I wrote this C code to find the value for 3 squared.
#include <stdio.h>
#include <math.h>
int main( void )
{
float a;
a = powf( 3, 2 );
printf( "\n%f\n", a );
return 0;
}
I get the warning implicit declaration of function 'powf' even with math.h library included and -lm in the terminal command:
gcc -o test.exe -ansi -Wall test.c -lm
My gcc version is 4.2.2 if it helps.
powf is added in C99. Option -ansi is equivalent to -std=c89. You need to use -std=c99 flag.
gcc -o test.exe -std=c99 -Wall test.c -lm
The problem is the -ansi parameter. This is equivalent to -std=c90.
As the man page for powf states, you need to use -std=c99
Related
I have a statically linked library, containing a global variable barvar. I can compile the library with no problems with either gcc-10 or clang (this is on macOS Catalina). Interestingly, the behavior differs between the two when I try to link it into a program that uses the library. Here's the code:
In globvars.h, int barvar is declared:
#ifndef H_GLOBVARS_H
#define H_GLOBVARS_H
extern int barvar;
#endif
In globvars.c, int barvar is defined:
#include "globvars.h"
int barvar;
In foo.c, the function foo sets and prints barvar:
#include <stdio.h>
#include "globvars.h"
void foo()
{
barvar = 10;
printf("barvar is: %d\n", barvar);
return;
}
Here's test.c, the program that uses the library:
void foo();
int main(int argc, char **argv)
{
foo();
return 0;
}
When I compile and link with gcc-10, no problems:
gcc-10 -c foo.c -o foo.o
gcc-10 -c globvars.c -o globvars.o
gcc-10 -c test.c -o test.o
gcc-ar-10 rcs liblinktest.a foo.o globvars.o
gcc -o testlinkrun test2.o -L. -llinktest
When I compile and link with clang, I get an undefined symbol error at the last step:
cc -c foo.c -o foo.o
cc -c globvars.c -o globvars.o
cc -c test.c -o test.o
ar rcs liblinktest.a foo.o globvars.o
cc -o testlinkrun test2.o -L. -llinktest
with error:
Undefined symbols for architecture x86_64:
"_barvar", referenced from:
_foo in liblinktest.a(foo.o)
Any ideas? Interestingly, it appears the only step that has to be done with gcc-10 is compiling globvars.c. I can use clang and the clang linker for all other steps, and everything is fine. Is it possible that clang is optimizing away all the variables in globvars.c? How can I prevent this?
As #EricPostpischil observed in this comment, the issue is that clang defaults to treating barvar as a common symbol. Either changing int barvar; to int barvar = 0;, or compiling with -fno-common, fix the issue.
Beginning with gcc-10, gcc's default behavior is -fno-common instead of -fcommon.
I have a program which is linked (dynamically) with libm.
There are also several plugins for this program.
Plugins are loaded explicitely with dlopen().
Some of these plugins use round() from libm.
On one system (Linux Mint 19.1 gcc 7.5.0) the program
does not work because of unresolved round.
Here is simple example:
Library (lib.c)
#include <stdio.h>
#include <math.h>
void func(double a, double b)
{
double c;
c = round(a + b);
printf("c = %lf\n", c);
}
Main program (main.c)
#include <stdio.h>
#include <dlfcn.h>
void *dll;
void (*f)(double, double);
double a = 1.234, b = 4.321;
int main(void)
{
dll = dlopen("./lib.so", RTLD_LAZY);
f = dlsym(dll, "func");
f(a,b);
return 0;
}
Building (Makefile)
all:
gcc -Wall -Os -shared -fPIC lib.c -o lib.so
gcc -Wall -Os -rdynamic -fPIC main.c -o main -ldl -lm
Run on Debian 8, gcc 4.9.2
./main
c = 6.000000
Run on Linux Mint 19.1, gcc 7.5.0
./main
./main: symbol lookup error: ./lib.so: undefined symbol: round
Now, add -lm for dll compilation
gcc -Wall -Os -shared -fPIC lib.c -o lib.so -lm
./main
c = 6.000000
So, the question is - why on this particular system one must use -lm not only for main program but for plugin also?
Just like an executable program, shared libraries are linked entities (unlike static libraries which are archives of object files).
Since shared libraries are linked like executables, you also need to link with the libraries that your library depends on:
gcc -Wall -Os -shared -fPIC lib.c -o lib.so -lm
I have a very simple Hello World program, that doesn't have a return at the end of its main() function. If I understand correctly, this should throw a Wreturn-type warning, but when I compile it, no output at all is given. It simply compiles it and is done.
program:
#include <stdio.h>
int main() {
printf("Hello World!\n");
}
compilation commmand:
gcc -Wall -Wextra -o hello.o hello.c
I also tried specifically with the Wreturn-type option.
On Manjaro 18.0.0 with GCC 8.2.1
Use
gcc -std=c89 -pedantic ...
because in C99 1 main() does not need a return 0;. It's as if there was one right before the closing brace.
I am trying to compile the following source file using gcc -g -Wall -Wextra -pedantic -std=c99 -o main -lm.
source.h
void simple_sum(void)
source.c
#include "source.h"
#include <stdio.h>
void simple_sum(void)
{
int a, b;
scanf("%d %d", &a, &b);
printf("%d + %d = %d\n",a, b, a + b);
}
main.c
#include "source.h"
#include <stdio.h>
int main(void)
{
printf("\n");
simple_sum();
return 0;
}
I get following error:
gcc -g -Wall -Wextra -pedantic -std=c99 -o main -lm
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In function _start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
Makefile:6: recipe for target 'main' failed
make: *** [main] Error 1
Could someone please suggest how to fix this?
Edit
I get the following error when I run using gcc -g -Wall main.c -o main
/tmp/ccEAL4iG.o: In functionmain':
/home/a/aalto_university/functions/calculation/main.c:7: undefined reference to simple_sum'
collect2: error: ld returned 1 exit status
Compile with
gcc -g -Wall -Wextra -pedantic -std=c99 source.c main.c -o myprog -lm
(actually, -lm is not needed, you don't use <math.h> functions; but keeping -lm should not harm)
Later, learn to write your Makefile to do these things in several steps:
First, get the source.o object file with
gcc -g -Wall -Wextra -pedantic -std=c99 -c source.c
then get the main.o object file with
gcc -g -Wall -Wextra -pedantic -std=c99 -c main.c
At last, link both of them
gcc -g source.o main.o -lm -o myprog
Here
gcc -g -Wall -Wextra -pedantic -std=c99 -o main -lm
you are not providing source file name to linker, hence it throw error like
undefined reference to `main'
While compiling provide source file main.c and source.c. For e.g first run this
gcc -g -Wall -Wextra -pedantic -std=c99 -c main.c source.c -lm
to create the object.o files & then create the executable by running
gcc source.o main.o -o my_exe
And finally run the executable. Also declaration of simple_sum() missing ; it should be
void simple_sum(void); /* you miss ;*/
Also learn how to use Makefile for compilation as #Basile pointed, there you don't have to create .o file manually, your Makefile will create .o file & compile if it's written correctly.
I wrote a simple printf C code and made a simple makefile.
When I run make with CFLAGS, CPPFLAGS and LDFLAGS, the values of the variables goes into a cc execution, followed by a gcc execution without those values, like this:
$ CFLAGS="-I." CPPFLAGS="-D TESTEDEFINE" CXXFLAGS="TESTECXXFLAGS" LDFLAGS="-L." LFLAGS="TESTELFLAGS" make
cc -I. -D TESTEDEFINE -L. teste.c -o teste
gcc -o teste teste.c
When I run the built program, the define isn't defined since it gives me the printf of the not defined #else.
teste.c
#include <stdio.h>
int main()
{
#if defined(TESTEDEFINE)
printf("TESTEDEFINE!!!");
#else
printf("!!!");
#endif
return 0;
}
Makefile
all: teste
gcc -o teste teste.c
The variables are for consistency, readability, and ease of use. Neither your compile nor your makefile reference them. The compiler does not automatically reference those variables.
Try this instead:
$ export CFLAGS="-I." CPPFLAGS="-D TESTEDEFINE" CXXFLAGS="TESTECXXFLAGS" LDFLAGS="-L." LFLAGS="TESTELFLAGS"
$ gcc $CFLAGS $CPPFLAGS $CXXFLAGS $LDFLAGS $LFLAGS -o teste teste.c
You would also need to define them in your makefile and reference them in the compiler line.