Include an external library in C

Question

I'm attempting to use a C library for an opencourseware course from Harvard. The instructor's instructions for setting up the external lib can be found here.

I am following the instructions specific to ubuntu as I am trying to use this lib on my ubuntu box. I followed the instructions on the page to set it up, but when I run a simple helloWorld.c program using a cs50 library function, gcc doesn't want to play along.

Example:

helloWorld.c

#include <stdio.h>
#include <cs50.h>

int
main(void){
        printf("What do you want to say to the world?\n");
        string message = GetString();
        printf("%s!\n\n", message);
}

$ gcc helloWorld.c

/tmp/ccYilBgA.o: In function `main':
helloWorld.c:(.text+0x16): undefined reference to `GetString'
collect2: ld returned 1 exit status

I followed the instructions to the letter as stated in the instructions, but they didn't work for me. I'm runing ubuntu 12.04. Please let me know if I can clarify further my problem.

Duncandunce

THe header file inclusion just marks the prototype of the function. Hence your file compiles fine but there is a link error. Have you installed the libraries as suggested? If so, could you check if the libraries are properly installed and the path variable is properly set so that gcc can find these libraries using the path? – Fervency

Does gcc helloWorld.c -lcs50 work? – Standstill

yep, didn't realize i had to explicitly link to the lib at the cmd line. Thanks to all. – Duncandunce

How else is it going to know that you want to link with that library? – Pontonier

I was thinking #include <cs50.h> would work just like #include <stdio.h> . – Duncandunce

Answer 1

First, as a beginner, you should always ask GCC to compile with all warnings and debugging information enabled, i.e. gcc -Wall -g. But at some time read How to invoke gcc. Use a good source code editor (such as GNU emacs or vim or gedit, etc...) to edit your C source code, but be able to compile your program on the command line (so don't always use a sophisticated IDE hiding important compilation details from you).

Then you are probably missing some Harvard specific library, some options like -L followed by a library directory, then -l glued to the library name. So you might need gcc -Wall -g -lcs50 (replace cs50 by the appropriate name) and you might need some -Lsome-dir

Notice that the order of program arguments to gcc is significant. As a general rule, if a depends upon b you should put a before b; more specifically I suggest

1. Start with the gcc program name; add the C standard level eg -std=c99 if wanted
2. Put compiler warning, debugging (or optimizing) options, eg -Wall -g (you may even want to add -Wextra to get even more warnings).
3. Put the preprocessor's defines and include directory e.g. -DONE=1 and -Imy-include-dir/
4. Put your C source file hello.c
5. Put any object files with which you are linking i.e. bar.o
6. Put the library directories -Lmy-lib-dir/ if relevant
7. Pur the library names -laa and -lbb (when the libaa.so depends upon libbb.so, in that order)
8. End with -o your-program-name to give the name of the produced binary. Don't use the default name a.out

Directory giving options -I (for preprocessor includes) and -L for libraries can be given several times, order is significant (search order).

Very quickly you'll want to use build automation tools like GNU make (perhaps with the help of remake on Linux)

Learn also to use the debugger gdb.

Get the habit to always ask for warnings from the compiler, and always improve your program till you get no warnings: the compiler is your friend, it is helping you!

Read also How to debug small programs and the famous SICP (which teaches very important concepts; you might want to use guile on Linux while reading it, see http://norvig.com/21-days.html for more). Be also aware of tools like valgrind

Have fun.

Indulgence

Answer 2

I take this course and sometimes I need to practice offline while I am traveling or commuting. Under Windows using MinGW and Notepad++ as an IDE (because I love it and use it usually while codding python) I finally found a solution and some time to write it down.

Starting from scratch. Steps for setting up gcc C compiler, if already set please skip to 5

1. Download Git and install. It includes Git Bash, which is MINGW64 linux terminal. I prefer to use Git as I need linux tools such as sed, awk, pull, push on my Windows and can replace Guthub's terminal.
2. Once Git installed make sure that gcc packages are installed. You can use my configuration for reference...

3. Make sure your compiler works. Throw it this simple code,

   • by saving it in your working directory Documents/Harvard_CS50/Week2/ hello.c

   #include <stdio.h>
   
   int main(void)
   {
    printf("Hello StackOverflow\n");
   }
   

   • start Git Bash -> navigate to working directory

   cd Documents/Harvard_CS50/Week2/

   • compile it in bash terminal

   gcc helloworld.c -o helloworld.exe

   • execute it using bash terminal

   ./helloworld.exe

   Hello StackOverflow

   4. If you see Hello StackOverflow, your compiler works and you can write C code.

Now to the important bit, installing CS50 library locally and using it offline. This should be applicable for any other libraries introduced later in the course.

5. Download latest source code file cs50.c and header file cs50.h from https://github.com/cs50/libcs50/tree/develop/src and save them in Documents/Harvard_CS50/src

6. Navigate into src directory and list the files to make sure you are on the right location using

   ls

   cs50.c cs50.h

7. Cool, we are here. Now we need to compile object file for the library using

   gcc -c -ggdb -std=c99 cs50.c -o cs50.o

8. Now using the generated cs50.o object file we can create our cs50 library archive file.

   ar rcs libcs50.a cs50.o

9. After all this steps we ended with 2 additional files to our original files. We are interested in only 2 of them cs50.h libcs50.a

   ls

   cs50.c cs50.h cs50.o libcs50.a

10. Copy Library and header files to their target locations. My MinGW is installed in C:\ so I copy them there

    cs50.h --> C:\MinGW\include

    libcs50.a --> C:\MinGW\lib

Testing the cs50 Library

To make sure our library works, we can throw one of the example scripts in the lecture and see if we can compile it using cs50.h header file for the get_string() method.

#include <stdio.h>
#include <cs50.h>

int main(void) 
{
    printf("Please input a string to count how long it is: ");
    string s = get_string();
    int n = 0;
    while (s[n] != '\0')
        {
            n++;
        }
    printf("Your string is %i chars long\n", n); 
}

11. Compile cs50 code using gcc and cs50 library. I want to be explicit and use:

    gcc -ggdb -std=c99 -Wall -Werror test.c -lcs50 -o test.exe

    But you can simply point the source, output filename and cs50 library

    gcc test.c -o test.exe -lcs50

Here we go, program is compiled using header and methods can be used within.

If you want Notepad++ as an IDE you can follow this tip to set it up with gcc as a compiler and run your code from there. Just make sure your nppexec script includes the cs50 library

npp_save
gcc -ggdb -std=c99 -Wall -Werror "$(FULL_CURRENT_PATH)" -lcs50 -o "$(CURRENT_DIRECTORY)\$(NAME_PART).exe"
cmd /c "$(CURRENT_DIRECTORY)\$(NAME_PART).exe"

Biogen

Perfect answer for Windows users! Thanks for the thorough explanation w/ images. I would like to add that for step 10, the folder location you are looking for should contain the stdio.h file. For me, it was: C:\Program Files\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\x86_64-w64-mingw32 that contained the include and lib folders I needed. Also, thanks to faizan-rizvi, who provided a further clue in his answer (option 2, step 2). – Libation

Thanks so much, I was running MinGW through chocolatey so there were a couple of different lib and include folders, I just stuck them in both and ran the compile command and it worked like a charm! Thanks again! – Spangle

Just one further comment for clarification. I'm guessing you need to keep the parent src folder and the grandparent folder? Just trying to piece together what this process actually did and organise it a bit better in my brain.. – Spangle

Answer 3

You need to link against the library during compilation. The library should end in .a or .so if you are on Ubuntu. To link against a library:

gcc -o myProgram myProgram.c -l(library name goes here but no parentheses)

Conciliatory

Answer 4

1. Download the cs50 from: http://mirror.cs50.net/library50/c/library50-c-5.zip
2. Extract it. (You will get two files cs50.c and cs50.h)
3. Now copy both the files to your default library folder. (which includes your stdio.h file)
4. Now while writing your program use: #include < cs50.c >
5. You can also copy the files to the folder containing your helloWorld.c file.
6. You have to use: #include " cs50.c ".

OR =====================================================================>

1. Open cs50.c and cs50.h files in text editor.
2. In cs50.h, just below #include < stdlib.h > add #include < stdio.h > and #include < string.h > both on new line.
3. Now open cs50.c file, copy everything (from: /**Reads a line of text from standard input and returns the equivalent {from line 47 to last}) and paste it in cs50.h just above the #endif and save the files.
4. Now you can copy the file cs50.h to either your default library folder or to your current working folder.
5. If you copied the file to default folder then use: #include < cs50.h > and if you copied the files to current working folder then use: #include " cs50.h ".

Haemophiliac

Answer 5

You have to link against the library, how come GCC would know what library you want to use?

gcc helloWorld.c -lcs50

Obnubilate

Answer 6

Research Sources:

• building on the answers above given by Basile Starynkevitch, and Gunay Anach
• combined with instructions from some videos on youtube ^{1 2}

Approach:

• covering the minimum things to do, and sharing the "norms" separately
• avoiding any modification to anywhere else on the system
• including the basic breakdown of the commands used
• not including all the fine details, covering only the requirements absolute to task or for effective communication of instructions. leaving the other mundane details to the reader
• assuming that the other stuff like compiler, environment variable etc is already setup, and familiarity with shell's file navigation commands is there

My Environment:

• compiler: gcc via msys2
• shell: bash via msys2
• IDE: doesnt matter here

Plan:

1. getting the source files
2. building the required files: *.o (object) and *.a (archive)
3. telling the compiler to use it

Action:

0. Let's say, current directory = "desktop/cs50"
   It contains all the *.c files like test-file.c which I will be creating for assignments/problem sets/practise etc.

1. Get the *.h and *.c files
   Source in this particular case: https://github.com/cs50/libcs50/tree/main/src

   1. Go over each file individually
   2. Copy all the content of it
      Say using "Copy raw contents" icon of individual files
   3. Create the corresponding file locally in the computer
      Do it in a a separate folder just to keep things clean, let's say in "desktop/cs50/src" aka ./src

2. Build the required files using in the terminal after changing your current directory to "desktop/cs50/src" :

   1. gcc -c cs50.c to create the "cs50.o" object file from "cs50.c" using "gcc"
   2. ar cr libcs50.a cs50.o to create "libcs50.a" archive file which'll be containing "cs50.o" object file
      
   • Here, "libcs50" = "lib" prefix + "cs50" name (same as the header file's name)
   • This is the norm/standard way where the prefix "lib" is significant as well for a later step
   • However, prefix can be skipped, and it's not compulsory for name to match the header file's name either. Though, Skipping prefix is not recommended. And I can't say for sure about the name part

3. To tell the compiler to be able to use this infrastructure, the commands will be in following syntax after going to the parent directory (i.e. to "desktop/cs50"):

   • gcc test-file.c -Isrc -Lsrc -lcs50 if you used "lib" prefix in step 2.2 above
   • here, -I flag is for specifying the directory of *.h header file included in your test_file.c
   • and -L flag is for specifying the directory to be used for -l
   • and -l is for the name of the *.a file. Here the "lib" prefix talked about earlier, and ".a" extension is not mentioned
   • the order of these flags matter, keep the -I -L -l flags after the "test-file.c"

Some more notees:

• don't forget to use the additional common flags (like those suggested above for errors etc)
• if you skipped the "lib" prefix, then you can't use -L -l flags
  so, syntax for command will become: gcc test-file.c -Isrc src/libcs50.a
• say i created my test-file.c file in "desktop/cs50/psets", so, it can be handled in 2 notable ways (current dir = "desktop/cs50/") :
  • cd psets then changing the relative address correspondingly in -I -L, so result:
    gcc test-file.c -I../src -L../src -lcs50
  • keeping current directory same, but then changing the file's relative address correspondingly, so result:
    gcc psests/test-file.c -Isrc -Lsrc -lcs50
  • or use absolute addresses 😜
• as it can be seen that this becomes quite long, that's when build automation tools such as make kick in (though i am accomplishing that using a shell script 😜)

Colatitude