This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Why Am I Getting Link Errors When Calling Function in Math.h?
Hello, I have a problem in C (gcc), when I compile a program that includes Math.h and uses functions from there, it says "undefined reference to...". it happens for sqrt, log, but for others not (for instance pow). How can I fix this library or reinstall it ? thanks in advance.
[I am using Ubuntu]
You shold link with -lm option: gcc -o test test.c -lm lm means link math.
Related
This question already has answers here:
CMake link to external library
(6 answers)
Closed 4 years ago.
I have a libname.a static library that works fine when I use gcc:
gcc -c main.c -o main.o ;
gcc main.o libname.a main
But now I would like to use CMake as the project is getting big, but I got this message and and I don't know how to include it in an appropriate way. (I tried link_target_library and/or link_directories and/or set(CMAKE_CC_FLAGS "absolute_path/libname.a").
Note that I don't have any source code for libname.a.
/usr/bin/ld: cannot find -llibname
As said in my comment target_link_libraries(${PROJECT_NAME} ${CMAKE_SOURCE_DIR}/libname.a) works. I can't really tell what the reason for your problem is (maybe you used target_link_library instead of target_link_libraries ?).
This question already has answers here:
What is the use of _start() in C?
(4 answers)
Closed 6 years ago.
Take the following C program:
int main(){}
It's not using anything from the C standard library.
I assumed we could disable linkage through -nostdlib.
However, this results in the following error:
$ gcc -nostdlib -o main main.c
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000400150
Can someone clarify what the _start symbol is and who is responsible for providing it?
How would one go about using -nostdlib?
Per the docs, I have also tried:
$ gcc -nostdlib -o main main.c -lgcc
Same behaviour is seen with clang.
_start is the entry point of the program, cf. this answer. The libc contains a little stub that reads the command line arguments off the place where the operating system puts them and then calls main. Thus, it is required to link against the libc in order to run a normal C program on a normal platform.
You can try to write a program without the libc, but then you have to provide your own _start. You can't do much though, you can't even return as there is nothing you return to. Your only option is to use assembly to painstakingly recreate the parts of the libc you need, like the _exit function.
This question already has answers here:
Undefined reference to `sin` [duplicate]
(4 answers)
Closed 9 years ago.
I have include the math.h in my c file, however it keeps prompts undefined reference to truncand undefined reference to ceil. And I didn't even use trunc in my file. Could anyone tell what was the problem?
If you compile using gcc, you might need to add -lm option to link to the math library (libm) like
gcc test.c -lm
It seems that you will need the info about how C source code are compiled and linked together.
The link contains some basic info:
Compile & Link
You use -I option of gcc to specify where to find headers, and -l option to link standard libraries
This question already has answers here:
Undefined reference to sqrt (or other mathematical functions)
(5 answers)
Closed 3 years ago.
Part of my program is to calculate sqrt of float number.
When I write sqrt(1.0f); I success to compile the program,but when I write sqrt(-1.0f);
the compilation fails with undefined reference to 'sqrt' - I suppose that in this case the nan value will be returned...
I compile the program uing gcc.
When I compile it with visual studio it is compiled successfuly with negative argument to sqrt.
How the problem could be solved
Thank you
You have to add the -lm flag on most Unix-based systems, as in:
Compile using:
gcc -c file.c
and then link using:
gcc -o program file.o -lm
Or if you don't want to separate the two compilation steps, simply write:
gcc -o program file.c -lm
Link with -lm to link with the math library
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Using GCC to produce readable assembly?
For example, when I try:
gcc -c myprogram
it will give me an object file, which is in binary form. What I would like to get is just an assembly file (right before it is processed by an assembler to produce the object file). Is there a way to do it in gcc?
just write gcc -S file.c, and you will get file.s which is assembly.
Yes. It's -S. See the manual.