Sample code for fmod:
#include <stdio.h>
#include <math.h>
int main(void)
{
double x = 0.14527, y = 3.14159;
printf("fmod(x, y) = %.6lf\n", fmod(x, y));
return 0;
}
Compiling:
$ gcc main.c -o main
I get
/tmp/ccztJO01.o: In function `main':
main.c:(.text+0x4d): undefined reference to `fmod'
collect2: ld returned 1 exit status
Then I found this in Google:
$ gcc -lm main.c -o main
Why should I use -lm, what is it exactly? From where I can get more information about gcc in detail?
-lm is simply telling it to link libm, which contains all the floating point math routines, including (no surprise here) fmod.
When I input gcc -lm main.c -o main I still get a linker error. I need to write gcc main.c -lm -o main for it work right. If it's working for you the other way, that's a bit odd. I understand that the linker will find the symbol declared in main.c (i.e. double fmod(double,double)), but only resolve it if it finds its definition later on (i.e. in libm.a).
Long story short, the libraries must be placed (at least once) "to the right of" the place where they are used.
It's not the compiler, but the linker, ld, that is complaining. It cannot find the routine fmod in your program. You have to tell it to link with math library libm with the -l flag.
[Much] more info: GCC, the GNU Compiler Collection.
Related
Sample code for fmod:
#include <stdio.h>
#include <math.h>
int main(void)
{
double x = 0.14527, y = 3.14159;
printf("fmod(x, y) = %.6lf\n", fmod(x, y));
return 0;
}
Compiling:
$ gcc main.c -o main
I get
/tmp/ccztJO01.o: In function `main':
main.c:(.text+0x4d): undefined reference to `fmod'
collect2: ld returned 1 exit status
Then I found this in Google:
$ gcc -lm main.c -o main
Why should I use -lm, what is it exactly? From where I can get more information about gcc in detail?
-lm is simply telling it to link libm, which contains all the floating point math routines, including (no surprise here) fmod.
When I input gcc -lm main.c -o main I still get a linker error. I need to write gcc main.c -lm -o main for it work right. If it's working for you the other way, that's a bit odd. I understand that the linker will find the symbol declared in main.c (i.e. double fmod(double,double)), but only resolve it if it finds its definition later on (i.e. in libm.a).
Long story short, the libraries must be placed (at least once) "to the right of" the place where they are used.
It's not the compiler, but the linker, ld, that is complaining. It cannot find the routine fmod in your program. You have to tell it to link with math library libm with the -l flag.
[Much] more info: GCC, the GNU Compiler Collection.
How to change the entry point of a C program compiled with gcc ?
Just like in the following code
#include<stdio.h>
int entry() //entry is the entry point instead of main
{
return 0;
}
It's a linker setting:
-Wl,-eentry
the -Wl,... thing passes arguments to the linker, and the linker takes a -e argument to set the entry function
You can modify your source code as:
#include<stdio.h>
const char my_interp[] __attribute__((section(".interp"))) = "/lib/ld-linux.so.2";
int entry() //entry is the entry point instead of main
{
exit(0);
}
The ".interp" section will let your program able to call external shared library.
The exit call will make your entry function to exit program instead of return.
Then build the program as a shared library which is executable:
$ gcc -shared -fPIC -e entry test_main.c -o test_main.so
$ ./test_main
If you are on a system that provides GNU Binutils (like Linux),
you can use the objcopy command
to make an arbitrary function the new entry point.
Suppose a file called program.c containing the entry function:
$ cat > program.c
#include <stdio.h>
int entry()
{
return 0;
}
^D
You first compile it using -c to generate a relocatable object file:
$ gcc -c program.c -o program.o
Then you redefine entry to be main:
$ objcopy --redefine-sym entry=main program.o
Now use gcc to compile the new object file:
$ gcc program.o -o program
NOTE: If your program already has a function called main, before step 2, you can perform a separate objcopy invocation:
objcopy --redefine-sym oldmain=main program.o
Minimal runnable example and notes on other answers
main.c
#include <stdio.h>
#include <stdlib.h>
int mymain(void) {
puts("hello");
exit(0);
}
compile and run:
gcc -nostartfiles -Wl,--entry=mymain -o main.out main.c
# or -Wl,-emymain
./main.out 1 2 3
The notes:
without -nostartfiles, the link fails with:
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
presumably because the glibc setup code that runs before main in _start normally calls main.
command line arguments are not setup for you, presumably because they would be setup by the glibc code that runs before main, so trying to use them prints undefined values. I haven't found a method that works for them.
Tested in Ubuntu 20.10.
How to change the entry point of a C program compiled with gcc ?
Just like in the following code
#include<stdio.h>
int entry() //entry is the entry point instead of main
{
return 0;
}
It's a linker setting:
-Wl,-eentry
the -Wl,... thing passes arguments to the linker, and the linker takes a -e argument to set the entry function
You can modify your source code as:
#include<stdio.h>
const char my_interp[] __attribute__((section(".interp"))) = "/lib/ld-linux.so.2";
int entry() //entry is the entry point instead of main
{
exit(0);
}
The ".interp" section will let your program able to call external shared library.
The exit call will make your entry function to exit program instead of return.
Then build the program as a shared library which is executable:
$ gcc -shared -fPIC -e entry test_main.c -o test_main.so
$ ./test_main
If you are on a system that provides GNU Binutils (like Linux),
you can use the objcopy command
to make an arbitrary function the new entry point.
Suppose a file called program.c containing the entry function:
$ cat > program.c
#include <stdio.h>
int entry()
{
return 0;
}
^D
You first compile it using -c to generate a relocatable object file:
$ gcc -c program.c -o program.o
Then you redefine entry to be main:
$ objcopy --redefine-sym entry=main program.o
Now use gcc to compile the new object file:
$ gcc program.o -o program
NOTE: If your program already has a function called main, before step 2, you can perform a separate objcopy invocation:
objcopy --redefine-sym oldmain=main program.o
Minimal runnable example and notes on other answers
main.c
#include <stdio.h>
#include <stdlib.h>
int mymain(void) {
puts("hello");
exit(0);
}
compile and run:
gcc -nostartfiles -Wl,--entry=mymain -o main.out main.c
# or -Wl,-emymain
./main.out 1 2 3
The notes:
without -nostartfiles, the link fails with:
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
presumably because the glibc setup code that runs before main in _start normally calls main.
command line arguments are not setup for you, presumably because they would be setup by the glibc code that runs before main, so trying to use them prints undefined values. I haven't found a method that works for them.
Tested in Ubuntu 20.10.
This question already has answers here:
Undefined reference to 'pow' even though -lm is a compile flag. [C]
(2 answers)
Closed 3 years ago.
I noticed that when I use sin inside function the compiler don't recognize it, here is an example:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
float sinus(float a){
return sin(a);}
int main(int argc, char **argv)
{
double a = sinus(2);
printf("%f \n", sin(2));
printf("%f", a);
return 0;
}
If I use it directly in main it works fine, but inside a user defined function it gives me this error undefined reference to sin.
For compiling I use gcc -Wall -lm -lc -lgcc -o "%e" "%f".
References to libraries typically go to the end of the command line, in particular after the sources have been specified:
gcc -Wall -o "%e" "%f" -lm
(specifing the C lib is not necessary, it is linked implicilty)
From the documentation:
-l library
[...]
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘foo.o -lz bar.o’ searches library ‘z’ after file foo.o but before bar.o. If bar.o refers to functions in ‘z’, those functions may not be loaded.
I am unable to compile the following simple C code and I don't know why.
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <math.h>
int main(){
double result;
result = cos(0.5);
printf("asin(0.5) is %f\n", result);
return 0;
}
The error message I receive after I try to compile is -
In function
'main':
test.c:(.text+0xlc): undefined reference to 'cos'
collect2: ld
returned 1 exit status
You need to link with the math library (-lm).
gcc -Wall -Wextra -o test test.c -lm
See this C FAQ.
In general whenever you get undefined reference error it's due to the compiler is not able to find your function definition. So it may be your function ( and you have not typed the spelling of function correctly so you will get this error ) or may be built-in function like you have encountered in this case.
to explore there are various library and their linking are necessary at the time of compilation
whenever you use math function use -lm ( l stands for link and m is for math )
in pthread built-in functions use -lpthread
and so on ...
In this case indeed use -lm
gcc -lm test.c
will be able to compile your program .