Input Parsing in C & deciding where to forward - c

Question

So I have the following project i need to work on for school. It involves a server/client communicating where the client sends requests to the server to get certain information. The server gets the req, parses it, and then sends a response based on the type of the request. For example:
GET /apple/fn/tom/ln/sawyer/a/25/id/1234 : This is a request to get the info for the following person who works at the Apple company (/apple):
fn (first name): Tom
ln (last name): Sawyer
a (age): 25
id (ID): 1234
Now the server should accept input, parse it, and return the information requested from its own database. implementing the server/client is not an issue. I need to know the best way to implement the algorithm to deal with input since not all requests would look like the one above. Other examples of requests:
GET /apple/i: This should return info about Apple company (i.e. address, phone number)
GET /apple/e: return number of employees in Apple company
GET /apple/e/id/1234: return info of employee in Apple company with the following id=1234 (which in our example would be Tom Sawyer) i.e. return first name, last name, age, address.
GET /apple/fn/tom/ln/sawyer/a/25/id/1234 : discussed above
SET /apple/fn/tom/ln/sawyer/a/25/id/5678 : update id of this employee to 5678
SET /apple/fn/tom/ln/sawyer/a/23 : update age of this employee to 23
...
I will have to implement different structs for req/response (i.e. a struct for req and another for response) as well as a different function for each of the different request/responses. BUT what is the best way to deal with parsing input && decide which function to send it to? I was told to look into using a parser-generator like Bison but to my understanding this would only help parse the input and break it up into pieces which is not that hard for me since I know I always have the "/" between fields so I can use the function:
strtok( input, "/" );
So main issue I have is how to decide where to send each request. So Assuming I have the following functions:
struct GetEmployeeInfoReq
{
char *fn;
char *ln;
int age;
};
struct GetEmployeeInfoResp
{
int house_num;
int street_num;
char *street_name;
char * postal_code;
int years_worked_here;
};
void GetEmployeeInfo( struct GetEmployeeInfoResp *resp, struct GetEmployeeInfoReq *req );
struct GetCompanyInfoReq
{
...
}
struct GetCompanyInfoResp
{
...
}
void GetCompanyInfo( struct GetCompanyInfoResp *resp, struct GetCompanyInfoReq *req );
Now I know that to call the first function I need the following request:
GET /apple/fn/tom/ln/sawyer/a/25/id/1234
and to call 2nd function I need the following:
GET /apple/i
My question is how to get this done? Off the top of my mind I'm thinking defining a variable for each possible field in the input req and using that so if this is my request:
GET /apple/e/id/1234
then I would have the following values defined and set to true:
bool is_apple = true;
bool is_employee = true;
bool is_id = true;
After I know that this request is:
GET /apple/e/id/<id_num>
AND NOT
GET /apple/e
so i can send it to the proper function.
Is this the correct approach as I'm lost on how to tackle this issue.
Thanks,

Answer 1

Get yourself a large piece of paper and make a diagram about the logic to get a grammar (not actually neede here, you could just parse it, but being a school assignment I assume that it is meant to be build up upon).
Some observations
every input starts with a /
every entry ends with a / except the last one
entries consist of characters and/or digits
empty entries gets you in trouble if you insist on using strtok (thanks to wildplasser, I missed that)
order matters?
Entries are
either key (one entry) or key/value (two consecutive entries).
In other words: the may or may not have an argument
Entries with a meaning
first entry is the company name (what do you do if that's all? Check assignment)
next entry is either
i print company info
e
without arguments: print #employees
with argument: print information about argument, argument must be correct
fn first name as a key, must have a value because of strtok, argument must be a string
ln last name as a key, must have a value because of strtok, argument must be a string
id id as a key, must have a value because of strtok, argument must be a string of digits (an integer so to say but it is still a string at that point) only
a age as a key, must have a value because of strtok, argument must be a string of digits (an integer so to say but it is still a string at that point) only
Examples:
/apple/e
/ start of input
apple company name. Must have an argument, so go on
e something with employers, my or may not have an argument so check.
EOI (end of input) that means that e has no arguments, so print the number of employees.
/apple/id/134
/ start of input
apple company name. Must have an argument, so go on
e something with employers, my or may not have an argument so check.
id is a key and must have a value, that means we need an argument, so check
1234 all digits which is correct value for id
EOI (end of input) no other things, so print the information you have about id-1234
/apple/fn/tom/ln/sawyer/a/25/id/1234
/ start of input
apple company name. Must have an argument, so go on
fn is a key, must have a value, check
tom the value for fn must be a string which it is, safe
ln is a key, must have a value, check
sawyer the value for ln must be a string which it is, safe
a is a key, must have a value, check
25 the value for ln must be a string of digits which it is, safe
id is a key, must have a value, check
1234 the value for ln must be a string of digits which it is, safe
EOI (end of input). The key for the database entry seems to be the name fn and ln, so read the entry tomsawyer and check if any of id or a is different and change accordingly. If nothing is different: check your assignment.
It might be good idea to build a struct with all of the informations and fill it while parsing (where I wrote "safe"). You need two main functions printIt(keys) and changeIt(key, value). Some things can be done immediately, like in my first example, some need further ado.
That's it, should be straightforward to implement.
EDIT a short example
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// ALL CHECKS OMMITTED!
#define DELIMITER '/'
// should resemble a row from the DB
typedef struct company {
char *comp_name;
unsigned int num_empl;
unsigned int empl_id;
unsigned int empl_age;
char *first_name;
char *last_name;
} company;
int main(int argc, char **argv)
{
company *entries;
char *input, *token;
size_t ilen;
if (argc < 2) {
fprintf(stderr, "Usage: %s stringtoparse \n", argv[0]);
exit(EXIT_FAILURE);
}
// work on copy
ilen = strlen(argv[1]);
input = malloc(ilen + 1);
strcpy(input, argv[1]);
entries = malloc(sizeof(company));
// skip first delimiter
if (*input == DELIMITER) {
input++;
ilen--;
}
token = strtok(input, "/");
if (token == NULL) {
fprintf(stderr, "Usage : %s stringtoparse \n", argv[0]);
exit(EXIT_FAILURE);
}
// first entry is the company name
entries->comp_name = malloc(strlen(token));
strcpy(entries->comp_name, token);
// mark empty entries as empty
entries->first_name = NULL;
entries->last_name = NULL;
entries->empl_age = -1;
entries->empl_id = -1;
// F(23)
entries->num_empl = 28657;
// only very small part of grammar implemented for simplicity
for (;;) {
token = strtok(NULL, "/");
if (token == NULL) {
break;
}
// "e" [ "/" "id" "/" number <<EOF>> ]
if (strcmp(token, "e") == 0) {
token = strtok(NULL, "/");
if (token == NULL) {
puts("Info about number of employees wanted\n");
// pure info, pull from DB (not impl.) and stop
break;
} else {
if (strcmp(token, "id") != 0) {
fprintf(stderr, "Only \"id\" allowed after \"e\" \n");
// free all heap memory here
exit(EXIT_FAILURE);
}
token = strtok(NULL, "/");
if (token == NULL) {
fprintf(stderr, "ERROR: \"id\" needs a number \n");
// free all heap memory here
exit(EXIT_FAILURE);
}
// does not check if it really is a number, use strtol() in prod.
entries->empl_id = atoi(token);
printf("Info about employee with id %d wanted\n", entries->empl_id);
// pure info, pull from DB (not impl.) and stop
break;
}
}
// "a" "/" number
else if (strcmp(token, "a") == 0) {
token = strtok(NULL, "/");
if (token == NULL) {
fprintf(stderr, "ERROR: \"a\" needs a number \n");
// free all heap memory here
exit(EXIT_FAILURE);
}
// does not check if it actually is a number, use strtol() in prod.
entries->empl_age = atoi(token);
printf("Age given: %d\n", entries->empl_age);
}
// "id" "/" number
else if (strcmp(token, "id") == 0) {
token = strtok(NULL, "/");
if (token == NULL) {
fprintf(stderr, "ERROR: \"id\" needs a number \n");
// free all heap memory here
exit(EXIT_FAILURE);
}
// does not check if it actually is a number, use strtol() in prod.
entries->empl_id = atoi(token);
printf("ID given: %d\n", entries->empl_id);
}
// "fn" "/" string
else if (strcmp(token, "fn") == 0) {
token = strtok(NULL, "/");
if (token == NULL) {
fprintf(stderr, "ERROR: \"fn\" needs a string \n");
// free all heap memory here
exit(EXIT_FAILURE);
}
entries->first_name = malloc(strlen(token));
strcpy(entries->first_name, token);
printf("first name given: %s\n", entries->first_name);
}
// "ln" "/" string
else if (strcmp(token, "ln") == 0) {
token = strtok(NULL, "/");
if (token == NULL) {
fprintf(stderr, "ERROR: \"ln\" needs a string \n");
// free all heap memory here
exit(EXIT_FAILURE);
}
entries->last_name = malloc(strlen(token));
strcpy(entries->last_name, token);
printf("last name given: %s\n", entries->last_name);
} else {
fprintf(stderr, "ERROR: Unknown token \"%s\" \n", token);
// free all heap memory here
exit(EXIT_FAILURE);
}
}
printf("\n\nEntries:\nCompany name: %s\nFirst name: %s\nLast name: %s\n",
entries->comp_name, entries->first_name, entries->last_name);
printf("Age: %d\nID: %d\nNumber of employees: %d\n",
entries->empl_age, entries->empl_id, entries->num_empl);
/*
* At this state you have information about what is given (in "entries")
* and what is wanted.
*
* Connect to the DB.
*
* If firstnamelastname is the DB-id and in the DB, you can check if
* the given ID is the same as the one in the DB and change if not.
*
* You can do the same for age.
*
* If firstnamelastname is not in the DB but ID is given check if the
* ID is in the DB, change firstname and/or lastname if necessary and
* congratulate on the wedding (many other reasons are possible, please
* check first or it might get really embarassing)
*/
// free all heap memory here
/* Disconnect from the DB */
exit(EXIT_SUCCESS);
}
Compiled with:
gcc -g3 -std=c11 -W -Wall -pedantic jjadams.c -o jjadams
try with
./jjadams "/Apple/fn/Tom/ln/Sawyer/a/10/id/3628800"
./jjadams "/Apple/e"
./jjadams "/Apple/e/id/1234"
./jjadams "/Apple/e/1234"