You need to link with the math library:

gcc -o sphere sphere.c -lm

The error you are seeing: error: ld returned 1 exit status is from the linker ld (part of gcc that combines the object files) because it is unable to find where the function pow is defined.

Including math.h brings in the declaration of the various functions and not their definition. The def is present in the math library libm.a. You need to link your program with this library so that the calls to functions like pow() are resolved.

The problem here is the order you link with the math library (the -lm option). Libraries should be after the sources or object files on the command line when building.

So if you ran the command to build manually, it should look like

gcc p1.c -o p1 -lm

The problem is that your makefile doesn't really do anything, and it lives on implicit rules alone. The implicit rules use certain variables in a certain order which do not place the library in the right position in your makefile.

Try instead something like this makefile:

# The C compiler to use.
CC = gcc

# The C compiler flags to use.
# The -g flag is for adding debug information.
# The -Wall flag is to enable more warnings from the compiler
CFLAGS = -g -Wall

# The linker flags to use, none is okay.
LDFLAGS = 

# The libraries to link with.
LDLIBS = -lm

# Define the name of the executable you want to build.
EXEC = p1

# List the object files needed to create the executable above.
OBJECTS = p1.o

# Since this is the first rule, it's also the default rule to make
# when no target is specified. It depends only on the executable
# being built.
all: $(EXEC)

# This rule tells make that the executable being built depends on
# certain object files. This will link using $(LDFLAGS) and $(LDLIBS).
$(EXEC): $(OBJECTS)

# No rule needed for the object files. The implicit rules used
# make together with the variable defined above will make sure
# they are built with the expected flags.

# Target to clean up. Removes the executable and object files.
# This target is not really necessary but is common, and can be
# useful if special handling is needed or there are many targets
# to clean up.
clean:
    -rm -f *.o $(EXEC)

If you run make using the above makefile, the make program should first build the object file p1.o from the source file p1.c. Then is should use the p1.o object file to link the executable p1 together with the standard math library.

It means that es3.c does not define a main function, and you are attempting to create an executable out of it. An executable needs to have an entry point, thereby the linker complains.

To compile only to an object file, use the -c option:

gcc es3.c -c
gcc es3.o main.c -o es3

The above compiles es3.c to an object file, then compiles a file main.c that would contain the main function, and the linker merges es3.o and main.o into an executable called es3.

You have to compile all your .cpp files (sample1.h is included by preprocessor from information in app1.cpp only):

g++ app1.cpp sample1.cpp -o app1

I have figured it out.

The problem was I had not properly set up the project with the proper libraries. I thought I was doing it right manually but I was not. PlatformIO has a really convenient Library manager that helps with set up. You just select a library and it will automatically add it to the project you choose and make sure the correct dependencies are listed in the .ini file. I was trying to manually drop the files I needed into the correct directories, and was not doing the proper adjustments to the .ini file. Thanks for the help @Mat!

-lpthread is an option to specify linking against the posix thread library. If your compilation works without specifying that library, it is probably included by default already, so you don't "miss" anything.

According to this, your original makefile is probably incorrect, that is why you get the error message.