My simple program compTest.c
#include<stdio.h>
#include<complex.h>
int main(void)
{
double complex z = 1.0 + 1.0 * I;
printf("|z| = %.4f\n", cabs(z));
return 0;
}
When using the standard library and compiling with gcc on a Linux system do I need to include the -lm flag for it to work?
Example:
gcc -o executableName fileName.c -lm
When I don't I get the following: /tmp/cc1o7rtt.o: In function `main':
comTest.c:(.text+0x35): undefined reference to `cabs'
collect2: error: ld returned 1 exit status
It seems that you've already discovered that the answer is yes.
The -lm flag tells the linker to link the math library, which contains, among other things, the code for the cabs function. (This is a gcc/Linux issue, not a C language issue.)
The Linux man page for cabs specifically says Link with -lm.
(In general, if you want to call any library function and you're not 100% certain how to use it, read the man page.)