GCC Compiler

GCC C Compiler flags

-c

gcc -c compiles source files without linking.

Example

gcc -c myfile.c would generate myfile.o object file.

-D

gcc -D defines a macro to be used by the preprocessor.

Example

#include <stdio.h>
void main()
{
#ifdef DEBUG
printf("Debug run\n");
#else
printf("Release run\n");
#endif
}

Build myfile.c and run it with DEBUG defined:

$ gcc -DDEBUG myfile.c -o myfile
$ ./myfile
Debug run
$

-g

gcc -g generates debug information to be used by GDB debugger.

  • -g0: no debug information
  • -g1: minimal debug information
  • -g : default debug information
  • -g3: maximal debug information

Example

// myfile.c
#include <stdio.h>
void main()
{
printf("Program run!!\n");
}

Build myfile.c on terminal and run gdb to debug:

$ gcc -g myfile.c -o myfile
$ gdb myfile
(gdb) run
Starting program: /home/ubuntu/myfile
Program run!!
Program exited with code 012.
(gdb) quit
$

-I

gcc -I adds include directory of header files.

Example

proj/src/myheader.h:

// myheader.h
#define NUM1 5

myfile.c

// myfile.c
#include <stdio.h>
#include "myheader.h"
void main()
{
int num = NUM1;
printf("num=%d\n", num);
}

Build myfile.c without include directory proj/src:

$ gcc myfile.c -o myfile
myfile.c:3:10: fatal error: myheader.h: No such file or directory
compilation terminated.
$

Build myfile.c with include directory proj/src:

$ gcc -Iproj/src myfile.c -o myfile
$ ./myfile
num=5
$

-L/-l

gcc -l links with a library file.

gcc -L looks in directory for library files.

Example

For static library file libmath.a use -lmath:

$ gcc -static myfile.c -lmath -o myfile

For shared library file libmath.so use -lmath:

$ gcc myfile.c -lmath -o myfile

-O

Set the compiler's optimization level.

  • -O0: optimization for compilation time (default)
  • -O1/-O: optimization for code size and execution time
  • -O2: optimization more for code size and execution time
  • -O3: optimization more for code size and execution time
  • -Os: optimization for code size
  • -Ofast: O3 with fast none accurate math calculations

-shared

Generate shared object file for shared library.

-w

Disable all warning messages.

-Wall

Enables all compiler's warning messages.

References

RapidTables: GCC C Compiler

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