only adopting it from ISO  so this makes about as much sense as saying  quot English as standardized by France quot   However GCC still seems to accept it for C    as stupid as it sounds

User · Answer

Basically  it will make your code a lot easier to compile under other compilers which also implement the ANSI standard  and  if you are careful in which libraries api calls you use  under other operating systems platforms   The first one  turns off SPECIFIC features of GCC   -ansi  The second one  will complain about ANYTHING at all that does not adhere to the standard  not only specific features of GCC  but your constructs too    -pedantic

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I use it all the time in my coding   The -ansi flag is equivalent to -std c89   As noted  it turns off some extensions of GCC   Adding -pedantic turns off more extensions and generates more warnings   For example  if you have a string literal longer than 509 characters  then -pedantic warns about that because it exceeds the minimum limit required by the C89 standard   That is  every C89 compiler must accept strings of length 509  they are permitted to accept longer  but if you are being pedantic  it is not portable to use longer strings  even though a compiler is permitted to accept longer strings and  without the pedantic warnings  GCC will accept them too

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If your code needs to be portable then you can test that it compiles without any gcc extensions or other non-standard features  If your code compiles with -pedantic -ansi then in theory it should compile OK with any other ANSI standard compiler

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Others have answered sufficiently  I would just like to add a few examples of frequent extensions   The main function returning void  This is not defined by the standard  meaning it will only work on some compilers  including GCC   but not on others  By the way  int main   and int main int  char    are the two signatures that the standard does define   Another popular extension is being able to declare and define functions inside other functions   void f         void g                                             g                   This is nonstandard  If you want this kind of behavior  check out C  11 lambdas

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If you re writing code that you envisage is going to be compiled on a wide variety of platforms  with a number of different compilers  then using these flags yourself will help to ensure you don t produce code that only compiles under GCC

User · Answer

GCC compilers always try to compile your program if this is at all possible  However  in some cases  the C and C   standards specify that certain extensions are forbidden  Conforming compilers such as gcc or g   must issue a diagnostic when these extensions are encountered  For example  the gcc compiler   s -pedantic option causes gcc to issue warnings in such cases  Using the stricter -pedantic-errors option converts such diagnostic warnings into errors that will cause compilation to fail at such points  Only those non-ISO constructs that are required to be flagged by a conforming compiler will generate warnings or errors

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