I have a C program that I am running on my MacOS terminal. All command line tools and GCC compiler have been installed. However for using functions like getpid() or execv() it gives the following error:
execv-test.c:7:35: error: implicit declaration of function 'getpid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
printf("Pid before execv: %d\n", getpid());
^
execv-test.c:8:2: error: implicit declaration of function 'execv' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
execv("print",NULL);
^
2 errors generated.
The code:
#include <stdio.h>
#include <stdlib.h>
int main (void)
{
printf("The game is never over, John. But there may be some new players now.\n");
printf("Pid before execv: %d\n", getpid());
execv("print",NULL);
printf("Returned from execv call.\n");
return 0;
}
The following Stack Overflow exchange suggested that I write helper functions for the ones that were taken as implicit declarations. However, I am not sure you could do the same with getpid() or execv(). What should I do to make sure this doesn't happen?
PLEASE NOTE: "print" is just another helper file that is supposed to be run once execv() is called.
Note that you are using system calls that are defined in the unistd.h header file. Therefore calling them without including the std library #include <unistd.h> amounts to "implicit declaration" = "calling a function without defining it first".
#include<stdio.h>
main()
{
printf("Sleep for 5 milisecond to exit.\n");
sleep(0.005);
printf("usleep for 5 milisecond to exit.\n");
usleep(5000);
return 0;
}
the sleep function takes more time to execute comparatively to the usleep function which takes seconds?
The sleep system call is defined in <unistd.h> on POSIX systems as:
unsigned int sleep(unsigned int seconds);
If you do not include <unistd.h>, the sleep function is not defined and the compiler, using obsolete pre-ansi semantics, infers a prototype of int sleep(double). This has undefined behavior. The actual unsigned int value received by the system could be anything at all, this can cause a long pause as you observe, but it could also crash depending on the system's ABI...
If you include a proper definition by including <unistd.h>, the 0.005 double argument will be implicitly converted to 0 and the program will not pause at all.
Note also that omitting the return type of main() is an obsolete feature. Avoid this old style programming and upgrade your compiler.
It is recommended to enable all warnings to avoid such silly bugs: use the -Wall -Wextra -pedantic options with gcc and clang.
Note also that both sleep and usleep may cause the program to stop for longer than specified in the argument, depending on current system load.
I was doing a research into the contents of another StackOverflow question and I thought it was a good time to brush up my knowledge of unix system calls.
While experimenting with execvp (WITHOUT fork on purpose) I ran into something that confuses me
I wrote 4 test programs
Program 1
#include <stdio.h>
int main() {
//printf("Doge\n");
execvp("ls");
printf("Foo\n");
return 0;
}
The program works as expected, the contents of the directory are printed and the Foo print statement is not
Program 2
However when I uncomment the first print statement and have the program be this
#include <stdio.h>
int main() {
printf("Doge\n");
execvp("ls");
printf("Foo\n");
return 0;
}
execvp returns a -1 and both print statements are issued. why?
Program 3
I vaguely remember having to use unistd.h when experimenting with unix system calls from college.
So I included it, but not execvp has a different signature and it needed some more args than just the name of the program. So I did this
#include <stdio.h>
#include <unistd.h>
int main() {
printf("Doge\n");
char *const parmList[] = {"ls", NULL};
execvp("ls", parmList);
printf("Foo\n");
return 0;
}
And this works. This has confused me. Why did exec work in the first program?
I also used This as a reference to the system calls.
Finally I wrote
Program 4
#include <stdio.h>
//#include <unistd.h>
int main() {
printf("Doge\n");
char *const parmList[] = {"ls", NULL};
execvp("ls", parmList);
printf("Foo\n");
return 0;
}
Which also works as expected.
Can someone explain what's going on?
With this snippet
#include <stdio.h>
int main() {
execvp("ls");
printf("Foo\n");
return 0;
}
you're invoking undefined behaviour. You're not providing the prototype for execvp which requires an argument list (null terminated) as a second parameter.
Using gcc without any warning option silently uses execvp as implicitly declared, and doesn't check parameters. It just calls the function. The function then looks for a second parameter and encounters... whatever is left of the call stack (or registers, depending on call conventions), that's why a previous printf call can change the behaviour.
Using gcc -Wall gives the following warning:
test.c:5:9: warning: implicit declaration of function 'execvp' [-Wimplicit-function-declaration]
execvp("ls");
Including the proper include (#include <unistd.h>) leads to:
test.c:6:9: error: too few arguments to function 'execvp'
execvp("ls");
^~~~~~
That's why you've got strange behaviour. Don't look further. Use execvp with 2 arguments, period. In your case "Program 3" is the way to go, and always set warning level to the maximum, if possible (gcc and clang: -Wall -Wextra -pedantic -Werror)
I'm trying to use the signal function (I know it's deprecated and there are a lot of problems with its portability but I can't use sigaction).
I also need to compile with -ansi and -D_POSIX_C_SOURCE=200112L
If I compile with one of these flags, signal only works one time.
How can I get the same behavior with these flags without using sigaction please ?
#include <signal.h>
#include <stdio.h>
static void signal_handler(int nbr)
{
(void)nbr;
puts("\nHi ! ");
}
int main(void)
{
signal(SIGINT, signal_handler);
puts("Hi ! ");
while (42);
return (0);
}
Be aware, the above code contains an infinite loop.
Thanks :)
Looks like your system has the Unix/System V signal mechanism, which resets the signal action to SIG_DFL after the first signal.
So you have to reinstall the handler in the signal handler itself:
static void signal_handler(int nbr)
{
signal(SIGINT, signal_handler);
(void)nbr;
puts("\nHi ! ");
}
From signal linux man:
* On glibc 2 and later, if the _BSD_SOURCE feature test macro is not
defined, then signal() provides System V semantics. (The default
implicit definition of _BSD_SOURCE is not provided if one invokes
gcc(1) in one of its standard modes (-std=xxx or -ansi) or defines
various other feature test macros such as _POSIX_SOURCE,
_XOPEN_SOURCE, or _SVID_SOURCE; see feature_test_macros(7).)
I tried just at random and compiled with -D_BSD_SOURCE and on Ubuntu it works as intended.
I want to know how and when can I use the exit() function like the program in my book:
#include<stdio.h>
void main()
{
int goals;
printf("enter number of goals scored");
scanf("%d",&goals);
if(goals<=5)
goto sos;
else
{
printf("hehe");
exit( );
}
sos:
printf("to err is human");
}
When I run it, it shows ERROR: call to undefined function exit().
Also, I want to know how I can create an option to close the window in which the program runs? For example, I made a menu-driven program which had several options and one of them was "exit the menu". How can I make this exit the program (i.e. close the window)?
Try using exit(0); instead. The exit function expects an integer parameter. And don't forget to #include <stdlib.h>.
The exit function is declared in the stdlib header, so you need to have
#include <stdlib.h>
at the top of your program to be able to use exit.
Note also that exit takes an integer argument, so you can't call it like exit(), you have to call as exit(0) or exit(42). 0 usually means your program completed successfully, and nonzero values are used as error codes.
There are also predefined macros EXIT_SUCCESS and EXIT_FAILURE, e.g. exit(EXIT_SUCCESS);
exit(int code); is declared in stdlib.h so you need an
#include <stdlib.h>
Also:
- You have no parameter for the exit(), it requires an int so provide one.
- Burn this book, it uses goto which is (for everyone but linux kernel hackers) bad, very, very, VERY bad.
Edit:
Oh, and
void main()
is bad, too, it's:
int main(int argc, char *argv[])
Try man exit.
Oh, and:
#include <stdlib.h>
int main(void) {
/* ... */
if (error_occured) {
return (EXIT_FAILURE);
}
/* ... */
return (EXIT_SUCCESS);
}
The exit() function is a type of function with a return type without an argument. It's defined by the stdlib header file.
You need to use ( exit(0) or exit(EXIT_SUCCESS)) or (exit(non-zero) or exit(EXIT_FAILURE) ).
The following example shows the usage of the exit() function.
#include <stdio.h>
#include <stdlib.h>
int main(void) {
printf("Start of the program....\n");
printf("Exiting the program....\n");
exit(0);
printf("End of the program....\n");
return 0;
}
Output
Start of the program....
Exiting the program....
You must add a line with #include <stdlib.h> to include that header file
and exit must return a value so assign some integer in exit(any_integer).
In addition to return an exit code to parent process -
In UNIX, an important aspect that I think has been left out is, that exit() at first calls (in reverse order) all those functions, which were registered by atexit() call.
Please refer to SUSv4 for details.
on unix like operating systems exit belongs to group of system calls. system calls are special calls which enable user code (your code) to call kernel code. so exit call makes some OS specific clean-up actions before returning control to OS, it terminates the program.
#include <stdlib.h>
// example 1
int main(int argc, char *argv){
exit(EXIT_SUCCESS);
}
// example 2
int main(int argc, char *argv){
return 0;
}
Some compilers will give you the same opcode from both of these examples but some won't. For example opcode from first function will not include any kind of stack positioning opcode which will be included in the second example like for any other function. You could compile both examples and disassemble them and you will see the difference.
You can use exit from any part of your code and be sure that process terminates. Don't forget to include integer parameter.
Write header file #include<process.h> and replace exit(); with exit(0);. This will definitely work in Turbo C; for other compilers I don't know.
Bad programming practice. Using a goto function is a complete no no in C programming.
Also include header file stdlib.h by writing #include <iostream.h>for using exit() function. Also remember that exit() function takes an integer argument . Use exit(0) if the program completed successfully and exit(-1) or exit function with any non zero value as the argument if the program has error.
Include stdlib.h in your header, and then call abort(); in any place you want to exit your program. Like this:
switch(varName)
{
case 1:
blah blah;
case 2:
blah blah;
case 3:
abort();
}
When the user enters the switch accepts this and give it to the case 3 where you call the abort function. It will exit your screen immediately after hitting enter key.
Use process.h instead of stdlib and iostream... It will work 100%.