What does the fpermissive flag do

Question

I m just wondering what the -fpermissive flag does in the g   compiler  I am getting      error  taking address of temporary  -fpermissive    which I can solve by giving the  -fpermissive flag to the compiler    EDIT  I just found what was causing the temporary address error part  I m going to fix that part right now

User · Answer

When you ve written something that isn t allowed by the language standard  and therefore can t really be well-defined behaviour  which is reason enough to not do it  but happens to map to some kind of executable if fed na  vely to the compiling engine  then -fpermissive will do just that instead of stopping with this error message  In some cases  the program will then behave exactly as you originally intended  but you definitely shouldn t rely on it unless you have some very special reason not to use some other solution

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A common case for simply setting -fpermissive and not sweating it exists  the thoroughly-tested and working third-party library that won t compile on newer compiler versions without -fpermissive   These libraries exist  and are very likely not the application developer s problem to solve  nor in the developer s schedule budget to do it  Set -fpermissive and move on in that case

User · Answer

Right from the docs      -fpermissive       Downgrade some diagnostics about nonconformant code from errors to warnings         Thus  using -fpermissive will allow some nonconforming code to compile    Bottom line  don t use it unless you know what you are doing

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The -fpermissive flag causes the compiler to report some things that are actually errors  but are permitted by some compilers  as warnings  to permit code to compile even if it doesn t conform to the language rules  You really should fix the underlying problem  Post the smallest  compilable code sample that demonstrates the problem      -fpermissive       Downgrade some diagnostics about nonconformant code from errors to warnings         Thus  using -fpermissive will allow some nonconforming code to compile

User · Answer

If you want a real-world use case for this  try compiling a very old version of X Windows-- say  either XFree86 or XOrg from aboout 2004  right around the split-- using a  modern   cough  version of gcc  such as 4 9 3   You ll notice the build CFLAGS specify both  -ansi  and  -pedantic    In theory  this means   blow up if anything even slightly violates the language spec    In practice  the 3 x series of gcc didn t catch very much of that kind of stuff  and building it with 4 9 3 will leave a smoking hole in the ground unless you set CFLAGS and BOOTSTRAPCFLAGS to  -fpermissive    Using that flag  most of those C files will actually build  leaving you free to move on to the version-dependent wreckage the lexer will generate

[c++] What does the fpermissive flag do?

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