In my programming book, it shows me exit used without any parameters(exit();).
Unfortunately it does not work.
Some people have said use exit(0); while some say exit(1); exit(2); and exit(3);
What is the difference between them and is there even an exit(4); ?
The funny thing is that my compiler does not need stdlib.h to execute exit(0); and the rest.
void exit( int exit_code );
Here, exit_code is the exit status of the program. After calling this, control is returned to the host environment. If exit_code is EXIT_SUCCESS, an implementation-defined status, indicating successful termination is returned. If exit_code is EXIT_FAILURE, an implementation-defined status, indicating unsuccessful termination is returned. In other cases implementation-defined status value is returned.
Check out here for more info.
P.S.: The reason that your compiler does not need stdlib.h to execute exit(0); maybe either it has been include by other headers that included in your code or, as #devnull mentioned, when building using gcc where exit() is one of the built-in functions.
The funny thing is that my compiler does not need stdlib.h to execute
exit(0);and the rest.
You seem to be using gcc. exit is one of the built-in functions provided by gcc, due to which you do not need the specified header.
The parameter passed to exit() is used to indicate termination status.
Prior to the 1999 version of the ISO C standard, it was legal to call a function with no visible declaration. The compiler would assume that the function exists, creating an implicit declaration. (It would also assume that it returns a result of type int, which exit() does not.) If this implicit declaration doesn't match the actual definition of the function, the behavior is undefined.
As of the 1999 standard, the "implicit int" rule was dropped, and a call without a visible declaration (as provided, in this case, by #include <stdlib.h>) became invalid. Even though it's invalid, a compiler may still issue a non-fatal warning and handle it under the older rules; gcc does this by default.
Under any version of the language, exit requires a single argument of type int. Passing 0 or EXIT_SUCCESS (a macro defined in <stdlib.h> causes the program to terminate and pass a status to the environment indicating success. Passing EXIT_FAILURE causes the program to terminate with a status indicating failure.
The meanings of other argument values are not specified by the C language. You'll commonly see exit(1) to denote failure, but that's not entirely portable.
(exit may be some kind of built-in function in gcc, but that doesn't affect the rules of the language; it's still invalid to call exit with no visible declaration, or to call it without an int argument. If it's built-in, that might affect the level of detail in the diagnostic message.)
exit(0) with a 0 is sent by a process if it has ended correctly. The other numbers are used when the exit has been produced with some kind of error.
At the same time that you allow the parent process to catch exit signals from its child processes with waitpid:
in this way if you make processes end concurrently: while(waitpid(1,&exit_code,0) > 0), where exit_code is an integer that gets the exit code of the finished process
or in this way: waitpid(-1,NULL,0) if you make them finish sequentially
you can get the exit status of child processes if you put this code after waitpid or into waitpid's loop:
pid = waitpid(-1, &exit_code, 0); //getting the finished process' PID
if (WIFEXITED(exit_code)) {
int statcode = WEXITSTATUS(exit_code);
printf(“Process with PID %d has finished with status %d, pid, statcode);
}
try this if you want to make sure that exit() works
Just use the <stdlib.h> header file and things will work!
Related
What does standard say about main return values range?
Say only up to 255?
Because
int main(void){
return 256;
}
echo $? ; # out 0
The standard doesn't say. 0, EXIT_SUCCESS and EXIT_FAILURE have (sort of) specified meanings. Anything else depends on the implementation.
At the present time, most Unix-based systems only support 8-bit return values. Windows supports (at least) a 32-bit return value. I haven't checked whether 64-bit Windows supports a 64-bit return value, but I rather doubt it, since even 64-bit Windows normally still uses a 32-bit int.
As others have stated, the C & C++ Standards don't constrain return values at all other than to state that
main() returns an int (which is of an implementation defined size), and
zero (or EXIT_SUCCESS) is a successful return and EXIT_FAILURE is a non-successful return.
It does specify that a main() that does explicitly not return a value is treated as if it had returned zero.
In this case, the interpretation of the return value is up to the process that waits on the process to complete (by calling wait(), waitpid(), or waitid()). wait() and waitpid() are the older POSIX functions and they specify that only the least significant eight bits of the return value shall be available to a waiting parent process. The POSIX:2008 standard added waitid() as a generalized wait method that has access to the full exit status of a child process.
After forking off a subprocess, the parent process calls one of the wait*() functions to sleep until the forked process is completed (e.g., returns from main(), calls exit() or abort() or something). The wait() and waitpid() functions return the status by way of a pointer to an integer. The caller extracts the actual exit status using the WIFEXITED(status_val) and WEXITSTATUS(status_val) macros. The latter is defined by POSIX and required to return the low-order 8 bits of the status argument. The waitid() function uses a pointer to a siginfo_t structure to return the process's status information. The si_status member contains the full status value as described in Status Information.
Basically, the values of the exit status are in the eye of the beholder. The ANSI/ISO specifications are open-ended. The POSIX suite has multiple ways to wait on a process to finish and fetch it's exit status. POSIX also defines spawn() as a lighter-weight version of exec() which has its own set of constraints on exit status values. Shells have a habit of further restricting result values -- GNU's bash limits the return status to 7 bits and a POSIX-compliant shell limits exit status values to 8 bits. FWIW, most people agree that restricting your return values to be lower than 64 seems to be safe.
Exit codes are a number between 0 and 255 inclusive on Unix like system. You can return anything but in Linux it's modded 256. Take a peek here for a good explanation on Linux return codes. There is also a Wikipedia article on the topic which talks breifly about exit codes for Windows.
The C standard do not impose particular limitation on exit codes, the paragraph about the return value of main delegates to the documentation about the exit() function, which in turn says:
If the value of status is zero or EXIT_SUCCESS, an implementation-defined form of the status successful termination is returned. If the value of status is EXIT_FAILURE, an implementation-defined form of the status unsuccessful termination is returned. Otherwise the status returned is implementation-defined.
which, apart from the EXIT_SUCCESS/EXIT_FAILURE guidelines, basically means "do whatever you want". :)
As said in one comment, the fact that on POSIX systems only the lower 8 bits of the exit code are actually considered is just a UNIXism, deriving from how the wait syscall is designed (the exit status has to be packed in the lower 8 bits of the wait return value), and has nothing to do with the C standard.
A counterexample is Windows, where the whole value passed to exit/return is considered (as long as it's not bigger than a DWORD1, but I don't think they'll ever make int be bigger than a DWORD, it would break a lot of code).
1. Because the GetExitCodeProcess parameter reserved for returning this value is a DWORD *.
On Unix, the wait system call sets a
status value of type int packed as a
bitfield with various types of child
termination information. If the child
terminated by exiting (as determined
by the WIFEXITED macro; the usual
alternative being that it died from an
uncaught signal), SUS specifies that
the lower 8 bits of the status value
contain the exit status; this can
be retrieved using the WEXITSTATUS
macro in wait.h. As such, on Unix
exit statuses are restricted to values
0-255, the range of an unsigned 8-bit
integer.
Unix like systems typically use a
convention of zero for success and non
zero for error. Some conventions
have developed as to the relative
meanings of various error codes; for
example GNU recommend that codes with
the high bit set be reserved for
serious errors, and FreeBSD have
documented an extensive set of
preferred interpretations.
C99 standard defines only 0 and 1. However, allows other values to be used.
See Exit Status wiki for more.
You return type int. You should be able to return any value that can be stored in an int. The exact size of an int is implementation-dependent, so I can't give you an exact range.
5.1.2.2.3 Program termination 1 If the return type of the main function is a
type compatible with int, a return
from the initial call to the main
function is equivalent to calling the
exit function with the value returned
by the main function as its argument;
10) reaching the } that terminates the
main function returns a value of 0. If
the return type is not compatible with
int, the termination status returned
to the host environment is unspecified
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf
ie, there is no requirement for it to return anything. However it strongly states just up from that what the usual definitions are. Almost implying they are they standard, but has a get out of jail free card which means it can be anything.
I have sort of a homework and it asks me to end the program gracefully without explicit termination such as calling exit() or killing the threads.
However I cannot think of any other methods than return 0, so what are the methods to end a program gracefully?
Killing the threads is absolutely not a graceful way to terminate a program. I think what your instructor means is that all your parent threads should wait on their child threads before terminating themselves.
Ideally, an explicit call pthread_exit from the main thread would ensure that all it's children continue running even after it exits. Refer to this link. But, the safest way to wait on your child threads before exiting is to use pthread_join.
Nevertheless, exit(0) is the graceful return for a process as such.
I think you are missing to tell us that you have a multi-threaded program. I suppose that the the idea of gracefully terminating the program is meant to terminate all your threads by setting a flag or something like that. And then only to terminate your main after all your threads have provably ended. The way you actually then terminate your main is of lesser importance.
exit(0) generally indicates that process (your program) terminated gracefully. In case of error it would exit(-1) or some other error code.
See my comment.
main will by default return 0 in the lack of a return 0; statement.
See §5.1.2.2.3¶1 of the C99 standard.
If the return type of the main function is a type compatible with int, a return from the
initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument; reaching the } that terminates the
main function returns a value of 0. If the return type is not compatible with int, the
termination status returned to the host environment is unspecified.
So, the following terminates gracefully although implicitly, distinguished from an explicit exit or return in main.
main() { }
They may be referring to how you handle errors in lower-level routines. Rather than doing something like
printf("ERROR: couldn't initialize the claveman\n");
exit(1);
You would return from that routine (possibly printing the error message at that level or waiting to do it at a higher level like main()).
return CLAVEMAN_INITIALIZE_ERROR;
All your routines would return zero for success or non-zero error codes, up until the code in main was able to return either EXIT_SUCCESS or an error code indicating the failure.
No idea what they could mean with gracefully but my first idea was just a return 0.
Or exit(0)
Can anyone tell me? What is the difference between exit(0) and exit(1) in C language?
What is the difference between exit(0) and exit(1) in C language?
exit(0) indicates successful program termination & it is fully portable, While
exit(1) (usually) indicates unsucessful termination. However, it's usage is non-portable.
Note that the C standard defines EXIT_SUCCESS and EXIT_FAILURE to return termination status from a C program.
0 and EXIT_SUCCESS are the values specified by the standard to indicate successful termination, however, only EXIT_FAILURE is the standard value for returning unsucessful termination. 1 is used for the same in many implementations though.
Reference:
C99 Standard: 7.20.4.3 The exit function
Para 5
Finally, control is returned to the host environment. If the value of status is zero or
EXIT_SUCCESS, an implementation-defined form of the status successful termination is
returned. If the value of status is EXIT_FAILURE , an implementation-defined form
of the status unsuccessful termination is returned. Otherwise the status returned is
implementation-defined.
exit in the C language takes an integer representing an exit status.
Exit Success
Typically, an exit status of 0 is considered a success, or an intentional exit caused by the program's successful execution.
Exit Failure
An exit status of 1 is considered a failure, and most commonly means that the program had to exit for some reason, and was not able to successfully complete everything in the normal program flow.
Here's a GNU Resource talking about Exit Status.
As #Als has stated, two constants should be used in place of 0 and 1.
EXIT_SUCCESS is defined by the standard to be zero.
EXIT_FAILURE is not restricted by the standard to be one, but many systems do implement it as one.
exit(0) indicates that the program terminated without errors. exit(1) indicates that there were an error.
You can use different values other than 1 to differentiate between different kind of errors.
The difference is the value returned to the environment is 0 in the former case and 1 in the latter case:
$ ./prog_with_exit_0
$ echo $?
0
$
and
$ ./prog_with_exit_1
$ echo $?
1
$
Also note that the macros value EXIT_SUCCESS and EXIT_FAILURE used as an argument to exit function are implementation defined but are usually set to respectively 0 and a non-zero number. (POSIX requires EXIT_SUCCESS to be 0). So usually exit(0) means a success and exit(1) a failure.
An exit function call with an argument in main function is equivalent to the statement return with the same argument.
exit is a system call used to finish a running process from which it is called. The parameter to exit is used to inform the parent process about the status of child process. So, exit(0) can be used (and often used) to indicate successful execution of a process and exit(1) to flag an error. reference link
exit(0) is equivalent to exit(EXIT_SUCCESS).
exit(1) is equivalent to exit(EXIT_FAILURE).
On failure normally any positive value get returned to exit the process, that you can find on shell by using $?.
Value more than 128 that is caused the termination by signal. So if any shell command terminated by signal the return status must be (128+signal number).
For example:
If any shell command is terminated by SIGINT then $? will give 130 ( 128+2)
(Here 2 is signal number for SIGINT, check by using kill -l )
exit function. In the C Programming Language, the exit function calls all functions registered with at exit and terminates the program.
exit(1) means program(process) terminate unsuccessfully.
File buffers are flushed, streams are closed, and temporary files are deleted
exit(0) means Program(Process) terminate successfully.
When the executable ends (exits) it returns a value to the shell that ran it. exit(0) usually indicates that all is well, whilst exit(1) indicates that something has gone amiss.
exit() should always be called with an integer value and non-zero values are used as error codes.
See also: Use of exit() function
exit(0) means Program(Process) terminate normally successfully..
exit(1) means program(process) terminate normally unsuccessfully..
If you want to observe this thing you must know signal handling and process management in Unix ...
know about sigaction, watipid()..for()...such....API...........
exit(0) behave like return 0 in main() function, exit(1) behave like return 1. The standard is, that main function return 0, if program ended successfully while non-zero value means that program was terminated with some kind of error.
I want to return a unique status code to a waiting parent process from a child process through exit(), based on the execution of child's code. If execvp fails, then the exit() is used. I assume that if execvp is successful, the command executed will send its status code.
pid=fork();
if(pid==0)
{
if(execvp(cmdName,cmdArgs)==-1)
{
printf("Exec failed!\n");
exit(K); //K?
}
}
waitpid(pid,&status,0);
Suppose the command passed to execvp() is "ls", the man page says that it may return 0(success), 1 or 2(failure).
What safe unique value K can I use to indicate the return status of a child process, which won't clash with any value returned by the command executed by execvp()?
For obvious reasons, there cannot be such a value of K that will never clash with the return status of any other program.
Proof: Suppose there was such a K, and you make your program call itself...
There is no safe unique value as every program chooses its return values of which there are only a limited number.
You have to document your program and say what it returns and also provide some form of log to give more details.
I believe anything above 127 (or negative, if you use signed byte) is reserved for OS (on Unix) for reporting segfaults, parity errors and such (any exit due to signal handler and some other things besides). All other exit codes you can use.
Update: found a link for Linux: http://www.tldp.org/LDP/abs/html/exitcodes.html
if(pid == 0)
{
execvp(cmd, args);
// printf("hello"); // apparently, putting this or not does not work.
_exit(-1);
}
else
{
// parent process work
}
"execvp()" replaces the current program with the to-be-execed program (of course in the same process context). So, putting, say, any printf() calls after execvp() won't work. That is what the docs say, and I have verified it as well.
But then, why is _exit() needed..? Does it so happen that the control DOES return to statements post execvp() ?
I will be grateful for any pointers.
Thanks
The function will return if it has failed.
If one of the exec functions returns to the calling process image, an error has occurred; the return value shall be -1, and errno shall be set to indicate the error.
The _exit() allows terminating the process properly and return an exit code, even if exec fails.
The execve() syscall can fail. The classic reason for doing this would be if the file isn't there or isn't executable. execvp() wraps around execve() to add path searching and default environment handling (virtually always what you want!) and so it adds in another few failure modes, notably trying to run something with a simple name that's not on the user's path. In any case, failure is failure and there's not a lot you can do when it happens except report that it has gone wrong and Get the (now useless) child process Out Of Dodge. (The simplest error reporting method is to print an error message, perhaps with perror(), but there are others.)
The reason why you need _exit() as opposed to the more normal exit() is because you want to quit the child process but you do not want to run any registered cleanup code associated with the parent process. OK, a lot of it might be harmless, but doing things like writing goodbye messages to a socket or something would be bad, and it's often not at all obvious what has been registered with atexit(). Let the parent process worry about its resources; the child basically owns nothing other than its stack frame!
If execvp fails, _exit will be called.
execvp's man page says:
Return Value
If any of the exec() functions returns, an error will have occurred. The return value is -1, and the global variable errno will be set to indicate the error.
One thing to note, you generally don't want a process' exit status to be signed (if portability matters). While exec() is free to return -1 on failure, its returning that so you can handle that failure within the child code.
The actual _exit() status of the child should be 0 - 255, depending on what errno was raised.