(.text + 0x20): undefined reference to "main" and undefined reference to a function

Question

My question is how to make makefile work without errors. The first question I have is with an undefined link to the main one. I have the main file my.pro.exe as a function. The second problem is the undefined link to SearchCustomer ().

Mistake:

bash-4.1$ make gcc -Wall -c producer.c shared.h gcc -Wall -c consumer.c shared.h gcc -Wall -c AddRemove.c shared.h gcc -pthread -Wall -o producer.o consumer.o AddRemove.o /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../lib64/crt1.o: In function `_start': (.text+0x20): undefined reference to `main' AddRemove.o: In function `AddRemove': AddRemove.c:(.text+0xb1): undefined reference to `SearchCustomer' AddRemove.c:(.text+0x1e9): undefined reference to `SearchCustomer' AddRemove.c:(.text+0x351): undefined reference to `SearchCustomer' collect2: ld returned 1 exit status make: *** [producer] Error 1 

Makefile:

 COMPILER = gcc CCFLAGS = -Wall all: main debug: make DEBUG=TRUE main: producer.o consumer.o AddRemove.o $(COMPILER) -pthread $(CCFLAGS) -o producer.o consumer.o AddRemove.o producer.o: producer.c shared.h $(COMPILER) $(CCFLAGS) -c producer.c shared.h consumer.o: consumer.c shared.h $(COMPILER) $(CCFLAGS) -c consumer.c shared.h AddRemove.o: AddRemove.c shared.h $(COMPILER) $(CCFLAGS) -c AddRemove.c shared.h ifeq ($(DEBUG), TRUE) CCFLAGS += -g endif clean: rm -f *.o 

Answer 1

This rule is

 main: producer.o consumer.o AddRemove.o $(COMPILER) -pthread $(CCFLAGS) -o producer.o consumer.o AddRemove.o 

not this way. It says to create a file called Manufacturer.o (with -o producer.o ), but you want to create a file called main . I apologize for the scream, but ALWAYS USE $ @ TO PURPOSE THE TASK :

 main: producer.o consumer.o AddRemove.o $(COMPILER) -pthread $(CCFLAGS) -o $@ producer.o consumer.o AddRemove.o 

As Shabaz rightly notes, gmake experts will also use $^ , which applies to all the premises of the rule. As a rule, if you repeat a line or a name, you are doing it wrong and should use a variable, whether built-in or created by you.

 main: producer.o consumer.o AddRemove.o $(COMPILER) -pthread $(CCFLAGS) -o $@ $^ 

Answer 2

This error means that when linking, the compiler cannot find the definition of the main() function anywhere.

In your makefile, the main rule expands to something like this.

 main: producer.o consumer.o AddRemove.o gcc -pthread -Wall -o producer.o consumer.o AddRemove.o 

According to the gcc manual page man page , using the -o switch is described below

File -o Put output in file. This applies regardless of which output is output, whether it is an executable file, an object file, an assembler file, or pre-processed C code. If -o not specified, the default executable is placed in a.out .

This means that gcc will put the output in the file name indicated immediately next to the -o switch. So here, instead of linking all the .o files together and creating a binary [ main , in your case], it creates a binary like producer.o , linking the other .o files. Please fix this.