I have seen similar posts in stackoverflow and other sites but I cannot find solution to my problem.
I have the following consoleout.sh file:
#!/bin/sh
#this way works in c:
#echo "Hello World!"
#but in function does not work:
a(){
echo "Hello World!"
}
Following C code:
#include <stdio.h>
#include <stdlib.h>
int main()
{
system(". consoleout.sh");
system("a");
return 0;
}
Without system("./consoleout.sh"), it works fine.
system() invokes a shell and waits for it to terminate. Another call to system() will create a different shell that starts from scratch.
To run your shell function, you need to do it from the shell where it was defined:
#include <stdio.h>
#include <stdlib.h>
int main()
{
return system( ". consoleout.sh; a" );
}
Each system calls a new instance of the shell, the second one doesn't know anything about the functions defined in the first one. You can, though, call the function in the first shell:
system(". consoleout.sh ; a");
Related
Hey so I was trying to solve a problem for beginners ctf event.
And this is the code that I am trying to run.
#include <stdio.h>
#include <stdlib.h>
int main(){
int (*func)();
func = getenv("MYENV");
func();
return 0;
}
I created a MYENV environment like this :
export MYENV=ls
but on running the code, it throws a segmentation fault (core dumped). I don't understand why.
The func function is basically calling the environment variable whose value is a simple command that I set. Why is it throwing an error.
I'm very new at linux and shell, so I'm sorry if this is too naive.
In C, if you want to run a system command, you have to use the system function (or one of the exec functions but that's more complicated):
#include <stdio.h>
#include <stdlib.h>
int main() {
char* cmd = getenv("MYENV");
system(cmd);
return 0;
}
If you're looking to run arbitrary code, you can inject shell code into it:
export MYENV=$'\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80'
You can learn more here.
I have the following .c file:
/home/eamorr/project1/eamorr.c
I compiles fine and its exe is located at:
/home/eamorr/project1/a.out
Now, I have a php file at:
/home/eamorr/project1/a/b/c/eamorr.php
It needs to call a.out
<?php
$cmd=__DIR__."../../../a.out";
$result=`$cmd`;
?>
Here's the eamorr.c program:
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
int main (int argc, char *argv[]){
setuid (0);
char temp[2048];
char pwd[1024];
realpath(argv[0],pwd);
sprintf(temp,"/bin/bash %s/doMagic.sh",pwd);
system((char *)temp);
return 0;
}
Unfortunately the pwd variable contains the wrong path!!!
/home/eamorr/project1/a.out/doMagic.sh
How do I get rid of the a.out bit from the path? I don't program in C very often and I've been at this for over an hour now...
If I understand correctly, what you would like to get is something like:
/home/eamorr/project1/doMagic.sh
First of all, I dont generally do this kind of path handling in C. However, I had a quick look and it seems that you could use the dirname() functionality. Have a look here http://man7.org/linux/man-pages/man3/basename.3.html. Please be careful with this because I would imagine that these are Linux stuff, not sure how you would do it in DOS.
I wrote little program to print "Hello world" in C. I'm not a C programmer, but I liked to try it. In my program there is an error. Please tell me what is it?
This is my program:
int main(){
printf("Hello World");
}
I wrote this with my Java Experiences. I can't find what is wrong.
You can't directly use printf() function as in Java. You should tell the compiler that you are going to use the input/output stream. You can tell it in this line:
#include <stdio.h>
and also you should enter this line at the end of the source code:
return 0;
this will tell the compiler :
"If the program succeed It will return 0 otherwise It will return any other number"
This means if your program is successful main() function will return 0. Then the compile know the program is Ok.
Then at last your complete code is:
#include <stdio.h>
int main() {
printf("Hello world");
return 0;
}
To compile this and see the word "Hello World", just save this file as a .c file and Open cmd in your program directory and type
gcc hello.c -o hello && hello
(Replace the 'hello.c' with your filename, and 'hello' with the name you want to put with your .exe file)
Remember My computer is Windows. And this compile code is for windows. If your OS is UNIX like OS. then use this code to compile:
gcc hello.c -o hello
./hello
A full hello world program in C:
#include <stdio.h>
int main(void) {
printf("Hello World\n");
return 0;
}
Then compile (assuming gcc) and execute it:
gcc -o test test.c
./test
First, you have to use a header file.
#include <stdio.h>
What that does is bring up a header file with a bunch of commands in them. That will make it recognize the "printf" piece of code.
Next, you have to close your program. By not having a closing statement, the program will not compile because it doesn't know if that is the end of the code. Use this at the end of your program...
return 0;
That closes the program, meaning that the compiler can stop looking for other code. You may also want to look at some programming manuals (there are dozens of free online ones) to learn about the syntax.
One last thing: Most pieces of C code require a semicolon at the end. This is not true for the "int main" statement, nor is it for the header file which I have defined above. The "return" function that closes the program, does however, need a semicolon.
Hoped this helped.
Should also include a pause at the end:
#include <stdio.h>
int main(void) {
printf("Hello World\n");
//Read a character from the console
getchar();
return 0;
}
Just like import in Java programs, in here you have to include libraries you're using in your program. You have used library function printf, but not included stdio.h.
I agree there are many ways to write one of the simplest way is
#include<stdio.h>
int main(void){
printf("Hello World\n");
return 0;
}
You can even use different ways as suggested above.
You should first look at the structure of "main". Try to understand the various parts as already explained so well in the above answers.
"#include" : The preprocessing directives to be included in the program. But why? Because you are trying to use the functions defined inside them.
int : The return type of "main" program. But why? Because the function calling "main" needs to know if the "main" program has functioned correctly.
main : The entry point of your code. Dont ask why here :-)
main( void ) : To tell the compiler that we are not passing any arguments to program "main"
return 0 : Beacuse you promised "main" that you will return something if "main" will function properly.
Finally the code:
#include <stdio.h>
int main( void )
{
printf( "Hello World\n" ) ; //Notice the '\n' here. Good coding practice.
return 0 ;
}
#include <stdio.h> //Pre-processor commands<br/>
void main() //Starting point of the program<br/>{ //Opening Braces
printf("Hello World\n"); //Print Hello World on the screen<br/>
return 0;
} //Ending Braces
Check it once it will work, I have written it with comments:
#include<stdio.h> //Pre-processor commands
void main() {
printf("Hello World\n"); //Print Hello World on the screen
}
A full hello world program in C:
#include <stdio.h>
int main(void) {
printf("Hello World\n");
return 0;
}
Then compile (assuming gcc) and execute it:
gcc -o test test.c
./test
You can't use printf() function as in Java. You have to tell the compiler what you are going to use.
You can tell this as follows:-
#include <stdio.h>
You must enter this line in last:-
return 0;
Then Your complete code is:-
#include <stdio.h>
int main(){
printf("Hello World");
return 0;
}
For compiling this and see the word "Hello World", just save this file as a .c file and Open cmd in your program directory and type:-
gcc hello.c -o hello && hello
(Replace the 'hello.c' with your filename, and 'hello' with the name you want to put with your .exe file)
Remember My computer is Windows. So I can compile only for Windows OS.
#include <stdio.h>
int main() {
// printf, used to print (display) Hello World
printf("Hello World ! ");
// return 0, as the main function is of type int so it must return an integer value
return 0;
}
I have a c program running. I want to make the program sleep for certain period say 5 sec. I want this sleep to be induced from a text file containing command "sleep(5)". I want to pass this file through redirection operator (<) while executing the program
say ./a.out < samplefile.txt
This samplefile.txt contains sleep(5) command in it. I tried the above scenario but the c program is reading it as stream of characters like s,l,e,e,p which is not our intention.
could some one please help me on this issue.
You may make your program read the commands from the text file, parse them and behave as the commands say.
Instead of parsing, you may use the environment variables.
In your program, call to getenv in any place requires configuration. When calling to the program, make sure the environment is set with the required variables, or use this notation:
VAR1=VALUE1 VAR2=VALUE2 ./a.out
I'm not very clear what you want to achieve, but this may be a prompt.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
char buf[512];
scanf("%s", buf);
if (!strcmp(buf, "sleep(5)")) {
printf("sleep...\n");
sleep(5);
}
printf("over\n");
return 0;
}
Today , When i coding, met a question..my Code as follow:
#include<stdlib.h>
void main()
{
system("dir");
getch();
}
The question : The user Screen is nothing..Why ? where is my result?
If you want the output when using system, at least into something you can read in your application, you need to pipe the output:
system("dir > /tmp/output.txt");
FILE *f = fopen("/tmp/output.txt", "r");
char text[1024]; // max sizeof of 1 kb, any more and I'd consider using `malloc()` instead.
fread(text, 1, 1024, f);
printf("%s\n", text);
fclose(f);
There are some problems in your program, at least one of which has already been mentioned.
void main() should be int main(void).
As I recall, the Windows/DOS getch function is declared in <conio.h>; you should have a #include directive for it. Be aware that both <conio.h> and getch are non-standard.
Since main returns int, you should return an int result.
But none of these problems explain the problem you're seeing.
With these changes:
#include <stdlib.h>
#include <conio.h>
int main(void)
{
system("dir");
getch();
return 0;
}
This should work; it should show a directory listing of whatever directory your program runs in (which is determined by TC; I don't know the details).
It's possible that the program is running in an empty directory, which means the dir command wouldn't show any files, but it should still produce some output.
Try commenting out the system() call and adding a printf call (note the added #include <stdio.h>):
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
int main(void)
{
printf("Hello, world\n");
getch();
return 0;
}
This should open a console window, print "Hello, world" in it, and wait for you to type Enter.
If you still don't see any output (either no console window, or a console window with nothing in it), then you have a problem that's not related to the system() call. Most likely the problem has to do with the way you're using Turbo C (I presume that's what "TC" stands for).
The main function in every C program is supposed to return an int you are returning void
Change void to int:
#include<stdlib.h>
int main()
{
system("dir");
getch();
}
When I tested, the dir command ran in my console and printed to standard out.
May be he is the running the program directly in the Turbo C IDE and hence his output is not visible. If he runs the program directly from cmd line it works. I remember you need to run Alt - F5 or some other combination to see the output window in Turbo C++