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Declaring function on another .c file makes float multiplication always be 0


Unsuccessful fread() of int stored in binary file, segmentation faultMake a float only show two decimal placesWhy is “while (!feof(file))” always wrong?printf by given pointer and format string. Issue with floatsHow do I achieve the theoretical maximum of 4 FLOPs per cycle?Using structs in multiple filesC++ Truncating Floats in Function Return16bit Float Multiplication in CInitialize multiple arrays in function C programmingImplicit function declarations and linkageThis C function should always return false, but it doesn’t






.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty height:90px;width:728px;box-sizing:border-box;








0















If I define the function in the main, everything works fine. But if I define the function in another c file, this weir float multiplication problem happens: it gives 0 always.



example1/main.c:



int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);


int main()

multiply_by_2(3);



example1/CMakeLists.txt



cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c)


Output:



result: 6.000000


example2/main.c:



 int main()

multiply_by_2(3);



example2/a.c:



int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



example2/CMakeLists.txt:



cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c a.c)


Output:



result: 0.000000









share|improve this question






















  • You don't need cmake for such a simple example

    – Basile Starynkevitch
    Mar 9 at 2:36






  • 1





    Clearly this doesn't compile, or at least shouldn't. printf() isn't declared in a.c, and multiply_by_2() isn't declared in main.c... show the full code.

    – Havenard
    Mar 9 at 3:07


















0















If I define the function in the main, everything works fine. But if I define the function in another c file, this weir float multiplication problem happens: it gives 0 always.



example1/main.c:



int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);


int main()

multiply_by_2(3);



example1/CMakeLists.txt



cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c)


Output:



result: 6.000000


example2/main.c:



 int main()

multiply_by_2(3);



example2/a.c:



int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



example2/CMakeLists.txt:



cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c a.c)


Output:



result: 0.000000









share|improve this question






















  • You don't need cmake for such a simple example

    – Basile Starynkevitch
    Mar 9 at 2:36






  • 1





    Clearly this doesn't compile, or at least shouldn't. printf() isn't declared in a.c, and multiply_by_2() isn't declared in main.c... show the full code.

    – Havenard
    Mar 9 at 3:07














0












0








0








If I define the function in the main, everything works fine. But if I define the function in another c file, this weir float multiplication problem happens: it gives 0 always.



example1/main.c:



int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);


int main()

multiply_by_2(3);



example1/CMakeLists.txt



cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c)


Output:



result: 6.000000


example2/main.c:



 int main()

multiply_by_2(3);



example2/a.c:



int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



example2/CMakeLists.txt:



cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c a.c)


Output:



result: 0.000000









share|improve this question














If I define the function in the main, everything works fine. But if I define the function in another c file, this weir float multiplication problem happens: it gives 0 always.



example1/main.c:



int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);


int main()

multiply_by_2(3);



example1/CMakeLists.txt



cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c)


Output:



result: 6.000000


example2/main.c:



 int main()

multiply_by_2(3);



example2/a.c:



int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



example2/CMakeLists.txt:



cmake_minimum_required(VERSION 2.6.0)

project(example C)

add_executable(example main.c a.c)


Output:



result: 0.000000






c floating-point






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Mar 9 at 2:28









Guerlando OCsGuerlando OCs

827




827












  • You don't need cmake for such a simple example

    – Basile Starynkevitch
    Mar 9 at 2:36






  • 1





    Clearly this doesn't compile, or at least shouldn't. printf() isn't declared in a.c, and multiply_by_2() isn't declared in main.c... show the full code.

    – Havenard
    Mar 9 at 3:07


















  • You don't need cmake for such a simple example

    – Basile Starynkevitch
    Mar 9 at 2:36






  • 1





    Clearly this doesn't compile, or at least shouldn't. printf() isn't declared in a.c, and multiply_by_2() isn't declared in main.c... show the full code.

    – Havenard
    Mar 9 at 3:07

















You don't need cmake for such a simple example

– Basile Starynkevitch
Mar 9 at 2:36





You don't need cmake for such a simple example

– Basile Starynkevitch
Mar 9 at 2:36




1




1





Clearly this doesn't compile, or at least shouldn't. printf() isn't declared in a.c, and multiply_by_2() isn't declared in main.c... show the full code.

– Havenard
Mar 9 at 3:07






Clearly this doesn't compile, or at least shouldn't. printf() isn't declared in a.c, and multiply_by_2() isn't declared in main.c... show the full code.

– Havenard
Mar 9 at 3:07













2 Answers
2






active

oldest

votes


















2














int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



you have an int returning function which does not return any integer. This triggers undefined behavior (once you call that multiply_by_2 function elsewhere). Be scared, very bad things could happen (even outside of that multiply_by_2 function or outside of its caller).



Next time, compile with all warnings and debug info, so with gcc -Wall -Wextra -g if using GCC. Improve your code to get no warnings.



Read also How to debug small programs.



If you use floating point numbers, be very cautious. They are difficult to understand and behave counter-intuitively (e.g. addition is not associative). Read http://floating-point-gui.de/ first.



If you have several translation units (e.g. source files such as a.c and main.c) you should in practice at least have some common header file and #include it in every *.c file. At a first approximation, your header file should declare all your  [global] types and functions. And you'll need to configure your build automation tool (e.g. write your Makefile) to take into account such dependencies: your header file should somehow be mentioned.



Read much more about C programming. See also some C reference site. Read the documentation of every function you are using (e.g. of printf)



You'll learn a lot by studying the source code of existing small free software programs (e.g. on github, gitlab, in a Linux distribution, etc).






share|improve this answer




















  • 3





    you have an int returning function which does not return any integer. This is undefined behavior. No, this is not undefined behavior. The behavior is undefined when the value of the function call is used by the caller in this case. The code shown in example2 nowhere uses the value of multiply_by_2() function call.

    – H.S.
    Mar 9 at 2:52












  • @H.S.: Huh. Learn something new every day. N1570, 5.9.1 Function definitions, item #12: "If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined."

    – ShadowRanger
    Mar 13 at 13:37











  • @ShadowRanger Of course and that what I wrote in my comment - The behavior is undefined when the value of the function call is used by the caller in this case. In this sentence by "in this case", I am referring the case where the function is suppose to return some value but the return statement is missing.

    – H.S.
    Mar 13 at 13:53












  • @H.S.: Yeah, I was thanking you for pointing that out and providing the associated reference. I was surprised, and looked it up for myself; figured I'd save others the effort. :-)

    – ShadowRanger
    Mar 13 at 14:38


















0














The problem was that I should add a declaration of the functions in main.c, even though my compiler wouldn't warne me about it. With the declarations, all bugs are gone






share|improve this answer


















  • 1





    I suspect turning up your warning level would get a warning output. Undeclared functions are kind of basic.

    – ShadowRanger
    Mar 9 at 2:31






  • 2





    Actually, you should have some common header file and include it everywhere. Copy & pasting the same declaration in several files is not scalable and should be avoided (unless perhaps your C source files are generated from something else; then the generator could duplicate declarations)

    – Basile Starynkevitch
    Mar 9 at 3:01












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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









2














int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



you have an int returning function which does not return any integer. This triggers undefined behavior (once you call that multiply_by_2 function elsewhere). Be scared, very bad things could happen (even outside of that multiply_by_2 function or outside of its caller).



Next time, compile with all warnings and debug info, so with gcc -Wall -Wextra -g if using GCC. Improve your code to get no warnings.



Read also How to debug small programs.



If you use floating point numbers, be very cautious. They are difficult to understand and behave counter-intuitively (e.g. addition is not associative). Read http://floating-point-gui.de/ first.



If you have several translation units (e.g. source files such as a.c and main.c) you should in practice at least have some common header file and #include it in every *.c file. At a first approximation, your header file should declare all your  [global] types and functions. And you'll need to configure your build automation tool (e.g. write your Makefile) to take into account such dependencies: your header file should somehow be mentioned.



Read much more about C programming. See also some C reference site. Read the documentation of every function you are using (e.g. of printf)



You'll learn a lot by studying the source code of existing small free software programs (e.g. on github, gitlab, in a Linux distribution, etc).






share|improve this answer




















  • 3





    you have an int returning function which does not return any integer. This is undefined behavior. No, this is not undefined behavior. The behavior is undefined when the value of the function call is used by the caller in this case. The code shown in example2 nowhere uses the value of multiply_by_2() function call.

    – H.S.
    Mar 9 at 2:52












  • @H.S.: Huh. Learn something new every day. N1570, 5.9.1 Function definitions, item #12: "If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined."

    – ShadowRanger
    Mar 13 at 13:37











  • @ShadowRanger Of course and that what I wrote in my comment - The behavior is undefined when the value of the function call is used by the caller in this case. In this sentence by "in this case", I am referring the case where the function is suppose to return some value but the return statement is missing.

    – H.S.
    Mar 13 at 13:53












  • @H.S.: Yeah, I was thanking you for pointing that out and providing the associated reference. I was surprised, and looked it up for myself; figured I'd save others the effort. :-)

    – ShadowRanger
    Mar 13 at 14:38















2














int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



you have an int returning function which does not return any integer. This triggers undefined behavior (once you call that multiply_by_2 function elsewhere). Be scared, very bad things could happen (even outside of that multiply_by_2 function or outside of its caller).



Next time, compile with all warnings and debug info, so with gcc -Wall -Wextra -g if using GCC. Improve your code to get no warnings.



Read also How to debug small programs.



If you use floating point numbers, be very cautious. They are difficult to understand and behave counter-intuitively (e.g. addition is not associative). Read http://floating-point-gui.de/ first.



If you have several translation units (e.g. source files such as a.c and main.c) you should in practice at least have some common header file and #include it in every *.c file. At a first approximation, your header file should declare all your  [global] types and functions. And you'll need to configure your build automation tool (e.g. write your Makefile) to take into account such dependencies: your header file should somehow be mentioned.



Read much more about C programming. See also some C reference site. Read the documentation of every function you are using (e.g. of printf)



You'll learn a lot by studying the source code of existing small free software programs (e.g. on github, gitlab, in a Linux distribution, etc).






share|improve this answer




















  • 3





    you have an int returning function which does not return any integer. This is undefined behavior. No, this is not undefined behavior. The behavior is undefined when the value of the function call is used by the caller in this case. The code shown in example2 nowhere uses the value of multiply_by_2() function call.

    – H.S.
    Mar 9 at 2:52












  • @H.S.: Huh. Learn something new every day. N1570, 5.9.1 Function definitions, item #12: "If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined."

    – ShadowRanger
    Mar 13 at 13:37











  • @ShadowRanger Of course and that what I wrote in my comment - The behavior is undefined when the value of the function call is used by the caller in this case. In this sentence by "in this case", I am referring the case where the function is suppose to return some value but the return statement is missing.

    – H.S.
    Mar 13 at 13:53












  • @H.S.: Yeah, I was thanking you for pointing that out and providing the associated reference. I was surprised, and looked it up for myself; figured I'd save others the effort. :-)

    – ShadowRanger
    Mar 13 at 14:38













2












2








2







int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



you have an int returning function which does not return any integer. This triggers undefined behavior (once you call that multiply_by_2 function elsewhere). Be scared, very bad things could happen (even outside of that multiply_by_2 function or outside of its caller).



Next time, compile with all warnings and debug info, so with gcc -Wall -Wextra -g if using GCC. Improve your code to get no warnings.



Read also How to debug small programs.



If you use floating point numbers, be very cautious. They are difficult to understand and behave counter-intuitively (e.g. addition is not associative). Read http://floating-point-gui.de/ first.



If you have several translation units (e.g. source files such as a.c and main.c) you should in practice at least have some common header file and #include it in every *.c file. At a first approximation, your header file should declare all your  [global] types and functions. And you'll need to configure your build automation tool (e.g. write your Makefile) to take into account such dependencies: your header file should somehow be mentioned.



Read much more about C programming. See also some C reference site. Read the documentation of every function you are using (e.g. of printf)



You'll learn a lot by studying the source code of existing small free software programs (e.g. on github, gitlab, in a Linux distribution, etc).






share|improve this answer















int multiply_by_2(float scalar) 
printf("result: %fn", scalar*2);



you have an int returning function which does not return any integer. This triggers undefined behavior (once you call that multiply_by_2 function elsewhere). Be scared, very bad things could happen (even outside of that multiply_by_2 function or outside of its caller).



Next time, compile with all warnings and debug info, so with gcc -Wall -Wextra -g if using GCC. Improve your code to get no warnings.



Read also How to debug small programs.



If you use floating point numbers, be very cautious. They are difficult to understand and behave counter-intuitively (e.g. addition is not associative). Read http://floating-point-gui.de/ first.



If you have several translation units (e.g. source files such as a.c and main.c) you should in practice at least have some common header file and #include it in every *.c file. At a first approximation, your header file should declare all your  [global] types and functions. And you'll need to configure your build automation tool (e.g. write your Makefile) to take into account such dependencies: your header file should somehow be mentioned.



Read much more about C programming. See also some C reference site. Read the documentation of every function you are using (e.g. of printf)



You'll learn a lot by studying the source code of existing small free software programs (e.g. on github, gitlab, in a Linux distribution, etc).







share|improve this answer














share|improve this answer



share|improve this answer








edited Mar 9 at 2:59

























answered Mar 9 at 2:33









Basile StarynkevitchBasile Starynkevitch

180k13174375




180k13174375







  • 3





    you have an int returning function which does not return any integer. This is undefined behavior. No, this is not undefined behavior. The behavior is undefined when the value of the function call is used by the caller in this case. The code shown in example2 nowhere uses the value of multiply_by_2() function call.

    – H.S.
    Mar 9 at 2:52












  • @H.S.: Huh. Learn something new every day. N1570, 5.9.1 Function definitions, item #12: "If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined."

    – ShadowRanger
    Mar 13 at 13:37











  • @ShadowRanger Of course and that what I wrote in my comment - The behavior is undefined when the value of the function call is used by the caller in this case. In this sentence by "in this case", I am referring the case where the function is suppose to return some value but the return statement is missing.

    – H.S.
    Mar 13 at 13:53












  • @H.S.: Yeah, I was thanking you for pointing that out and providing the associated reference. I was surprised, and looked it up for myself; figured I'd save others the effort. :-)

    – ShadowRanger
    Mar 13 at 14:38












  • 3





    you have an int returning function which does not return any integer. This is undefined behavior. No, this is not undefined behavior. The behavior is undefined when the value of the function call is used by the caller in this case. The code shown in example2 nowhere uses the value of multiply_by_2() function call.

    – H.S.
    Mar 9 at 2:52












  • @H.S.: Huh. Learn something new every day. N1570, 5.9.1 Function definitions, item #12: "If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined."

    – ShadowRanger
    Mar 13 at 13:37











  • @ShadowRanger Of course and that what I wrote in my comment - The behavior is undefined when the value of the function call is used by the caller in this case. In this sentence by "in this case", I am referring the case where the function is suppose to return some value but the return statement is missing.

    – H.S.
    Mar 13 at 13:53












  • @H.S.: Yeah, I was thanking you for pointing that out and providing the associated reference. I was surprised, and looked it up for myself; figured I'd save others the effort. :-)

    – ShadowRanger
    Mar 13 at 14:38







3




3





you have an int returning function which does not return any integer. This is undefined behavior. No, this is not undefined behavior. The behavior is undefined when the value of the function call is used by the caller in this case. The code shown in example2 nowhere uses the value of multiply_by_2() function call.

– H.S.
Mar 9 at 2:52






you have an int returning function which does not return any integer. This is undefined behavior. No, this is not undefined behavior. The behavior is undefined when the value of the function call is used by the caller in this case. The code shown in example2 nowhere uses the value of multiply_by_2() function call.

– H.S.
Mar 9 at 2:52














@H.S.: Huh. Learn something new every day. N1570, 5.9.1 Function definitions, item #12: "If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined."

– ShadowRanger
Mar 13 at 13:37





@H.S.: Huh. Learn something new every day. N1570, 5.9.1 Function definitions, item #12: "If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined."

– ShadowRanger
Mar 13 at 13:37













@ShadowRanger Of course and that what I wrote in my comment - The behavior is undefined when the value of the function call is used by the caller in this case. In this sentence by "in this case", I am referring the case where the function is suppose to return some value but the return statement is missing.

– H.S.
Mar 13 at 13:53






@ShadowRanger Of course and that what I wrote in my comment - The behavior is undefined when the value of the function call is used by the caller in this case. In this sentence by "in this case", I am referring the case where the function is suppose to return some value but the return statement is missing.

– H.S.
Mar 13 at 13:53














@H.S.: Yeah, I was thanking you for pointing that out and providing the associated reference. I was surprised, and looked it up for myself; figured I'd save others the effort. :-)

– ShadowRanger
Mar 13 at 14:38





@H.S.: Yeah, I was thanking you for pointing that out and providing the associated reference. I was surprised, and looked it up for myself; figured I'd save others the effort. :-)

– ShadowRanger
Mar 13 at 14:38













0














The problem was that I should add a declaration of the functions in main.c, even though my compiler wouldn't warne me about it. With the declarations, all bugs are gone






share|improve this answer


















  • 1





    I suspect turning up your warning level would get a warning output. Undeclared functions are kind of basic.

    – ShadowRanger
    Mar 9 at 2:31






  • 2





    Actually, you should have some common header file and include it everywhere. Copy & pasting the same declaration in several files is not scalable and should be avoided (unless perhaps your C source files are generated from something else; then the generator could duplicate declarations)

    – Basile Starynkevitch
    Mar 9 at 3:01
















0














The problem was that I should add a declaration of the functions in main.c, even though my compiler wouldn't warne me about it. With the declarations, all bugs are gone






share|improve this answer


















  • 1





    I suspect turning up your warning level would get a warning output. Undeclared functions are kind of basic.

    – ShadowRanger
    Mar 9 at 2:31






  • 2





    Actually, you should have some common header file and include it everywhere. Copy & pasting the same declaration in several files is not scalable and should be avoided (unless perhaps your C source files are generated from something else; then the generator could duplicate declarations)

    – Basile Starynkevitch
    Mar 9 at 3:01














0












0








0







The problem was that I should add a declaration of the functions in main.c, even though my compiler wouldn't warne me about it. With the declarations, all bugs are gone






share|improve this answer













The problem was that I should add a declaration of the functions in main.c, even though my compiler wouldn't warne me about it. With the declarations, all bugs are gone







share|improve this answer












share|improve this answer



share|improve this answer










answered Mar 9 at 2:29









Guerlando OCsGuerlando OCs

827




827







  • 1





    I suspect turning up your warning level would get a warning output. Undeclared functions are kind of basic.

    – ShadowRanger
    Mar 9 at 2:31






  • 2





    Actually, you should have some common header file and include it everywhere. Copy & pasting the same declaration in several files is not scalable and should be avoided (unless perhaps your C source files are generated from something else; then the generator could duplicate declarations)

    – Basile Starynkevitch
    Mar 9 at 3:01













  • 1





    I suspect turning up your warning level would get a warning output. Undeclared functions are kind of basic.

    – ShadowRanger
    Mar 9 at 2:31






  • 2





    Actually, you should have some common header file and include it everywhere. Copy & pasting the same declaration in several files is not scalable and should be avoided (unless perhaps your C source files are generated from something else; then the generator could duplicate declarations)

    – Basile Starynkevitch
    Mar 9 at 3:01








1




1





I suspect turning up your warning level would get a warning output. Undeclared functions are kind of basic.

– ShadowRanger
Mar 9 at 2:31





I suspect turning up your warning level would get a warning output. Undeclared functions are kind of basic.

– ShadowRanger
Mar 9 at 2:31




2




2





Actually, you should have some common header file and include it everywhere. Copy & pasting the same declaration in several files is not scalable and should be avoided (unless perhaps your C source files are generated from something else; then the generator could duplicate declarations)

– Basile Starynkevitch
Mar 9 at 3:01






Actually, you should have some common header file and include it everywhere. Copy & pasting the same declaration in several files is not scalable and should be avoided (unless perhaps your C source files are generated from something else; then the generator could duplicate declarations)

– Basile Starynkevitch
Mar 9 at 3:01


















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