What does wildcard mean in makefile?

Question

I found the following lines in a makefile tutorial, but I have some problem with the bold lines.

In 1 line, if I write

program_C_SRCS:=$(*.c)

it does not work. So please tell me what is wildcard word in doing here. Is this word is specific to the makefile only?

In tutorial it is written that second line will perform the test substitution. Can anyone tell me something about this text substitution?

Please excuse me if my questions are very basic because I am new to make filestuff.

link of tutorial

CC:=g++
program_NAME:=myprogram
**program_C_SRCS:=$(wildcard *.c)**  # 1 line 
program_CXX_SRCS:=$(wildcard *.cc)
**program_C_OBJ:=$(program_C_SRCS:.c=.o)** # 2 line
program_CXX_OBJ:=$(program_CXX_SRCS:.c=.o)
program_OBJ:= $(program_C_OBJ) $(program_CXX_OBJ)

Answer 1

Suppose you have two source files. foo.c and bar.c.

program_C_SRCS:=$(wildcard *.c) # 1 line

The wildcard function is Make syntax. The variable program_C_SRCS will now have the value foo.c bar.c (maybe not in that order).

program_C_OBJ:=$(program_C_SRCS:.c=.o) # 2 line

This is a substitution reference. It transforms text, replacing one substring with another. The variable program_C_OBJ now has the value foo.o bar.o.

Answer 2

The use of wildcard card function in make file is to list all the source files with a particular extension. For example:

program_C_SRCS:=$(*.c) // In this the variable program_C_SRCS will have all the files with ".c" extension.

Suppose if you want to convert .c files to .o files then the following syntax may be useful:

program_C_OBJS:=$(patsubst %.c,%.o,$(wildcard *.c))