How's GNU extended function running when didn't defined _GNU

How's GNU extended function running when didn't defined _GNU_SOURCE

I test GNU extended function such as "wcstoq" at different platforms.
At first,I didn't compile the testcase with D_GNU_SOURCE，so I got a compile warning as follows:

wcstoq.c:31:12: warning: implicit declaration of function 'wcstoq'; did you mean 'wcstol'? [-Wimplicit-function-declaration]
 retval=wcstoq(nptr,endptr,base);
 ^~~~~~
 wcstol

when I use gdb to debug this testcase,it goes into the correct function which is same as the case I defined _GNU_SOURCE.
But the function goes wrong when I did't define _GNU_SOURCE.

for example:

When I test the out of range case in x86_64, it should set retval= LLONG_MAX(0x7FFFFFFFFFFFFFFF),but it actually set retval= -1(0xFFFFFFFFFFFFFFFF).
The result also confused me when I test it in other platforms ppc, it actually set retval=0x000000007FFFFFFF.
when I defined _GNU_SOURCE, function runs ok in x86_64 and ppc, it both returns LLONG_MAX(0x7FFFFFFFFFFFFFFF).

my question is:

1 Why when I don't define _GNU_SOURCE,but gcc still can find the right function？

2 Why the fuction failed even the gcc find the right one,and in some cases the function can still be used,but in other cases it goes wrong？

3 Why the failed result of fuction is different in different platforms, how architecture platforms or other factors impact the different results？

4 The right way to use GNU extend function？

Thks!

edited Nov 26 '18 at 14:43

asked Nov 13 '18 at 9:13

qiqi

1

There's no prototype definition in the case you've shown, so the compile does not know the sizes (or numbers) of parameters. The external symbol exists, since it is not a macro or inline.

– Thomas Dickey
Nov 13 '18 at 9:31

I think the best answer to 4) is that you should define _GNU_SOURCE to get the library headers to include prototypes for GNU extensions. Without declarations at all, the compiler assumes the return type is int.

– Peter Cordes
Nov 13 '18 at 15:23

Thanks,@PeterCordes，Do you know any more information about the behavior of function with implicit declaration, not only the return type but also some input or output parameters, is it mean the behavior of this function is unpredictable or undefined?Especially for GNU extended functions?

– qiqi
Nov 14 '18 at 5:04

Thanks,@ThomasDickey but what I am confused and curious about why the function could also be called even if not defined _GNU_SOURCE，and why the behavior or output of the function is not the same in different platforms

– qiqi
Nov 14 '18 at 5:05

add a comment |

I test GNU extended function such as "wcstoq" at different platforms.
At first,I didn't compile the testcase with D_GNU_SOURCE，so I got a compile warning as follows:

wcstoq.c:31:12: warning: implicit declaration of function 'wcstoq'; did you mean 'wcstol'? [-Wimplicit-function-declaration]
 retval=wcstoq(nptr,endptr,base);
 ^~~~~~
 wcstol

when I use gdb to debug this testcase,it goes into the correct function which is same as the case I defined _GNU_SOURCE.
But the function goes wrong when I did't define _GNU_SOURCE.

for example:

my question is:

1 Why when I don't define _GNU_SOURCE,but gcc still can find the right function？

2 Why the fuction failed even the gcc find the right one,and in some cases the function can still be used,but in other cases it goes wrong？

3 Why the failed result of fuction is different in different platforms, how architecture platforms or other factors impact the different results？

4 The right way to use GNU extend function？

Thks!

edited Nov 26 '18 at 14:43

asked Nov 13 '18 at 9:13

qiqi

1

There's no prototype definition in the case you've shown, so the compile does not know the sizes (or numbers) of parameters. The external symbol exists, since it is not a macro or inline.

– Thomas Dickey
Nov 13 '18 at 9:31

I think the best answer to 4) is that you should define _GNU_SOURCE to get the library headers to include prototypes for GNU extensions. Without declarations at all, the compiler assumes the return type is int.

– Peter Cordes
Nov 13 '18 at 15:23

Thanks,@PeterCordes，Do you know any more information about the behavior of function with implicit declaration, not only the return type but also some input or output parameters, is it mean the behavior of this function is unpredictable or undefined?Especially for GNU extended functions?

– qiqi
Nov 14 '18 at 5:04

Thanks,@ThomasDickey but what I am confused and curious about why the function could also be called even if not defined _GNU_SOURCE，and why the behavior or output of the function is not the same in different platforms

– qiqi
Nov 14 '18 at 5:05

add a comment |

I test GNU extended function such as "wcstoq" at different platforms.
At first,I didn't compile the testcase with D_GNU_SOURCE，so I got a compile warning as follows:

wcstoq.c:31:12: warning: implicit declaration of function 'wcstoq'; did you mean 'wcstol'? [-Wimplicit-function-declaration]
 retval=wcstoq(nptr,endptr,base);
 ^~~~~~
 wcstol

when I use gdb to debug this testcase,it goes into the correct function which is same as the case I defined _GNU_SOURCE.
But the function goes wrong when I did't define _GNU_SOURCE.

for example:

my question is:

1 Why when I don't define _GNU_SOURCE,but gcc still can find the right function？

2 Why the fuction failed even the gcc find the right one,and in some cases the function can still be used,but in other cases it goes wrong？

3 Why the failed result of fuction is different in different platforms, how architecture platforms or other factors impact the different results？

4 The right way to use GNU extend function？

Thks!

edited Nov 26 '18 at 14:43

asked Nov 13 '18 at 9:13

qiqi

I test GNU extended function such as "wcstoq" at different platforms.
At first,I didn't compile the testcase with D_GNU_SOURCE，so I got a compile warning as follows:

wcstoq.c:31:12: warning: implicit declaration of function 'wcstoq'; did you mean 'wcstol'? [-Wimplicit-function-declaration]
 retval=wcstoq(nptr,endptr,base);
 ^~~~~~
 wcstol

when I use gdb to debug this testcase,it goes into the correct function which is same as the case I defined _GNU_SOURCE.
But the function goes wrong when I did't define _GNU_SOURCE.

for example:

my question is:

1 Why when I don't define _GNU_SOURCE,but gcc still can find the right function？

2 Why the fuction failed even the gcc find the right one,and in some cases the function can still be used,but in other cases it goes wrong？

3 Why the failed result of fuction is different in different platforms, how architecture platforms or other factors impact the different results？

4 The right way to use GNU extend function？

Thks!

gcc compilation gnu cpu-architecture glibc

edited Nov 26 '18 at 14:43

asked Nov 13 '18 at 9:13

qiqi

edited Nov 26 '18 at 14:43

asked Nov 13 '18 at 9:13

qiqi

edited Nov 26 '18 at 14:43

asked Nov 13 '18 at 9:13

qiqi

asked Nov 13 '18 at 9:13

qiqi

asked Nov 13 '18 at 9:13

qiqi

1

There's no prototype definition in the case you've shown, so the compile does not know the sizes (or numbers) of parameters. The external symbol exists, since it is not a macro or inline.

– Thomas Dickey
Nov 13 '18 at 9:31

I think the best answer to 4) is that you should define _GNU_SOURCE to get the library headers to include prototypes for GNU extensions. Without declarations at all, the compiler assumes the return type is int.

– Peter Cordes
Nov 13 '18 at 15:23

Thanks,@PeterCordes，Do you know any more information about the behavior of function with implicit declaration, not only the return type but also some input or output parameters, is it mean the behavior of this function is unpredictable or undefined?Especially for GNU extended functions?

– qiqi
Nov 14 '18 at 5:04

Thanks,@ThomasDickey but what I am confused and curious about why the function could also be called even if not defined _GNU_SOURCE，and why the behavior or output of the function is not the same in different platforms

– qiqi
Nov 14 '18 at 5:05

add a comment |

1

There's no prototype definition in the case you've shown, so the compile does not know the sizes (or numbers) of parameters. The external symbol exists, since it is not a macro or inline.

– Thomas Dickey
Nov 13 '18 at 9:31

I think the best answer to 4) is that you should define _GNU_SOURCE to get the library headers to include prototypes for GNU extensions. Without declarations at all, the compiler assumes the return type is int.

– Peter Cordes
Nov 13 '18 at 15:23

Thanks,@PeterCordes，Do you know any more information about the behavior of function with implicit declaration, not only the return type but also some input or output parameters, is it mean the behavior of this function is unpredictable or undefined?Especially for GNU extended functions?

– qiqi
Nov 14 '18 at 5:04

Thanks,@ThomasDickey but what I am confused and curious about why the function could also be called even if not defined _GNU_SOURCE，and why the behavior or output of the function is not the same in different platforms

– qiqi
Nov 14 '18 at 5:05

There's no prototype definition in the case you've shown, so the compile does not know the sizes (or numbers) of parameters. The external symbol exists, since it is not a macro or inline.

– Thomas Dickey
Nov 13 '18 at 9:31

I think the best answer to 4) is that you should define _GNU_SOURCE to get the library headers to include prototypes for GNU extensions. Without declarations at all, the compiler assumes the return type is int.

– Peter Cordes
Nov 13 '18 at 15:23

Thanks,@PeterCordes，Do you know any more information about the behavior of function with implicit declaration, not only the return type but also some input or output parameters, is it mean the behavior of this function is unpredictable or undefined?Especially for GNU extended functions?

– qiqi
Nov 14 '18 at 5:04

Thanks,@ThomasDickey but what I am confused and curious about why the function could also be called even if not defined _GNU_SOURCE，and why the behavior or output of the function is not the same in different platforms

– qiqi
Nov 14 '18 at 5:05

add a comment |

1 Answer
1

active

oldest

votes

Implicit function declarations, a GCC extension which is enabled by default, use a return type of int, however the function is defined as returning long long. With such a type mismatch, anything can happen. In practice, the results depend on the calling convention, and that is why they vary across architectures.

You should really compile with -Werror=implicit-function-declaration (or use C++). Implicit function declarations were removed from C close to twenty years ago (in ISO/IEC 9899:1999), but we still cannot change the GCC default because too many autoconf checks would break, resulting in software accidentally losing features.

(I think you meant to write retval in your question instead of errno.)

edited Nov 23 '18 at 9:10

answered Nov 22 '18 at 21:32

Florian Weimer

15.4k3944

Maybe worth mentioning explicitly that implicit declarations are allowed in ISO C89. But yes, in later ISO C standards (and C++) it's not allowed, and agreed that it's generally a bad idea. There's a reason GCC warns about it even without -Wall.

– Peter Cordes
Nov 22 '18 at 23:04

Yes,I mean retval instead of errno and I reedit it.

– qiqi
Nov 27 '18 at 1:08

add a comment |

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(I think you meant to write retval in your question instead of errno.)

edited Nov 23 '18 at 9:10

answered Nov 22 '18 at 21:32

Florian Weimer

15.4k3944

Maybe worth mentioning explicitly that implicit declarations are allowed in ISO C89. But yes, in later ISO C standards (and C++) it's not allowed, and agreed that it's generally a bad idea. There's a reason GCC warns about it even without -Wall.

– Peter Cordes
Nov 22 '18 at 23:04

Yes,I mean retval instead of errno and I reedit it.

– qiqi
Nov 27 '18 at 1:08

add a comment |

(I think you meant to write retval in your question instead of errno.)

edited Nov 23 '18 at 9:10

answered Nov 22 '18 at 21:32

Florian Weimer

15.4k3944

Maybe worth mentioning explicitly that implicit declarations are allowed in ISO C89. But yes, in later ISO C standards (and C++) it's not allowed, and agreed that it's generally a bad idea. There's a reason GCC warns about it even without -Wall.

– Peter Cordes
Nov 22 '18 at 23:04

Yes,I mean retval instead of errno and I reedit it.

– qiqi
Nov 27 '18 at 1:08

add a comment |

(I think you meant to write retval in your question instead of errno.)

edited Nov 23 '18 at 9:10

answered Nov 22 '18 at 21:32

Florian Weimer

15.4k3944

(I think you meant to write retval in your question instead of errno.)

edited Nov 23 '18 at 9:10

answered Nov 22 '18 at 21:32

Florian Weimer

15.4k3944

edited Nov 23 '18 at 9:10

answered Nov 22 '18 at 21:32

Florian Weimer

15.4k3944

answered Nov 22 '18 at 21:32

Florian Weimer

15.4k3944

answered Nov 22 '18 at 21:32

Florian Weimer

15.4k3944

Maybe worth mentioning explicitly that implicit declarations are allowed in ISO C89. But yes, in later ISO C standards (and C++) it's not allowed, and agreed that it's generally a bad idea. There's a reason GCC warns about it even without -Wall.

– Peter Cordes
Nov 22 '18 at 23:04

Yes,I mean retval instead of errno and I reedit it.

– qiqi
Nov 27 '18 at 1:08

add a comment |

Maybe worth mentioning explicitly that implicit declarations are allowed in ISO C89. But yes, in later ISO C standards (and C++) it's not allowed, and agreed that it's generally a bad idea. There's a reason GCC warns about it even without -Wall.

– Peter Cordes
Nov 22 '18 at 23:04

Yes,I mean retval instead of errno and I reedit it.

– qiqi
Nov 27 '18 at 1:08

Maybe worth mentioning explicitly that implicit declarations are allowed in ISO C89. But yes, in later ISO C standards (and C++) it's not allowed, and agreed that it's generally a bad idea. There's a reason GCC warns about it even without -Wall.

– Peter Cordes
Nov 22 '18 at 23:04

Yes,I mean retval instead of errno and I reedit it.

– qiqi
Nov 27 '18 at 1:08

add a comment |

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