table of contents next: global

Video

Youtube: libcurl video tutorial: setup

setup a makefile that builds our code

Using Linux and gcc, we create a Makefile and a first C program embryo called mycurlapp.c.

When created, we can invoke make to build our program mycurlapp and we can invoke it from the command line.

We can reuse this Makefile in several future episodes easily as we will keep naming our source code mycurlapp.c.

curl-config

The Makefile uses the curl-config tool, which typically gets installed with curl. It is used to extract information about the curl build.

Makefile

This is not really a make school but here’s how the basic stuff in the file works:

CC = `curl-config --cc`

This sets the CC variable to hold the contents of what curl-config --cc outputs. That’s the name of the compiler that was used to build libcurl. Most commonly this is plain gcc. We could possibly make it CC = gcc too.

LIBS = `curl-config --libs`

This sets the LIBS variable to hold the contens of what curl-config --libs outputs. That’s a list of linker options necessary to link with libcurl on this machine. Most commonly, this is -lcurl. We could also probably get away with simply doing LIBS = -lcurl.

mycurlapp: mycurlapp.o

This is the first rule of the makefile and thus the “default” one that will be checked for a plain make invoke.

The output called mycurlapp depends on the files on the right side of the colon. There’s only one file there: mycurlapp.o. If the .o file is newer than mycurlapp, then the following command will be executed:

$(CC) -g -o $@ $< $(LIBS)

The CC variable (that is the compiler) is used and a command line is created with a few options like -g and -o. $@ is an automatic variable that holds the name from the left side of the colon in the line above (mycurlapp), $< holds the names on the ride side of the colon in the line above (mycurlapp.o) and $LIBS is often just -lcurl.

In a typical machine, the command line that runs is thus something like: gcc -g -o mycurlapp mycurlapp.o -lcurl.

mycurlapp.o: mycurlapp.c

The object file depends on the C file. If the object file is missing or if the C file is newer than the object file, run the following command:

$(CC) -g -c $<

Again, $< holds the names on the ride side of the colon in the line above, which one is mycurlapp.c which makes this run something like: cc -g -c mycurlapp.c. The -c flag to gcc makes it produce the object file from the C file.

clean:
    rm -f mycurlapp.o mycurlapp

Here’s a special target that lets us do make clean and then this command line is run. Handy when we want to wipe previous build results and start over, by force.

mycurlapp.c

The point in this episode is only that the code uses libcurl functions so we need to make sure the build succeeds. We don’t care very much what exactly the code does. Move on to simplest for that.