Learning C, segmentation fault - c

Question

I'm just learning C after learning Java and I'm having a tough time...
This program is supposed to be a simple command prompt program that takes in a command like "sum 1 2" and adds outputs "3". The program tokenizes the input by space into a 2d array. So the first element will have the command and the following integers will be for arithmetic. As soon as I type "sum 1 2" I get a segmentation fault error and the program crashes.
I set every row in my 2d array to "NULL" so that I could know when to stop iterating through the rows. I looked everywhere and I would like to know if this was an incorrect way of doing it, and if there's a more effective way.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define tLength 20
#define tRow 7
char input[tLength];
char tokens[tRow][tLength];
char p[tLength];
char y[tLength];
int counter = 0;
void getTokens(char inp[]) {
strcpy(y, inp);
strcpy(p, strtok(y," "));
strcpy(tokens[counter], p);
counter = 1;
while (p!=NULL){
strcpy(p,strtok(NULL, " "));
if (counter < tRow) {
strcpy(tokens[counter], p);
counter++;
}
else {
printf("Cannot process more lines");
counter = 0;
break;
}
}
counter = 0;
}
void commandLine() {
int count;
for (count=0;count<tRow;count++){
strcpy(tokens[count],"NULL");
}
printf(">");
fgets(input, tLength, stdin);
getTokens(input);
if (strcmp("quit", tokens[0]) == 0 || strcmp("Quit", tokens[0]) == 0) {
printf("Bye!");
}
else if (strcmp("sum", tokens[0]) == 0 || strcmp("Sum", tokens[0]) == 0) {
int sum = 0;
counter = 1;
while (strcmp("NULL",tokens[counter])!=0) {
sum += atoi(tokens[counter]);
counter++;
}
counter = 0;
printf("%d", sum);
}
else if (strcmp("prod", tokens[0]) == 0 || strcmp("Prod", tokens[0]) == 0) {
int temp = 0;
int prod = 1;
counter = 1;
if (atoi(tokens[1]) == 0) {
printf("%d", 0);
}
else {
while (strcmp("NULL",tokens[counter])!=0) {
prod *= atoi(tokens[counter]);
counter++;
}
}
printf("%d", prod);
}
else {
printf("Error, unknown command");
}
}
void main(void) {
commandLine();
}

Answer 1

Several tips:
global variables when unecessary, are bad
use capital letters for macros
better initialize variables, than left uninitialized
use descriptive variable names ( not, p, y etc )
empty string can't be compared to "NULL"
POSIX offers strcasecmp which is case insensitive version of strcmp
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXINPUTLEN 20
#define MAXTOKENS 7
void getTokens( char* input, char tokenized[][MAXINPUTLEN] )
{
int counter = 0;
char* token = strtok( input, " " );
while( token != NULL )
{
if( counter < MAXTOKENS )
{
strncpy( tokenized[counter], token, MAXINPUTLEN );
counter++;
}
else
{
printf( "Cannot process more lines" );
counter = 0;
break;
}
token = strtok( NULL, " " );
}
counter = 0;
}
void commandLine()
{
char input[MAXINPUTLEN] = {0};
printf( ">" );
fgets( input, MAXINPUTLEN, stdin );
char tokens[MAXTOKENS][MAXINPUTLEN] = {{0}};
getTokens( input, tokens );
if( strcasecmp( "quit", tokens[0] ) == 0 )
{
printf( "Bye!" );
}
else if( strcasecmp( "sum", tokens[0] ) == 0 )
{
int sum = 0;
for( int i = 1; tokens[i][0] != 0; ++i )
{
sum += atoi( tokens[i] );
}
printf( "%d", sum );
}
else if( strcasecmp( "prod", tokens[0] ) == 0 )
{
int prod = 1;
for( int i = 1; tokens[i][0] != 0; ++i )
{
prod *= atoi( tokens[i] );
}
printf( "%d", prod );
}
else
{
printf( "Error, unknown command" );
}
}
int main( void )
{
commandLine();
return 0;
}

Answer 2

Arrays in C work differently than in Java.
In C, arrays are primitive types. You cannot assign NULL to an array in C.
In Java, an array is an object reference; you need to create an array object and assign it to the array variable.
In fact, Java arrays are more like C pointers.
Your code makes excessive use of global variables and tries to call strcpy on a NULL pointer when you reach the last token.
You should use a pointer to capture the return value from strtok:
/* Returns the number of tokens captured. */
int getTokens(char inp[]) {
char *p;
int counter = 0;
strcpy(y, inp);
p = strtok(y," ");
while (p!=NULL){
if (counter < tRow) {
strcpy(tokens[counter], p);
counter++;
}
else {
printf("Cannot process more lines");
counter = 0;
break;
}
}
return counter;
}
Note: There may be more errors in the code. I never made it past getTokens.