Why can't I explicitly `-I /usr/include`?












1














Both gcc and clang seem to silently discard /usr/include from the list of include directories when explicitly included via -I.
Is there a specific reason as to why common compilers apparently don't allow to include the system's include dir?




background:



Suppose you depend on a header file located in /usr/include while having inherited a directory containing an incompatible version of the same header from your build system via the CPATH environment variable (effectively appending that directory to the -I list from the right).






gcc include clang







share|improve this question


















  • 1




    Create a new directory, symlink the files you want there, and point -I at that new directory. This avoids side effects that you do not control.
    – Marc Glisse
    Nov 22 at 21:36
















1














Both gcc and clang seem to silently discard /usr/include from the list of include directories when explicitly included via -I.
Is there a specific reason as to why common compilers apparently don't allow to include the system's include dir?




background:



Suppose you depend on a header file located in /usr/include while having inherited a directory containing an incompatible version of the same header from your build system via the CPATH environment variable (effectively appending that directory to the -I list from the right).






gcc include clang







share|improve this question


















  • 1




    Create a new directory, symlink the files you want there, and point -I at that new directory. This avoids side effects that you do not control.
    – Marc Glisse
    Nov 22 at 21:36














1












1








1







Both gcc and clang seem to silently discard /usr/include from the list of include directories when explicitly included via -I.
Is there a specific reason as to why common compilers apparently don't allow to include the system's include dir?




background:



Suppose you depend on a header file located in /usr/include while having inherited a directory containing an incompatible version of the same header from your build system via the CPATH environment variable (effectively appending that directory to the -I list from the right).






gcc include clang







share|improve this question













Both gcc and clang seem to silently discard /usr/include from the list of include directories when explicitly included via -I.
Is there a specific reason as to why common compilers apparently don't allow to include the system's include dir?




background:



Suppose you depend on a header file located in /usr/include while having inherited a directory containing an incompatible version of the same header from your build system via the CPATH environment variable (effectively appending that directory to the -I list from the right).






gcc include clang




gcc include clang






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Nov 22 at 20:40









vega8

5071320




5071320








  • 1




    Create a new directory, symlink the files you want there, and point -I at that new directory. This avoids side effects that you do not control.
    – Marc Glisse
    Nov 22 at 21:36














  • 1




    Create a new directory, symlink the files you want there, and point -I at that new directory. This avoids side effects that you do not control.
    – Marc Glisse
    Nov 22 at 21:36








1




1




Create a new directory, symlink the files you want there, and point -I at that new directory. This avoids side effects that you do not control.
– Marc Glisse
Nov 22 at 21:36




Create a new directory, symlink the files you want there, and point -I at that new directory. This avoids side effects that you do not control.
– Marc Glisse
Nov 22 at 21:36












1 Answer
1






active

oldest

votes


















1














GCC ignores -I/usr/include because it is a system header directory by default, and using -I would turn it into non-system header, leading to confusing behavior, particularly with system headers that are not entirely aligned with language standards. (GCC gives system headers more latitude and suppresses warnings, for example.)



If you use -isystem /usr/include, then /usr/include moves to the front of the search list. However, you will likely have to move the other default search path entries as well, to avoid breaking too many things. gcc -v will print the entire search path.






share|improve this answer























  • But please don't use -isystem /usr/include. The system headers are searched in a very specific order, and changing that order will break things.
    – Marc Glisse
    Nov 22 at 21:34










  • Good point, it will likely be necessary to move the other default search paths as well.
    – Florian Weimer
    Nov 22 at 21:38












  • -isystem wouldn't be able to override CPATH anyway since it has lower precedence than CPATH (which effectively is equivalent to -I).
    – vega8
    Nov 23 at 14:56










  • -nostdinc could help, though that's going to be a fragile hack.
    – Marc Glisse
    Nov 24 at 22:25











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1














GCC ignores -I/usr/include because it is a system header directory by default, and using -I would turn it into non-system header, leading to confusing behavior, particularly with system headers that are not entirely aligned with language standards. (GCC gives system headers more latitude and suppresses warnings, for example.)



If you use -isystem /usr/include, then /usr/include moves to the front of the search list. However, you will likely have to move the other default search path entries as well, to avoid breaking too many things. gcc -v will print the entire search path.






share|improve this answer























  • But please don't use -isystem /usr/include. The system headers are searched in a very specific order, and changing that order will break things.
    – Marc Glisse
    Nov 22 at 21:34










  • Good point, it will likely be necessary to move the other default search paths as well.
    – Florian Weimer
    Nov 22 at 21:38












  • -isystem wouldn't be able to override CPATH anyway since it has lower precedence than CPATH (which effectively is equivalent to -I).
    – vega8
    Nov 23 at 14:56










  • -nostdinc could help, though that's going to be a fragile hack.
    – Marc Glisse
    Nov 24 at 22:25
















1














GCC ignores -I/usr/include because it is a system header directory by default, and using -I would turn it into non-system header, leading to confusing behavior, particularly with system headers that are not entirely aligned with language standards. (GCC gives system headers more latitude and suppresses warnings, for example.)



If you use -isystem /usr/include, then /usr/include moves to the front of the search list. However, you will likely have to move the other default search path entries as well, to avoid breaking too many things. gcc -v will print the entire search path.






share|improve this answer























  • But please don't use -isystem /usr/include. The system headers are searched in a very specific order, and changing that order will break things.
    – Marc Glisse
    Nov 22 at 21:34










  • Good point, it will likely be necessary to move the other default search paths as well.
    – Florian Weimer
    Nov 22 at 21:38












  • -isystem wouldn't be able to override CPATH anyway since it has lower precedence than CPATH (which effectively is equivalent to -I).
    – vega8
    Nov 23 at 14:56










  • -nostdinc could help, though that's going to be a fragile hack.
    – Marc Glisse
    Nov 24 at 22:25














1












1








1






GCC ignores -I/usr/include because it is a system header directory by default, and using -I would turn it into non-system header, leading to confusing behavior, particularly with system headers that are not entirely aligned with language standards. (GCC gives system headers more latitude and suppresses warnings, for example.)



If you use -isystem /usr/include, then /usr/include moves to the front of the search list. However, you will likely have to move the other default search path entries as well, to avoid breaking too many things. gcc -v will print the entire search path.






share|improve this answer














GCC ignores -I/usr/include because it is a system header directory by default, and using -I would turn it into non-system header, leading to confusing behavior, particularly with system headers that are not entirely aligned with language standards. (GCC gives system headers more latitude and suppresses warnings, for example.)



If you use -isystem /usr/include, then /usr/include moves to the front of the search list. However, you will likely have to move the other default search path entries as well, to avoid breaking too many things. gcc -v will print the entire search path.







share|improve this answer














share|improve this answer



share|improve this answer








edited Nov 22 at 21:39

























answered Nov 22 at 20:55









Florian Weimer

14.5k2943




14.5k2943












  • But please don't use -isystem /usr/include. The system headers are searched in a very specific order, and changing that order will break things.
    – Marc Glisse
    Nov 22 at 21:34










  • Good point, it will likely be necessary to move the other default search paths as well.
    – Florian Weimer
    Nov 22 at 21:38












  • -isystem wouldn't be able to override CPATH anyway since it has lower precedence than CPATH (which effectively is equivalent to -I).
    – vega8
    Nov 23 at 14:56










  • -nostdinc could help, though that's going to be a fragile hack.
    – Marc Glisse
    Nov 24 at 22:25


















  • But please don't use -isystem /usr/include. The system headers are searched in a very specific order, and changing that order will break things.
    – Marc Glisse
    Nov 22 at 21:34










  • Good point, it will likely be necessary to move the other default search paths as well.
    – Florian Weimer
    Nov 22 at 21:38












  • -isystem wouldn't be able to override CPATH anyway since it has lower precedence than CPATH (which effectively is equivalent to -I).
    – vega8
    Nov 23 at 14:56










  • -nostdinc could help, though that's going to be a fragile hack.
    – Marc Glisse
    Nov 24 at 22:25
















But please don't use -isystem /usr/include. The system headers are searched in a very specific order, and changing that order will break things.
– Marc Glisse
Nov 22 at 21:34




But please don't use -isystem /usr/include. The system headers are searched in a very specific order, and changing that order will break things.
– Marc Glisse
Nov 22 at 21:34












Good point, it will likely be necessary to move the other default search paths as well.
– Florian Weimer
Nov 22 at 21:38






Good point, it will likely be necessary to move the other default search paths as well.
– Florian Weimer
Nov 22 at 21:38














-isystem wouldn't be able to override CPATH anyway since it has lower precedence than CPATH (which effectively is equivalent to -I).
– vega8
Nov 23 at 14:56




-isystem wouldn't be able to override CPATH anyway since it has lower precedence than CPATH (which effectively is equivalent to -I).
– vega8
Nov 23 at 14:56












-nostdinc could help, though that's going to be a fragile hack.
– Marc Glisse
Nov 24 at 22:25




-nostdinc could help, though that's going to be a fragile hack.
– Marc Glisse
Nov 24 at 22:25


















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