Segmentation Fault using Strtok() for 2d array in C - c

Question

I keep getting Segmentation Fault while trying to split a string such as "1,2,3;4,5,6;7,8,9" into a 2d Array why does it keep happening with this code could someone please explain this for me?
int main(void){
char string[100];
char *token;
char *end;
int num;
int row_counter;
int column_counter;
int counter_position;
char *rows[100];
int square_ints[40][40];
row_counter = 0;
column_counter = 0;
string[] = "1,2,3;4,5,6;7,8,9";
token = strtok(string, ";");
rows[row_counter] = token;
row_counter++;
while (token != NULL){
token = strtok(NULL, ";");
rows[row_counter] = token;
row_counter++;
}
counter_position = row_counter;
for (row_counter = 0; row_counter < counter_position; row_counter++) {
token = strtok(rows[row_counter], ",");
num = strtol(token, &end, 10);
square_ints[row_counter][column_counter] = num;
printf("%d\n", square_ints[row_counter][column_counter]);
column_counter++;
while (token != NULL) {
token = strtok(NULL, ",");
num = strtol(token, &end, 10);
square_ints[row_counter][column_counter] = num;
printf("%d\n", square_ints[row_counter][column_counter]);
column_counter++;
}
}
}
It should be printing: 1 2 3 4 5 6 7 8 9 instead I get 1 2 3 Segmentation Fault

Answer 1

To expand on what MikeCAT mentioned:
As the start of your for loop, you do:
token = strtok(rows[row_counter], ",");
num = strtol(token, &end, 10);
Later you do:
token = strtok(NULL, ",");
num = strtol(token, &end, 10);
The problem is that in both cases, strtok will return NULL, so the value of token is NULL. When you pass token to strtol, it will try to use/dereference this and it will produce a segfault.
So, to fix, after each strtok call, you need to add:
if (token == NULL)
break;
Here's the corrected code. I've added some debug printf, so you can see the issue.
I've used cpp conditionals to demarcate old vs new code:
#if 0
// old code
#else
// new code
#endif
#if 1
// new code
#endif
Anyway, here it is:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#ifdef DEBUG
#define dbgprt(_fmt...) \
do { \
printf(_fmt); \
} while (0)
#else
#define dbgprt(_fmt...) \
do { \
} while (0)
#endif
int
main(void)
{
#if 0
char string[100];
#else
char string[] = "1,2,3;4,5,6;7,8,9";
#endif
char *token;
char *end;
int num;
int row_counter;
int column_counter;
int counter_position;
char *rows[100];
int square_ints[40][40];
row_counter = 0;
column_counter = 0;
#if 0
string[] = "1,2,3;4,5,6;7,8,9";
#endif
token = strtok(string, ";");
rows[row_counter] = token;
row_counter++;
while (token != NULL) {
token = strtok(NULL, ";");
rows[row_counter] = token;
row_counter++;
}
counter_position = row_counter;
for (row_counter = 0; row_counter < counter_position; row_counter++) {
dbgprt("DEBUG: rows[%d]='%s'\n",row_counter,rows[row_counter]);
token = strtok(rows[row_counter], ",");
dbgprt("DEBUG: token='%s'\n",token);
#if 1
if (token == NULL)
break;
#endif
num = strtol(token, &end, 10);
square_ints[row_counter][column_counter] = num;
printf("%d\n", square_ints[row_counter][column_counter]);
column_counter++;
while (token != NULL) {
token = strtok(NULL, ",");
dbgprt("DEBUG2: token='%s'\n",token);
#if 1
if (token == NULL)
break;
#endif
num = strtol(token, &end, 10);
square_ints[row_counter][column_counter] = num;
printf("%d\n", square_ints[row_counter][column_counter]);
column_counter++;
}
}
return 0;
}
Note that while the above code works, it is replicating code before a given loop and inside the loop. If we add an extra pointer variable (e.g. bp), we can simplify the code a bit:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(void)
{
char string[] = "1,2,3;4,5,6;7,8,9";
char *token;
char *end;
int num;
int row_counter;
int column_counter;
int counter_position;
char *bp;
char *rows[100];
int square_ints[40][40];
row_counter = 0;
column_counter = 0;
bp = string;
while (1) {
token = strtok(bp,";");
bp = NULL;
if (token == NULL)
break;
rows[row_counter] = token;
row_counter++;
}
counter_position = row_counter;
for (row_counter = 0; row_counter < counter_position; row_counter++) {
bp = rows[row_counter];
if (bp == NULL)
break;
while (1) {
token = strtok(bp, ",");
bp = NULL;
if (token == NULL)
break;
num = strtol(token, &end, 10);
square_ints[row_counter][column_counter] = num;
printf("%d\n", square_ints[row_counter][column_counter]);
column_counter++;
}
}
return 0;
}
There is another similar function strsep that is considered by some to be "strtok done right". Here's a version that uses that function:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(void)
{
char string[] = "1,2,3;4,5,6;7,8,9";
char *token;
char *end;
int num;
int row_counter;
int column_counter;
int counter_position;
char *bp;
char *rows[100];
int square_ints[40][40];
row_counter = 0;
column_counter = 0;
bp = string;
while (1) {
token = strsep(&bp,";");
if (token == NULL)
break;
rows[row_counter] = token;
row_counter++;
}
counter_position = row_counter;
for (row_counter = 0; row_counter < counter_position; row_counter++) {
bp = rows[row_counter];
if (bp == NULL)
break;
while (1) {
token = strsep(&bp,",");
if (token == NULL)
break;
num = strtol(token, &end, 10);
square_ints[row_counter][column_counter] = num;
printf("%d\n", square_ints[row_counter][column_counter]);
column_counter++;
}
}
return 0;
}