Develop with GCC on Ubuntu¶

This tutorial shows how to build and debug C programs on Ubuntu using GCC and GDB. For instructions on installing GCC and related tooling (including IDEs, debuggers, and the build system), see the dedicated guide on How to set up a development environment for GCC on Ubuntu. This article assumes that the tooling suggested in that article has been installed.

The GNU Compiler Collection (GCC) provides support for several programming languages. Working with the GCC toolchain in Ubuntu is straightforward, so this tutorial is limited to showing a basic ‘Hello, world!’ program in the C language and an introductory debugging session.

Writing a sample C program¶

Create a project directory:

mkdir -p ~/c-projects/hello-world
cd ~/c-projects/hello-world

Write a ‘Hello, world!’ program that includes setting a variable. Create a hello.c file with the following content:

hello.c¶

#include <stdio.h>

int main() {
    int number = 42;
    printf("Hello, world!\nThe answer to everything is %d.\n", number);
    return 0;
}

Build the source code using the gcc compiler, specifying hello as the file name of the resulting executable:
```
gcc hello.c -o hello
```
This command incorporates the intermediate steps of preprocessing, compiling, and linking to generate an executable in one go. Use the -v option to display detailed information on how gcc invokes the individual tools that handle the intermediate steps.

Note

gcc can compile source code in many programming languages, including C, C++, and Assembly. The compiler guesses the language based on the file extension (for example, .c, .cpp, or .s). You can explicitly specify the language using the -x option as well.
Run the program executable:
dev@ubuntu:~/projects/hello-world$
./hello
```
Hello, world!
The answer to everything is 42.
```

Fixing simple bugs¶

The gcc compiler provides helpful output when encountering a problematic part of the source code.

Modify the ‘Hello, world!’ program source to omit an expected argument. Note the missing number argument on line 5:

hello.c¶

#include <stdio.h>

int main() {
    int number = 42;
    printf("Hello, world!\nThe answer to everything is %d.\n");
    return 0;
}

Build the source:

$ gcc hello.c -o hello
hello.c: In function ‘main’:
hello.c:5:57: warning: format ‘%d’ expects a matching ‘int’ argument [-Wformat=]
    5 |     printf("Hello, world!\nThe answer to everything is %d.\n");
      |                                                        ~^
      |                                                         |
      |                                                         int

gcc displays a warning but still compiles the source code.

Run the program to see how it behaves without the integer argument specified:
dev@ubuntu:~/projects/hello-world$
./hello
```
Hello, world!
The answer to everything is -1350680728.
```

Using the Make build system¶

Compiling manually by invoking gcc directly can help understand the build process. It is also adequate for simple projects like ‘Hello, world!’. A build system simplifies and automates the process.

See below for a simple example of using the GNU Make build system to build the ‘Hello, world!’ program from Writing a sample C program.

Create a basic Makefile (a file named Makefile in the project directory) with the following contents:
```
## Variables:

# Compiler
CC = gcc

# Target executable
TARGET = hello

# Source files
SRCS = hello.c

# Object files
OBJS = $(SRCS:.c=.o)

## Targets:

# Default target
all: $(TARGET)

# Linking
$(TARGET): $(OBJS)
	$(CC) -o $@ $^

# Compiling
%.o: %.c
	$(CC) -c $< -o $@

# Cleaning
clean:
	rm -f $(OBJS) $(TARGET)

.PHONY: all clean
```
The Makefile divides the build process into separate compilation and linking steps. It also uses “automatic variables” that allow for writing the compilation rules without specifying absolute file names. For an overview, see Automatic Variables in the GNU Make manual.

Important

Makefile rules must start with the tab character. See Rule Syntax.
Compile the program using make:
dev@ubuntu:~/projects/hello-world$
make
```
gcc -c hello.c -o hello.o
gcc -o hello hello.o
```
Checking the resulting files would show:
dev@ubuntu:~/projects/hello-world$
ls -1
```
hello
hello.c
hello.o
Makefile
```
Run the program executable:
dev@ubuntu:~/projects/hello-world$
./hello
```
Hello, world!
The answer to everything is 42.
```

Debugging with GDB¶

GDB is the standard debugger accompanying GNU GCC. While you can use the debugging capabilities of your preferred IDE, GDB offers adequate debugging support for the command line.

See the example below for a quick introduction to GDB use with the ‘Hello, world!’ program from Writing a sample C program.

Recompile the program with the -g option, which produces debugging information specifically for GDB:
```
gcc -g hello.c -o hello
```

Run the debugger:

$ gdb hello

#[snip]

Reading symbols from hello...
(gdb) start
Temporary breakpoint 1 at 0x1155: file hello.c, line 4.
Starting program: /home/rkratky/c-projects/hello-world/hello 

[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Temporary breakpoint 1, main () at hello.c:4
4     int number = 42;
(gdb) print number
$1 = 32767
(gdb) next
5     printf("Hello, world!\nThe answer to everything is %d.\n", number);
(gdb) print number
$2 = 42
(gdb)

In the above example:

Line 6: the hello program is started.
Line 15: number is queried before being assigned.
Line 17: program execution is stepped forward.
Line 19: number is queried again after being assigned.

Press Ctrl+X+A to switch to the text user interface (TUI) of GDB for a more interactive experience:

┌─hello.c───────────────────────────────────────────────────────────┐
│        1 #include <stdio.h>                                       │
│        2                                                          │
│        3 int main() {                                             │
│        4     int number = 42;                                     │
│ >     5     printf("Hello, world!\nThe answer to everything is %d│
│        6     return 0;                                            │
│        7 }                                                        │
│        8                                                          │
│                                                                   │
│                                                                   │
│                                                                   │
└───────────────────────────────────────────────────────────────────┘
Thread 0x7ffff7f897 (src) In: main          L5    PC: 0x55555555515c
(gdb) print number
$1 = 32767
(gdb) next
(gdb) print number
$2 = 42
(gdb)
Type help at any point to see available commands.
Press Ctrl+D to quit GDB.