I have this super basic C file:
#include <sqlite3.h>
#include <stdio.h>
int main(void) {
printf("%s\n", sqlite3_libversion());
return 0;
}
And in the same directory I have sqlite3.h, sqlite3.c and sqlite3ext.h downloaded from the downloads page on sqlite.org.
I then run gcc -c main.c. Then chmod +x main.o. Then ./main.o. And every time I get:
Killed: 9
What am I doing wrong?
You cannot execute a relocatable object file directly like that. Try this:
gcc main.c -o main -lsqlite3
This works on Ubuntu with libsqlite3-dev package installed. Running main results in:
3.8.2
The -o flag specifies the name of the executable file. If you ommit -o main, you'll get a file called a.out with gcc on most platforms (maybe a.exe on windows+cygwin?). Either way, this file will already be executable, so you can skip the chmod +x.
The -lsqlite3 flag tells the compiler to link in the sqlite3 library too.
If you've built sqlite3 from scratch, you may also need -I and -L flags to tell the compiler where to look for libraries and headers.
In your command, the "-c" flag skips the linking stage and produces a relocatable object, where otherwise, gcc will produce an executable file.
You can use readelf -h main.o using output of your original command and readelf -h main using output of my suggested command, or alternatively just file main.o and file main to see differences in file types.
Related
I use gcc compiled the hello.c:
dele-MBP:temp ldl$ ls
a.out hello.c
now, when I cat a.out:
$ cat a.out
??????? H__PAGEZERO?__TEXT__text__TEXTP1P?__stubs__TEXT??__stub_helper__TEXT???__cstring__TEXT??__unwind_info__TEXT?H??__DATA__nl_symbol_ptr__DATA__la_symbol_ptr__DATH__LINKEDIT ?"? 0 0h ? 8
P?
/usr/lib/dyldס??;K????t22
?*(?P
8??/usr/lib/libSystem.B.dylib&`)h UH??H?? ?E??}?H?u?H?=5??1ɉE??H?? ]Ð?%?L?yAS?%i?h?????Hello
P44?4
there shows the messy code.
I want to know what type of the a.out? is it assembly language? if is why there have so many ??? or %%%?
There are several intermediate file formats, depending on the compiler system you use. Most systems use the following steps, here shown with GCC as example:
Preprocessed C source (gcc -E test.c -o test.i), but this is before compilation, strictly speaking
Assembly source (gcc -S test.c -o test.s)
Object file containing machine code, not executable because calls to external functions are not resolved (gcc -c test.c -o test.o)
Executable file containing machine code (gcc test.c -o test)
Only the first two steps generate text files that you could read by cat or in a text editor. This is BTW a valuable source for insight. However, you can use objdump to see most informations contained in the other formats. Please read its documentation.
Each step does also all steps before it. So (gcc test.c -o test) generates assembly source and object file in temporary files that are removed automatically. You can watch that process by giving GCC the option -v.
Use gcc --help to see some entry points for further investigations.
There is at lot more to say about this process but it would fill a book.
I've watched tons of videos about how to use sublime text 3 I do what they say but it doesn't work. When i type "subl test.c" in terminal it opens up a a file called test.c in sublime text 3 when i use the command gcc -c test.c everything is fine too, but when I try to run the code using ./test it says bash: ./test: No such file or directory
Bash says that there is no such file or directory because you haven't created a file called 'test'. You should specify an output filename, i.e. you should type gcc test.c -o your_out_filename. Then you may run your program using ./your_out_filename. Without -o flag gcc will create a a.out by default, so your out_filename will be a.out.
You have to use the following command to create a file called test:
gcc test.c -o test
If you don't use the -o option (gcc test.c) your created file will be a.out.
The option -c of gcc only compiles your file and doesn't link it to a program which you can run. The result of the -c option is only an object file called test.o.
Therefore the easiest way is the one I have mentionend above (-o option).
You have to run:
gcc -o output test.c
output is the file you have to do ./output in terminal for it to execute
Suppose there are 2 c program named abc.c and xyz.c . Now we want to work with the 2 executables at a time. So we change the name of the ./a.out using
gcc -g abc.c -o abc
gcc -g xyz.c -o xyz
Even gcc -o abc abc.c works.
What does the -g and -o in the above commands specify or describe?
What is the significance of -g and -o in the command for renaming ./a.out file.
Thanks in advance.
-g means to leave debugging information in the output file, it's unrelated to renaming.
-o means to put the result in the specified file instead of the default filename (abc.o for object files, a.out for linked executable files).
From https://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html:
-g
Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information.
-o file
Place output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.
-g starts becoming useful once you use debuggers such as gdb and lldb. When you attach to a running program and advancing one line at a time printing/altering the state as it changes.
if we specify -g option while compiling, debugging symbols will be available in the output file which will be useful when you try to debug using GDB.
If we won't specify -o option, the output will be placed in default a.out file. So if we run
gcc a.c - output will be in a.out
gcc b.c - output is a.out which is replacing old a.out file
If you want the output not to be a.out file, you can give -o option while compiling
gcc abc.c -o a
-o and -g options are not related.
I'm trying to compile my first "Hello World" application using GCC (build-on clang and stand-alone GCC-4.9.2) without any success:
OS version is OS X Yosemite 10.10.2.
I'm using the following code (main.c):
#include <stdlib.h>
#include <stdio.h>
int main (){
printf("Hello World!");
getchar();
return 0;
}
Then I compile it in terminal with a command (clang shipped with XCode):
gcc -o hello -c main.c
As a result I got following error whe running compiled hello file:
MBP-Andrii:cTest andrii$ gcc -o hello -c main.c
MBP-Andrii:cTest andrii$ hello
-bash: /Users/andrii/avrTest/cTest/hello: Permission denied
If I change the permissions for hello file to 777 and a new error again:
MBP-Andrii:cTest andrii$ chmod 777 hello
MBP-Andrii:cTest andrii$ hello
Killed: 9
MBP-Andrii:cTest andrii$
The same thing happens with stand-alone GCC-4.9.2.
I guess It might be something related to the output binary format or some missing flags for compiler.
Remove the -c from the command you're using to compile the application.
The -c tells the compiler to only compile and assemble but not link. You are not actually creating an application when using -c.
See the GCC Manual:
-c
Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix ‘.c’, ‘.i’, ‘.s’, etc., with ‘.o’.
Unrecognized input files, not requiring compilation or assembly, are ignored.
Try executing it by typing ./hello
I have GCC running on my Ubuntu operating system. I wrote a small program in C and tried compiling it. Its output was an a.out file like it would do on Windows. How can I make it put out a Linux executable?
a.out is the executable (assuming you've done full compilation rather than just generation of object files but that's the most likely case). To run it, use (from a shell):
./a.out
If you want to give it a different name, simply rename it, or better:
gcc -o actualname myprog.c
to get an executable called actualname which is then run (of course) with:
./actualname
See the following transcript:
pax> cat testprog.c
#include <stdio.h>
int main (void) { printf("Hi.\n\n"); return 0; }
pax> gcc testprog.c ; ./a.out
Hi.
pax> gcc -o xyzzy testprog.c ; ./xyzzy
Hi.
Suppose your C file is f.c.
gcc f.c gives the a.out executable, and you can run it in a terminal as ./a.out.
gcc f.c -o myprog gives myprog as the executable, and you can run it in a terminal as ./myprog.
It is a Linux executable. a.out files (actual a.out format, not files named a.out by default) cannot be executed on Windows.
In both cases, most likely you get a standard executable usable in the local system, but named a.out. On Linux it's an ELF file.