Parse command line arguments in Lex - c

Suppose I want my Lex and Yacc program to parse the command line arguments like:
./a.out show memory
I want lex to parse the string "show memory". How do I accomplish this?

You'll need to concatenate all the arguments into a big string, by inserting whitespace between them. Then feed the remaining text buffer to Lex/Yacc, by re-defining the YY_INPUT macro so it reads input from your text buffer.
The start could be something like:
#include <stdio.h>
#include <string.h>
char *argbuf;
size_t arglen;
int main(int argc, char *argv[])
{
int i;
// Compute total length of all arguments, with a single space between.
arglen = 0;
for(i = 1; argv[i] != NULL; i++)
arglen += 1 + strlen(argv[i]);
// Allocate buffer space.
argbuf = malloc(arglen);
if(argbuf == NULL)
{
fprintf(stderr, "No memory for argument buffer, aborting");
exit(1);
}
// Concatenate all arguments. This is inefficient, but simple.
argbuf[0] = 0;
for(i = 1; argv[i] != NULL; i++)
{
if(i > 1)
strcat(argbuf, " ");
strcat(argbuf, argv);
}
// Here we should be ready to call yyparse(), if we had implemented YY_INPUT().
return 0;
}

int main(int argc, char **argv) {
if(argc > 1) {
if(argv[1])
yy_scan_string(argv[1]);
}
yyparse();
return 0;
}

What's wrong with doing it the old fashioned way?:
if(argc > 1 && !strcmp(argv[1],"show"))
{
if(argc > 2)
{
if(!strcmp(argv[2],"memory"))
...
else if(!strcmp(argv[2],"cpu"))
...
else ...
}
}
Besides, getopt() and friends are more appropriate.

My blog article Parsing command line parameters with Yacc & Flex explains this with a working example. There is no need to concatenate the argument string. The reason is given in the article.
The blurb is:
Every once in a while someone comes along and asks how to parse
command line parameters with Yacc & Flex. This is rather straight
forward, but requires some knowledge of the generated code to get
right.
Here we present a source template that does this. The user only has to
edit the grammar and scanning rules. Some knowledge of C, Yacc and
Flex is assumed.
The code is WTFPL licensed
The template is written for Berkeley Yacc and the reflex variant of
Flex. It may be made to work with GNU Bison and SourceForge Flex,
possibly with a few changes.
What you get is a template where you can just insert your lexical and grammar specification.
Please ask questions about the using and adapting the template itself to the blog comments.

Related

How do you generally scan/parse commands in interactive REPL programs in C, is there a standard way?

I am writing an interactive REPL program in c.
Some examples of commands (lines starting with >) I would like to handle are:
$ ./my_program // run the program
> add user
id: 123 // this is the output of above command
> update user 123 name "somename"
> remove user 123
> quit
So basically the command is a line with multiple strings.
This is how I am trying to handle the commands.
scan the whole line
parse the command and get a corresponding int value unique to command
do whatever needs to be done for the command
#include <stdio.h>
int parse_cmd(const char *buffer)
{
// parse command
}
int main(int argc, const char **argv)
{
// init code
char buffer[100];
int cmd;
while (1) {
printf("> ");
scanf("%[^\n]%*c", buffer);
cmd = parse_cmd(buffer);
if (cmd < 0) {
printf("error: invalid command\n");
continue;
}
switch (cmd) {
// handle commands
}
}
// deinit code
}
There are a lot of cli programs I have seen that take command inputs in similar way.
I wonder if there is a general way of writing cli programs?
I can write code to parse the commands, just wanted to know the standard approach for such situations?
While there's no real standard way, quite a lot of opensource console tools with an interactive mode use the GNU readline library (https://tiswww.case.edu/php/chet/readline/rltop.html).
It's actually quite easy to use, even simpler than implementing everything 100% correctly by yourself.
Your example rebased on readline:
int main(int argc, const char **argv)
{
// init code
int cmd;
char* line;
while (1) {
line = readline("> ");
if (line) {
cmd = parse_cmd(line);
switch (cmd) {
// handle commands
default:
printf("error: invalid command\n");
}
free(line);
} else {
break;
}
}
// deinit code
}
This isn't any more complex than your example, but you immediately gain:
command line editing at the interactive prompt, with correct handling of each and every possible terminal
correct handling of EOF (important if stdin is redirected)
unlimited input line size
And it's not very hard to add a command history, with arrow-up and down to repeat previous lines, incremental search, optionally persisted to a file, et et.
There's not really a standard way to do it. This is not a 100% fair comparison, but your question is kind of like if there is a standard way to construct a compiler, because you are in fact constructing a language, although a very simple one.
But one reasonably common way that works fairly well for simple programs is this approach. Let's assume that we have two commands add and del. Create a function for both these commands. First we search for one of the strings "add " or "del ". Notice the spaces. Put a pointer on the next character and call the corresponding function with the rest of the line as argument and allow them to determine things.
Here is some pseudo:
parse(bufferptr)
word = getFirstWord(bufferptr)
ptr = bufferptr + strlen(word)
if word == "add"
return add(ptr)
else if word == "del"
return del(ptr)
return -1
add(bufferptr)
word = getFirstWord(bufferptr)
if userExist(word)
return -1
else
return addUser(word)
del(bufferptr)
word = getFirstWord(bufferptr)
if not userExist(word)
return -1
else
return delUser(word)
buffer = input()
int res = parse(buffer)

Read input.txt file and also output.bmp file from terminal (C-programming)

I have to do an assignment where I have to write a C-Programm, where it gets the input-file-name from the console as command line parameter.
It should move the data from the input.txt file (the input file has the information for the bmp file - color etc.) to the generated output.png file. The 20 20 parameters stand for width and height for the output.png image.
So the console-request for example (tested on Linux) will look like this:
./main input.txt output.bmp 20 20
I know that this code reads an input.txt File and puts it on the screen.
FILE *input;
int ch;
input = fopen("input.txt","r");
ch = fgetc(input);
while(!feof(input)) {
putchar(ch);
ch = fgetc(input);
}
fclose(input);
And this would (for example) write it to the output.png file.
FILE *output;
int i;
output = fopen("ass2_everyinformationin.bmp", "wb+");
for( i = 0; i < 55; i++)
{
fputc(rectangle_bmp[i], output);
}
fclose(output);
But this code works only, if I hard-code the name directly in the code, not by using a command line parameters.
I don't have any clue, how to implement that and I also didn't find any helpful information in the internet, maybe someone can help me.
Greetings
The full prototype for a standard main() is
int main(int argc, char* argv[]);
You get an int with the number of arguments, argc and
a list of "strings" (as far as they exist in C), argv.
You can for example use
#include "stdio.h"
int main(int argc, char* argv[])
{
printf("Number: %d\n", argc);
printf("0: %s\n", argv[0]);
if (1<argc)
{
printf("1: %s\n", argv[1]);
}
}
to start playing with the arguments.
Note that this is intentionally not implementing anything but a basic example of using command line parameters. This matches an accpeted StackOverflow policy of providing help with assignments, without going anywhere near solving them.

built in command to change prompt in custom linux/unix shell

I have looked all over google and I find how to change in the bash config files, but my project requires a built in command to change the prompt.
I declared char pointer outside any function, my command modifies it, but when the function returns (int to continue a do while loop) and the prompt is displayed again, it is blank.
I have tried using a structure, union, and even a second char pointer and got the same issue.
I thought using a global char pointer that could be accessed and modified in any function would be the solution to this part of my project.
I would appreciate and will try any response.
Edit:
posted on my phone, tried to ask w/o code, but here it is
Code:
char *prmpt;
...
int main(int argc, char **argv)
prmpt="$$ ";
do
{
printf("%s ", prmpt);
}while(1)
int cmd_prompt(char **args)
{
prmpt = (char*)args[1];
return 1;
}
Essentially one needs to use fgets or getline or better yet they might use readline or editline. Here is an example using getline:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char *prompt;
size_t len = 256;
size_t i = 0;
if (!(prompt = malloc(256 * sizeof(char))))
return 1;
strcpy(prompt, "$");
while ( 1 )
{
printf("[[%s]] ", prompt);
i = getline(&prompt, &len, stdin);
prompt[i-1] = '\0';
}
}
Or if you might try implementing getline on your own using getchar ( perhaps if getline is not available on your system, or you just want to know how it works). And eventually move on to writing your own editline library if you continue to be interested in writing shells.

Forking with command line arguments

I am building a Linux Shell, and my current headache is passing command line arguments to forked/exec'ed programs and system functions.
Currently all input is tokenized on spaces and new lines, in a global variable char * parsed_arguments. For example, the input dir /usa/folderb would be tokenized as:
parsed_arguments[0] = dir
parsed_arguments[1] = /usa/folderb
parsed_arguments tokenizes everything perfectly; My issue now is that i wish to only take a subset of parsed_arguments, which excludes the command/ first argument/path to executable to run in the shell, and store them in a new array, called passed_arguments.
so in the previous example dir /usa/folderb
parsed_arguments[0] = dir
parsed_arguments[1] = /usa/folderb
passed_arguments[0] = /usa/folderb
passed_arguments[1] = etc....
Currently I am not having any luck with this so I'm hoping someone could help me with this. Here is some code of what I have working so far:
How I'm trying to copy arguments:
void command_Line()
{
int i = 1;
for(i;parsed_arguments[i]!=NULL;i++)
printf("%s",parsed_arguments[i]);
}
Function to read commands:
void readCommand(char newcommand[]){
printf("readCommand: %s\n", newcommand);
//parsed_arguments = (char* malloc(MAX_ARGS));
// strcpy(newcommand,inputstring);
parsed = parsed_arguments;
*parsed++ = strtok(newcommand,SEPARATORS); // tokenize input
while ((*parsed++ = strtok(NULL,SEPARATORS)))
//printf("test1\n"); // last entry will be NULL
//passed_arguments=parsed_arguments[1];
if(parsed[0]){
char *initial_command =parsed[0];
parsed= parsed_arguments;
while (*parsed) fprintf(stdout,"%s\n ",*parsed++);
// free (parsed);
// free(parsed_arguments);
}//end of if
command_Line();
}//end of ReadCommand
Forking function:
else if(strstr(parsed_arguments[0],"./")!=NULL)
{
int pid;
switch(pid=fork()){
case -1:
printf("Fork error, aborting\n");
abort();
case 0:
execv(parsed_arguments[0],passed_arguments);
}
}
This is what my shell currently outputs. The first time I run it, it outputs something close to what I want, but every subsequent call breaks the program. In addition, each additional call appends the parsed arguments to the output.
This is what the original shell produces. Again it's close to what I want, but not quite. I want to omit the command (i.e. "./testline").
Your testline program is a sensible one to have in your toolbox; I have a similar program that I call al (for Argument List) that prints its arguments, one per line. It doesn't print argv[0] though (I know it is called al). You can easily arrange for your testline to skip argv[0] too. Note that Unix convention is that argv[0] is the name of the program; you should not try to change that (you'll be fighting against the entire system).
#include <stdio.h>
int main(int argc, char **argv)
{
while (*++argv != 0)
puts(*argv);
return 0;
}
Your function command_line() is also reasonable except that it relies unnecessarily on global variables. Think of global variables as a nasty smell (H2S, for example); avoid them when you can. It should be more like:
void command_Line(char *argv[])
{
for (int i = 1; argv[i] != NULL; i++)
printf("<<%s>>\n", argv[i]);
}
If you're stuck with C89, you'll need to declare int i; outside the loop and use just for (i = 1; ...) in the loop control. Note that the printing here separates each argument on a line on its own, and encloses it in marker characters (<< and >> — change to suit your whims and prejudices). It would be fine to skip the newline in the loop (maybe use a space instead), and then add a newline after the loop (putchar('\n');). This makes a better, more nearly general purpose debug routine. (When I write a 'dump' function, I usually use void dump_argv(FILE *fp, const char *tag, char *argv[]) so that I can print to standard error or standard output, and include a tag string to identify where the dump is written.)
Unfortunately, given the fragmentary nature of your readCommand() function, it is not possible to coherently critique it. The commented out lines are enough to elicit concern, but without the actual code you're running, we can't guess what problems or mistakes you're making. As shown, it is equivalent to:
void readCommand(char newcommand[])
{
printf("readCommand: %s\n", newcommand);
parsed = parsed_arguments;
*parsed++ = strtok(newcommand, SEPARATORS);
while ((*parsed++ = strtok(NULL, SEPARATORS)) != 0)
{
if (parsed[0])
{
char *initial_command = parsed[0];
parsed = parsed_arguments;
while (*parsed)
fprintf(stdout, "%s\n ", *parsed++);
}
}
command_Line();
}
The variables parsed and parsed_arguments are both globals and the variable initial_command is set but not used (aka 'pointless'). The if (parsed[0]) test is not safe; you incremented the pointer in the previous line, so it is pointing at indeterminate memory.
Superficially, judging from the screen shots, you are not resetting the parsed_arguments[] and/or passed_arguments[] arrays correctly on the second use; it might be an index that is not being set to zero. Without knowing how the data is allocated, it is hard to know what you might be doing wrong.
I recommend closing this question, going back to your system and producing a minimal SSCCE. It should be under about 100 lines; it need not do the execv() (or fork()), but should print the commands to be executed using a variant of the command_Line() function above. If this answer prevents you deleting (closing) this question, then edit it with your SSCCE code, and notify me with a comment to this answer so I get to see you've done that.

How would I scan phrases from the command line?

I am currently trying to get my program to work the way I want. I am currently at the point where I can open up any text file from the command line (an unlimited amount) and display them. I'd like to expand on this and have users enter phrases that format the displayed text. I have previously posted a similar question and I've gotten some great answers, but I cannot use getopt(). How else would it be possible to scan the command line for a "-w" and a number, so "-w5" and a "-s" with no number. Those are the only two things I'd like to be able to detect. I don't mind if statements, I was hoping for the shortest program in my friends, but at this point, I'd just like to get it done. Any ideas? Multiple if-statements was my friend's idea, I personally think this is unneeded, but if that's what I have to do... If anyone else has any ideas, that would be really useful. I just want my program to detect those two characters from the command line. I'm fairly new to C (I've only made a few programs), but I'm edger to learn and I have tried googling and trying this on my own, but being new to C, trying to find what I need through all the other text and jargon is difficult.
Anything will be useful, thanks.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
int l = 1;
while(l != argc)
{
FILE *fp;
fp = fopen(argv[l], "rb");
l++;
if (fp != NULL)
{
int i = 1;
do
{
i = fgetc(fp);
printf("%c",i);
printf(" ");
}
while(i!=-1);
fclose(fp);
}
else
{
printf("Error.\n");
}
}
}
void scanningForWS(int argc, char **argv)
{
}
You should look at plan9's ARGBEGIN and ARGEND macros in their libc.h file (at the very end of the file), to see how it's done (for an example of its usage, see arg(3)).
Alernatively, you can check the suckless implementation of this mechanism, which is very nice (I have re-implemented a version of it which parses arguments even after incorrect flags have been found, but it's not published anywhere. I can publish it if you need that).
The command line arguments are in argv, and since argv is an array, the only way to find a specific element inside of it is to iterate through, checking each element until you get the one you want. If you don't want to write all that yourself, it looks like C has a method called 'lfind' in search.h that does this. Here is an example of how to use it. Hope that helps :3.
Also, the GNU documentation for it

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