I'm new to programming and currently I'm learning C programming. I'm writing codes on the code blocks and in it using GCC compiler. When I create a new project, (as you know it creates main.c file with it) and due to that I'm not able to compile another file in that project.
File 1:
#include<stdio.h>
int main()
{
int a,b,c,d;
printf("Enter three numbers\n");
scanf("%d%d%d",&a,&b,&c);
d=a;
if(b>d)
d=b;
if(c>d)
d=c;
printf("\n The maximum of three numbers is %d",d);
}
File 2: main.c
#include <stdio.h>
int main()
{
printf("Hello world!\n");
return 0;
}
When I compile the first programme, it shows the following error:
multiple definition of 'main'
first defined here
I've searched every where I could and I'm not able to solve this. In one of the answers here on stack overflow, someone had suggested to write this in
(Project->Build options...->Linker settings (tab))
-Wl,--allow-multiple-definition
When I wrote it, there were no errors. But it wasn't able to run my File 1 and instead, it runs that main.c file. Even when I close the main.c file, it opens there again and runs main.c file which gives the output "Hello World!".
Initially when I was using code blocks there were no such errors. I don't know why this is happening and I've not much knowledge about compilers.
As noted in comments you can only have one main function.
So when you start a new project you need to replace the main.c file with the main.c file you want to use. Or you can edit the 'hello world' main.c program.
When you start a new project in code::blocks you can get a new directory with a simple program that prints 'Hello World'. This file is usually main.c. You need to edit this file or replace it. The reason that code::blocks puts this simple main.c program in the new project is so that you can compile it and test your system without having to write a new program.
Some computer languages allow you to use the same function name for different functions ( which are identified by their parameters and sometimes return types ). That's called overloading. C does not allow this. Functions in C must have unique names.
The main() function is a special one in C as it is used as the standard entry point for applications. That is, the main() function will be called first and your application should start and (typically) end in that function.
As a beginner I would suggest you avoid automated editor features that create and build projects for you. You will miss out on learning how things work doing that. Use an editor to start from empty files and learn how they all connect and how to use the compiler from the command line. The command line is something every beginner should start from, IMO.
It may be harder to learn, but it will give you a much better feel for what is going on.
I guess what you maybe trying to do is have multiple sandbox "gists" that you may wanna run all as their own main function. If that is the case, then just close your project and open the files directly. As long as they are not in a project, they will run fine.
Related
I have a C program that needs to run when I turn on my machine (Red Pitaya).
the beginning of the program presented here:
//my_test program
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include "redpitaya/rp.h"
int main(int argc, char **argv){
int jj=1;
while(1) {
printf("Ready for experiment number %i\n",jj);
int i, D;
int32_t TrigDly;
and so on...
the program is executable with a run.sh file called uri_test.sh, that contains the following:
cat /opt/redpitaya/fpga/fpga_0.94.bit>/dev/xdevcfg
LD_LIBRARY_PATH=/opt/redpitaya/lib ./my_test
both files are located in a directory under /root. the program is working perfectly when run manually on PuTTY terminal-
/RedPitaya/Examples/C/Uri# ./my_test
or
/RedPitaya/Examples/C/Uri# ./uri_test.sh
I tried to follow the solution presented here :
https://askubuntu.com/questions/9853/how-can-i-make-rc-local-run-on-startup
without success.
any suggestions? Thank you.
There are several ways to have a program running at startup, and it depends upon your init subsystem (are you using systemd or a SysV-style init?).
BTW, a source program in C is not a script and you generally compile it (using gcc -Wall -Wextra -g) into some executable. In your case, you probably want to set up its rpath at build time (in particular to avoid the LD_LIBRARY_PATH madness), perhaps by passing something like -Wl,-rpath,/opt/redpitaya/lib to your linking gcc command.
Perhaps a crontab(5) entry with #reboot could be enough.
Whatever way you are starting your program at startup time, it generally is the case that its stdin, stdout, stderr streams are redirected (e.g. to /dev/null, see null(4)) or not available. So it is likely that your printf output go nowhere. You might redirect stdout in your script, and I would recommend using syslog(3) in your C program, and logger(1) in your shell script (then look also into some *.log file under /var/log/). BTW, its environment is not the same as in some interactive shell (see environ(7)...), so your program is probably failing very early (perhaps at dynamic linking time, see ld-linux.so(8), since LD_LIBRARY_PATH might not be set to what you want it to be...).
You should consider handing program arguments in your C program (perhaps with getopt_long(3)) and might perhaps have some option (e.g. --daemonize) which would call daemon(3).
You certainly should read Advanced Linux Programming or something similar.
I recommend to first be able to successfully build then run some "hello-world" like program at startup which uses syslog(3). Later on, you could improve that program to make it work with your Red Pitaya thing.
I'm working on an application which defines it's own printf() to get around differences between the different CRTs out there or because some other platforms don't have it.
When building the application with gcc this automatically seems to work and the custom printf is used instead of libc's one; if I understand it correctly this is because of the order in which object files/libraries appear in the link command or maybe because object files are always searched before CRT libs, correct?
I'd like to do the same using msvc. Just building the project gives the expected 'LNK2005: _printf already defined in printf.obj' because printf is also in msvcrtd.lib. Fair enough. I know about /NODEFAULTLIB but that excludes everything resulting in unresolved references for everything but printf. I scanned through the other linker settings but couldn't find anything which allows this (apart from /FORCE maybe, but the 'might produce an invalid executable' comment doesn't make it sound like a good idea). Also nothing in the module definition file docs; the latter got me thinking it might be possible to create a stub library which has all exports from msvcrt.lib except printf but that seems a brittle solution even if it works.
In the end the question is simple: how do I tell msvc's linker it should skip msvcrt's printf definition and use the one from my printf.obj instead. Basically /NODEFAULTFUNCTION:printf or so. Just an answer for one single executable is ok, though I'd also be interested to know if and how it can be done when building a dll instead where the custom printf is exported: how to tell the linker it should use the export from my .lib instead of msvcrt.lib?
edit simplest repo I could find: create a file main.c:
#include <stdio.h>
int main(int argc, char** argv)
{
printf("Hello");
return 0;
}
and a file printf.c:
int printf(const char *fmt, ...)
{
write(1, "ok\n", 3);
return 3;
}
For VS2013 (though the other versions might work as well): create a new empty C++ project and add both files then build. (For gcc: just gcc main.c printf.c and the resulting a.out prints 'ok')
The culptrit for VS is #include : without that it works ok but I have yet to find out if the original code allows getting rid of it in some way. But even if it does I'd still want to know if this can be solved at the link level.
So first I made a new project by going to File> OS X > Command Line Tool and chose C as the language. This is what I got:
Then I made a new C file called program.c:
Then I pressed run. It didn't build successfully:
However, when I deleted the first default file (main.c) and then pressed run program1.c....it built successfully.
My problem: Why can't I have several files under the same project and run them successfully?
Because you will have the main function declared in both files. I do not know how to tell XCode to build two separate applications..
You can't have 2 main functions
Edit: each c program has to contain only one main function.
Other files of that program can not implement another main function.
I created windows console program project in visual studio, and made two c language files. Both has same source:
#include<stdio.h>
int main() {
printf("hello");
}
When I try to compile, compiler screams about having two main()s. See below:
I can't understand. I thought two files in one project works separately. Am I wrong?
Yes, you are wrong. IIRC, the whole project is compiled and then linked to form a single executable.
In a single executable, there can be only one main() function.
These two main functions are within the same Project. You should have only one main function in your program.
Try to implement a new project, and then add the main function in there.
I wrote this in notepad and then compiled it with lcc-win, using the command lc hello.c
#include <stdio.h>
int main(void)
{
printf("Hello World\n");
return 0;
}
The resulting exe was 100 KB. Seems kind of huge for a program that just prints Hello World. Is this normal? Can I reduce the size? 100 KB isn't really an issue these days but it still seems kind of big for what it does. Wouldn't be too bad if every C code I write comes out as a 100 KB exe though.
1- Everytime you use the include <> tag you do make a link with a c library and load it in your programm.
That is also why it is important to include only in the files that actualy need the library functions.
2- On the other part, the binary that you generate is always full of important informations (cf : libelf or ASM), headers, steps that needs to be here if you want to programm to be run nicely. This does take space to.
This is a really simple question, what happens to the lcc-win is the same with the C Compiler Digital Mars, he do not link the exe with dlls containing the functions printf and etc., functions are linked together with EXE, so no requerindo that your computer has the DLLs.
Look, I created a simple Hello World EXE, and I opened hin in Hex Editor.... the printf function is stored in msvcrt.dll, and, the exe don't have this dll in import section...
And, u can found the source definition in this other picture:
Use this style of function definition is more fast then make a dll call....