Suggestion for parsing values from a particular format - c

Question

I'm working on a client-server program, where initial step involves parsing the request from client on the server side. The input would look like this.
INSERT A->B B->C; QUERY A B; RESET;
So there are three different commands and they are separated by ';'. RESET option has no parameters. INSERT might have any number of parameter(which are space separated and each value separated by "->"). QUERY is again space separated. The server has to build a acyclic graph based on the input. So my problem is to parse this string into subsequent requests. I planned on using 'strtok' and when the final value is reached(for example 'A'), create a linked list of INSERTS(since the number of request is unknown). But my code is too big and I'm looking for a more concise solution for this problem.
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
typedef struct insert {
char event1;
char event2;
struct insert * next;
}insert,*insPtr;
typedef struct query {
char event1;
char event2;
struct query * next;
}query,*queryPtr;
typedef struct reset {
int status;
}reset,*rPtr;
void create_node(char *events) {
char event[2];
char *a,*str;
char *pch = strtok(events,"->");
while(pch != NULL) {
printf("%s\n",pch);
pch = strtok(NULL,"->");
}
}
int insert_parser(char *string) {
char *a, *b;
a = string;
b = "INSERT ";
while(*a == *b){
a++;
b++;
}
char *pch = strtok(a," ");
while(pch){
printf("%s\n",pch);
pch = strtok(NULL," ");
}
return(0);
}
int parse_for_values (char* command) {
int value;
if (strstr(command, "INSERT") !=NULL ) {
printf("%s\n",command);
printf ("Insert command found\n");
value = 1;
} else if(strstr(command, "QUERY") !=NULL) {
printf("%s\n",command);
printf ("Query command found\n");
value = 2;
} else if(strstr(command, "RESET") != NULL) {
printf ("Reset command found\n");
printf("%s\n",command);
value = 3;
} else {
printf("unknown command:%s:\n",command);
printf("Unknown command\n");
return(1);
}
switch(value) {
case 1:
insert_parser(command);
break;
case 2:
break;
case 3:
break;
}
return(0);
}
int parse_for_command(char *input) {
char *ptr;
input = strtok(input,"\n");// this strtok is to remove the trailing '\n' in the string
ptr = strtok(input,";");
// printf("%s\n",ptr);
char *cmdPtr;
while(ptr != NULL) {
cmdPtr = ptr;
parse_for_values(cmdPtr);
// printf("%s",ptr);
ptr = strtok(NULL, ";"); //NULL as first argument tells strtok to work on internally held value
}
return(0);
}
int main() {
char input[100];
printf("Enter the input\n");
fgets(input, 100, stdin);
//printf("%s",input);
char *inp = input;
inp = strtok(inp,"\n");
create_node(inp);
//parser(input,"INSERT ", "->");
//parse_for_command(input);
return(0);
}

Answer 1

You have to find individual tokens (lexical analysis) and analyse this sequence of tokens (syntactic analysis). Either you write these procedures manually, or you formally specify syntax of your language and employ existing tools to create the required C code automatically (see flex and bison).

Answer 2

I'm not 100% clear on what you are asking, but the separation of the input commands into individual strings can be done with a single set of calls to strtok. Additionally, the time to remove the trailing newline resulting from the call to fgets is immediately after the call to fgets so you do not have dangling newlines hanging off the end of the string. (e.g.):
fgets(input, 100, stdin);
len = strlen (input); /* get length of str */
if (input[len - 1] != '\n') /* no '\n', input too long */
input[--len] = 0; /* null-terminate */
Separating the strings with strtok can make use of a for loop to handle the initial and subsequent calls to NULL in a single call. For purposes of the example, I've just used a statically declared character array to hold the separated strings, but you can just as well pass p to whatever type list or abstraction you desire. If your intent was to further separate the individual components of each command, then a simple while loop walking a pointer (or two) down the separated INSERT or QUERY command will work. Below is the example, if your intent was different, please let me know.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define NCMD 10
#define LCMD 64
int main (void) {
char input[] = "INSERT A->B B->C; QUERY A B; RESET;";
char command[NCMD][LCMD] = {{0}};
char *p = NULL;
size_t i = 0;
size_t ncmd = 0;
for (p = strtok (input, ";"); p; p = strtok (NULL, ";")) {
while (*p == ' ') p++;
strncpy (command[ncmd++], p, LCMD);
}
printf ("\n The separated commands are:\n\n");
for (i = 0; i < ncmd; i++)
printf (" command[%zu] : %s\n", i, command[i]);
printf ("\n");
return 0;
}
Output
$ ./bin/parseinput
The separated commands are:
command[0] : INSERT A->B B->C
command[1] : QUERY A B
command[2] : RESET
Note: the length validation on p as well as the limit check on ncmd were intentionally omitted for purposes of the example.