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.
Related
In Operating Systems Design and Implementation by Andrew S. Tanenbaum and Albert S. Woodhull, there's the following fragment:
"[...] consider exit, which processes should use when they are finished executing. It has one parameter, the exit status (0 to 255), which is returned to the parent via statloc [as in waitpid(int, int *statloc, int)] in the waitpid system call. The low-order byte of status contains the termination status, with 0 being normal termination and the
other values being various error conditions. The high-order byte contains the child's exit status (0
to 255)."
So, what's the difference between the concepts?
The exit status refers to the value that a processes passed to the exit function (or returned from main) on normal program termination.
The termination status refers to what caused the program to end or stop. In other words, whether it exited normally (as mentioned above), or was terminated by a signal. This can also indicate if the processes was stopped (but not terminated) by a signal, or if it continued after being stopped. This can happen when a process is being debugged.
The exit status is what the process itself provided in a call to exit (or the return value from main, which is defined to be the same thing as calling exit).
But that's not the only way a process can be terminated. In particular, it can be terminated by a signal, which is what will happen if the program segfaults, or divides by zero, or if some other process sends it a kill signal. In those cases, there is no exit status because the program was never able to call exit(). The termination status, then, indicates whether the program called exit(), or, if not, the number of the signal which resulted in termination. There are likely to also be some implementation-specific flags; for example, Linux sets a flag in the termination status if the program terminated by a signal created a core dump.
See main waitpid for the macros you can use to extract this information from the status value returned by waitpid.
What is the difference between errno available from <errno.h> and the return code which can be printed using echo $? in bash. Are they referring to the same code?
A process's exit code doesn't usually correspond to an errno code.
The most portable strategy is just to have 1 for failure exits (or even more portably EXIT_FAILURE as defined in stdlib.h) and 0 for success exits.
Some programs follow the bsd sysexits strategy for mapping mores specific exit conditions to exit codes.
Other programs may choose to return errno codes (relevant to the system call failure that ultimately led to a process exit) or e.g., errno+1 , and you'll usually be able to fit that into the 8 bits used by exit, but there's no system-wide-enforced exit code strategy or even an unenforced convention apart from a zero exit code meaning success and a nonzero exit code meaning (some kind of) failure.
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!
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)
I understand that exit(1) indicated an error , for example :
if (something went wrong)
exit(EXIT_FAILURE);
But what's the purpose of using exit(EXIT_SUCCESS); ?
When handling with processes maybe ? e.g. for fork() ?
thanks
This gives the part of the system that invokes the program (usually the command shell) a way to check if the program terminated normally or not.
Edit - start -
By the way, it is possible to query the exit code of an interactive command as well through the use of the $? shell variable. For instance this failed ls command yields an exit code of value 2.
$ ls -3
ls: invalid option -- '3'
Try `ls --help' for more information.
$ echo $?
2
Edit - end -
Imagine a batch file (or shell script) that invokes a series of programs and depending on the outcome of each run may choose some action or the other. This action may consist of a simple message to the user, or the invocation of some other program or set of programs.
This is a way for a program to return a status of its run.
Also, note that zero denotes no problem, any non-zero value indicates a problem.
Programs will often use different non-zero values to pass more information back (other than just non-normal termination). So the non-zero exit value then serves as a more specific error code that can identify a particular problem. This of course depends on the meanings of the code being available (usually/hopefully in the documentation)
For instance, the ls man page has this bit of information at the bottom:
Exit status is 0 if OK, 1 if minor problems, 2 if serious trouble.
For Unix/Linux man pages, look for the section titled EXIT STATUS to get this information.
you can only exit your program from the main function by calling return. To exit the program from anywhere else, you can call exit(EXIT_SUCCESS). For example, when the user clicks an exit button.
It's a system call. There's always good information on system calls if you check the man pages:
http://linux.die.net/man/3/exit
On a Linux box, you can simply type man exit into a terminal and this information will come up.
There are two ways of 'normally' exiting a program: returning from main(), or calling exit(). Normally exit() is used, and thought of, for signalling a failure. However, if you are not in main(), you must still exit somehow. exit(0) is usually used to terminate the process when not in main().
main() is actually not a special function to the operating system, only to the runtime environment. The 'function' that actually gets loaded is normally defined as _start() (this is handled by the linker, and beyond the scope of this answer), written in assembly, which simply prepares the environment and calls main(). Upon return from main(), it also calls exit() with the return value from main().