Handle segmentation fault accessing non-existent command line argument - c

I'm making a program in C in linux environment. Now, program runs with arguments which I supply in the command line.
For example:
./programName -a 45 -b 64
I wanted to handle the case when my command line parameters are wrongly supplied. Say, only 'a' and 'b' are valid parameters and character other than that is wrong. I handled this case. But suppose if my command line parameter is like this:
./programName -a 45 -b
It gives segmentation fault(core dumped). I know why it gives because there is no arguments after b. But how can I handle this situation such that when this condition arrives, I can print an error message on screen and exit my program.

As per the main function wiki page:
The parameters argc, argument count, and argv, argument vector, respectively
So you can use your argc parameter to check whether or not you have the right number of arguments. If you don't have 4, handle it and proceed without segfault.

You can, and quite probably should, use getopt() or its GNU brethren getopt_long().
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char **argv)
{
int b = 0;
int a = 0;
int opt;
while ((opt = getopt(argc, argv, "a:b:")) != -1)
{
switch (opt)
{
case 'a':
a = atoi(optarg);
break;
case 'b':
b = atoi(optarg);
break;
default:
fprintf(stderr, "Usage: %s -a num -b num\n", argv[0]);
exit(1);
}
}
if (a == 0 || b == 0)
{
fprintf(stderr, "%s: you did not provide non-zero values for both -a and -b options\n", argv[0]);
exit(1);
}
printf("a = %d, b = %d, sum = %d\n", a, b, a + b);
return(0);
}
You can make the error detection more clever as you wish, not allowing repeats, spotting extra arguments, allowing zeros through, etc. But the key point is that getopt() will outlaw your problematic invocation.
We can't see what went wrong with your code because you didn't show it, but if you go accessing a non-existent argument (like argv[4] when you run ./programName -a 42 -b), then you get core dumps. There are those who write out option parsing code by hand; such code is more vulnerable to such problems than code using getopt() or an equivalent option parsing function.

Related

getopt usage with/without option

I'm writing a simple code making use of *argv[] parameter. I'd like to know whether I can use getopt() function for the following intent.
./myprogram -a PATH
./myprogram PATH
The program can either take merely PATH (e.g. /usr/tmp) or take -a option in addition to PATH. Can getopt() be used for this state? If can, how?
The program can either take merely PATH (e.g. /usr/tmp) or take option in addition to PATH. Can getopt() be used for this state? If can, how?
Certainly. I'm not sure where you even see a potential issue, unless its that you don't appreciate POSIX's and getopt()'s distinction between options and arguments. They are related, but not at all the same thing.
getopt() is fine with the case that no options are in fact specified, and it gives you access to the non-option arguments, such as PATH appears to be for you, regardless of how many options are specified. The usual usage model is to call getopt() in a loop until it returns -1 to indicate that no more options are available from the command line. At each step, the global variable optind variable provides the index of the next argv element to process, and after getopt() (first) returns -1, optind provides the index of the first non-option argument. In your case, that would be where you expect to find PATH.
int main(int argc, char *argv[]) {
const char options[] = "a";
_Bool have_a = 0;
char *the_path;
int opt;
do {
switch(opt = getopt(argc, argv, options)) {
case -1:
the_path = argv[optind];
// NOTE: the_path will now be null if no path was specified,
// and you can recognize the presence of additional,
// unexpected arguments by comparing optind to argc
break;
case 'a':
have_a = 1;
break;
case '?':
// handle invalid option ...
break;
default:
// should not happen ...
assert(0);
break;
}
} while (opt >= 0);
}
Using an optstring of "a" allows an argument of -a to act as a flag.
optind helps detect that only one additional argument is present.
The program can be executed as ./program -a path or ./program path
#include <stdio.h>
#include <unistd.h>
int main(int argc, char **argv)
{
char op = ' ';//default value
int opt;
while ((opt = getopt(argc, argv, "a")) != -1)//optstring allows for -a argument
{
switch (opt)
{
case 'a':
op = 'a';//found option, set op
break;
default:
fprintf(stderr, "%s: unknown option %c\n", argv[0], optopt);
return 1;
}
}
if ( optind + 1 != argc)//one argument allowed besides optstring values
{
fprintf(stderr, "Usage: %s [-a] PATH\n", argv[0]);
return 1;
}
printf("%s %c\n", argv[optind], op);
return 0;
}

clarification on argc and argv

I have some doubts on what are argc and argv, i cant seem to grasp the concept, for what should i use them and how should i use them?
like i have this program that receives from the command line two integers between -100000 and 100000 computes thir addition and prints the result, while performing all needed check about te number of paramenters and their correctness.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
int a, b;
char ch;
if (argc != 4)
{
printf("ERROR - Wrong number of command line parameters.\n");
exit(1);
}
if (sscanf(argv[1], "%d", &a) != 1)
{
printf("ERROR - the first parameter (%s) is not a valid integer.\n",
argv[1]);
exit(2);
}
if (sscanf(argv[2], "%d", &b) != 1)
{
printf("ERROR - the second parameter (%s) is not a valid integer.\n",
argv[2]);
exit(2);
}
ch = argv[3][0];
if (ch == 'a')
printf("The sum result is %d\n", a + b);
else if (ch == 'b')
printf("The subtraction result is %d\n", a - b);
else if (ch == 'c')
printf("The multiplication result is %d\n", a * b);
else if (ch == 'd')
{
if (b != 0)
printf("The division result is %d\n", a / b);
else
printf("ERROR the second value shoulb be different than 0 \n");
}
else
printf("ERROR parameter (%c) does not correspond to a valid value.\n",
ch);
return 0;
}
but how does the program receive from the command line two arguments?? where do i input them?? i am using codeblocks.
argc is the number of parameters passed to your program when it's invoked from command line.
argv is the array of received parameters, and it is an array of strings.
Note that the name of the program is always passed automatically.
Assuming your program executable is test, when you invoke from terminal:
./text 145 643
argc will be 3: program name and the two numbers
argv will be the char* array {"./text","145","643"}
When you write a code, say hello.c, you can run it from the terminal, by going to that directory/folder from the terminal, and compile it using a compiler like gcc.
gcc hello.c -o hello
If you are using Windows, with a compiler like Turbo C or Visual Studio, then it would create a .exe file. This creates an executable file.
When you run the file from command line, you can give command-line arguments as a way of input to the program.
On terminal, you could ./hello arg1 arg2, where arg1 and arg2 are the command line arguments to it. To see how to do this in Windows using a compiler like Turbo C, see this link too.
So what are argc and argv[]?
Your main functions uses main(int argc, char *argv[]), to take the command line arguments.
argc is the number of command line arguments passed. In the above case, that is 3.
argv[] is an array of strings, which in this case are 3 strings. argv[1] will be equal to "arg1" and argv[2] will be equal to "arg2". "./hello" will be in argv[0].
So you give your command line arguments in the command line, be it Linux or Windows. The above explanation was more for Linux. See this and this for command line arguments in Turbo C (I do not recommend Turbo C), and this in case of Visual C.
To know more about command line arguments, you can read this.

How to use command line options in a c command line tool?

I am trying to understand how to use command line options with a command line c tool and I came accross this example.Can some one explain how the code flow works,I am not able to understand it,also I understand that it uses a getopt() function which is inbuilt.
The exe called is rocket_to and it has two command line options, e and a. e option takes 4 as an argument and a option takes Brasalia,Tokyo,London as argument.
Can some one explain how the code works?
This is the actual code:
command line:
rocket_to -e 4 -a Brasalia Tokyo London
code:
#include<unistd.h>
..
while((ch=getopt(argc,argv,"ae:"))!=EOF)
switch(ch){
..
case 'e':
engine_count=optarg;
..
}
argc -=optind;
argv +=optind;
There are many manual pages for getopt() including the POSIX specification. They describe what the getopt() function does. You can also read the POSIX Utility Conventions which describes how arguments are handled by most programs (but there are plenty of exceptions to the rules, usually because of historical, pre-POSIX precedent).
In the example outline code, the -e option takes an argument, and that is the 4 in the example command line. You can tell because of the e: in the third argument to getopt() (the colon following the letter indicates that the option takes an argument). The -a option takes no argument; you can tell because it is not followed by a colon in the third argument to getopt(). The names Brasilia, Tokyo, London are non-option arguments after the option processing is complete. They're the values in argv[0] .. argv[argc-1] after the two compound assignments outside the loop.
The use of EOF is incorrect; getopt() returns -1 when there are no more options for it to process. You don't have to include <stdio.h> to be able to use getopt().
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char **argv)
{
int ch;
int aflag = 0;
char *engine_count = "0";
while ((ch = getopt(argc, argv, "ae:")) != -1)
{
switch (ch)
{
case 'a':
aflag = 1;
break;
case 'e':
engine_count = optarg;
break;
default:
fprintf(stderr, "Usage: %s [-a][-e engine] [name ...]\n", argv[0]);
exit(EXIT_FAILURE);
}
}
argc -= optind;
argv += optind;
printf("A flag = %d\n", aflag);
printf("Engine = %s\n", engine_count);
for (int i = 0; i < argc; i++)
printf("argv[%d] = %s\n", i, argv[i]);
return 0;
}
That is working code which, if compiled to create a program rocket_to, produces:
$ ./rocket_to -e 4 -a Brasilia Tokyo London
A flag = 1
Engine = 4
argv[0] = Brasilia
argv[1] = Tokyo
argv[2] = London
$ ./rocket_to -a -e 4 Brasilia Tokyo London
A flag = 1
Engine = 4
argv[0] = Brasilia
argv[1] = Tokyo
argv[2] = London
$ ./rocket_to -e -a 4 Brasilia Tokyo London
A flag = 0
Engine = -a
argv[0] = 4
argv[1] = Brasilia
argv[2] = Tokyo
argv[3] = London
$
From the getopt man page:
The getopt() function parses the command-line arguments. Its arguments argc and argv are the argument count and array as passed to
the main() function on program invocation. An element of argv that starts with '-' (and is not exactly "-" or "--") is an option element. The characters of this element (aside from the initial '-') are option characters. If getopt() is called repeatedly, it
returns successively each of the option characters from each of the option elements.
The 3rd argument to getopt() are the valid options. If the option is followed by a colon it requires an argument. The argument can be accessed through the optarg variable. So in your example you have two options: 'a' which takes no argument and 'e' which takes an argument.
If getopt() finds an options it returns the character. If all options are parsed it returns -1 and if an unknown option is found it returns -1.
So your code loops through all options and processes them in a switch statement.
Next time when you have trouble understanding something like this try to run man <unknown function> first.

getopt with repeated and optional arguments

For a simple C project of a filesystem in a file, I have to make a command for writing the partitions table. It just contains the number of partitions and the size of them, pretty simple.
It should work like mk_part -s size [-s size ...] [name].
[name] is the filename of the disk, it's optionnal because there is a default one provided.
I don't know much getopt_long (and getopt) but all I read is that I while get options in a while so the two way of processing for me would be :
store all the sizes in an array and then write them in the table.
write size directly during parsing
For the first choice the difficulty is that I don't know the number of partitions. But I still could majorate this number by argc or better by (argc-1)/2 and it would work.
For the second choice I don't know which file to write.
So what is the best alternative to get all those arguments and how can I get this optionnal name ?
getopt can handle both repeated and optional args just fine. For repeated args each invocation of getopt will give you the next arg. getopt doesn't care that it is repeated. For the arg at the end, just need to check for its presence once all the options are parsed. Below is code modified from the example in the getopt man page to handle your scenario:
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
int opt;
while ((opt = getopt(argc, argv, "s:")) != -1) {
switch (opt) {
case 's':
printf("size=%d\n", atoi(optarg));
break;
default: /* '?' */
exit(EXIT_FAILURE);
}
}
if (optind < argc) {
printf("name=%s\n", argv[optind]);
} else {
printf("optional name arg not present\n");
}
exit(EXIT_SUCCESS);
}
And here is some sample runs of the program showing it handling repeated options and the arg at the end.
$ ./a.out -s 10 -s 20 -s 30
size=10
size=20
size=30
optional name arg not present
$ ./a.out -s 1 my_name
size=1
name=my_name
I think you're overthinking this. I know it's always tempting to try to avoid malloc's, but the efficiency of option parsing is never (*) important. You only parse options once, and in the cost of initializing a new process, finding the executable, linking and loading it, and all the rest of the process of starting up a command, the time it takes to parse options is probably not even noise.
So just do it in the simplest way possible. Here's one possible outline:
int main(int argc, char* argv) {
/* These variables describe the options */
int nparts = 0; // Number of partitions
unsigned long* parts = NULL; // Array of partitions (of size nparts)
const char* diskname="/the/default/name"; // Disk's filename
for (;;) {
switch (getopt(argc, argv, "s:")) {
case '?':
/* Print usage message */
exit(1);
case 's':
/* Some error checking missing */
parts = realloc(parts, ++nparts * sizeof *parts);
parts[nparts - 1] = strtoul(optarg, NULL, 0);
continue;
case -1:
break;
}
break;
}
if (optind < argc) diskname = argv[optind++];
if (optind != argc) {
/* print error message */
exit(1);
}
return do_partitions(diskname, parts, nparts);
}
The above code is missing a lot of error checking and other niceties, but it's short and to the point. It simply reallocs the partition array every time a new size is found. (That's probably not as awful as you think it is, because realloc itself is probably clever enough to increase the allocation size exponentially. But even if it were awful, it's not going to happen often enough to even notice.)
The trick with continue and break is a common way of nesting a switch inside a for. In the switch, continue will continue the for loop, while break will break out of the switch; since all the switch actions which do not terminate the for loop continue, whatever follows the switch block is only executed for a switch action which explicitly breaks. So the break following the switch block breaks the for loop in precisely those cases where the switch action did a break.
You might want to check that there was at least one partition size defined before call the function which does the repartitioning.

how to parse command line arguments in C?

I am trying to parse command line arguments in C. Currently, I am using getopt do the parse. I have something like this:
#include <unistd.h>
int main(int argc, char ** argv)
{
while((c=getopt(argc, argv, "abf:")) != -1)
{
switch(c)
{
case 'a':
break;
case 'b':
break;
case 'f':
puts(optarg);
break;
case ':':
puts("oops");
break;
case '?'
puts("wrong command");
break;
}
}
}
then need to use ./a.out -fto run the program, and -f is the command element, but looks like -f must start with a '-', if I do not want the command element starts with '-', i.e, using ./a.out f instead of ./a.out -f, how to achieve it?
if getopt does not support parsing a command line in this way, are there any other library to use in C?
The argc and argv variables give you access to what you're looking for. argc is "argument count" and argv is "argument vector" (array of strings).
getopt is a very useful and powerful tool, but if you must not start with a dash, you can just access the argument array directly:
int main( int argc, char** argv) {
if( argc != 1) { /* problem! */ }
char * argument = argv[1]; // a.out f ... argv[1] will be "f"
}
You could use (on Linux with GNU libc) for parsing program arguments:
getopt with getopt_long; you might skip some arguments using tricks around optind
argp which is quite powerful
and of course you could parse program arguments manually, since they are given thru main(int argc, char**argv) on Linux (with the guarantee that argc>0, that argv[0] is "the program name" -e.g. to find it in your $PATH when it contains no / ..., that argv[argc] is the NULL pointer, and that before that every argv[i] with i<argc and i>0 is a zero-terminated string. See execve(2) for more.
GNU coding standards: command line interfaces document quite clearly some conventions. Please, obey at least the --help and --version conventions!
You might also be concerned by customizing the shell auto-completion facilities. GNU bash has programmable completion. zsh has a sophisticated completion system.
Remember that on Posix and Linux the globbing of command words is done by the shell before starting your program. See glob(7).
The getopt library will stop parsing at the first non-option argument. For command-based programs, this will be at the command name. You can then set optind to the index to start at and run getopt again with the command-specific arguments.
For example:
// general getopts
if (optind >= argc) return 0; // error -- no command
if (strcmp(argv[optind], "command") == 0)
{
++optind; // move over the command name
// 'command'-specific getopts
if (optind >= argc) return 0; // error -- no input
}
This should allow git-like command-line parsing.

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