strtok and storage in arrays: output not as expected - c

Question

In the below code, the file test.txt has the following data :
192.168.1.1-90
192.168.2.2-80
The output of this is not as expected.
I expect the output to be
192.168.1.1
90
192.168.2.2
80
Any help would be much appreciated.
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp;
char *result[10][4];
int i=0;
const char s[2] = "-";
char *value,str[128];
fp = fopen("test.txt", "r");
if (fp == NULL)
printf("File doesn't exist\n");
else{
while(!feof(fp)){
if(fgets(str,sizeof(str),fp)){
/* get the first value */
value = strtok(str, s);
result[i][0]=value;
printf("IP : %s\n",result[i][0]); //to be removed after testing
/* get second value */
value = strtok(NULL, s);
result[i][1]=value;
printf("PORT : %s\n",result[i][1]); //to be removed after testing
i++;
}}
for (int k=0;k<2;k++){
for (int j=0;j<2;j++){
printf("\n%s\n",result[k][j]);
}
}
}
return(0);
}

Answer 1

You can try this solution. It uses dynamic memory instead, but does what your after.
The code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFFSIZE 128
void exit_if_null(void *ptr, const char *msg);
int
main(int argc, char const *argv[]) {
FILE *filename;
char buffer[BUFFSIZE];
char *sequence;
char **ipinfo;
int str_size = 10, str_count = 0, i;
filename = fopen("ips.txt", "r");
if (filename == NULL) {
fprintf(stderr, "%s\n", "Error Reading File!");
exit(EXIT_FAILURE);
}
ipinfo = malloc(str_size * sizeof(*ipinfo));
exit_if_null(ipinfo, "Initial Allocation");
while (fgets(buffer, BUFFSIZE, filename) != NULL) {
sequence = strtok(buffer, "-\n");
while (sequence != NULL) {
if (str_size == str_count) {
str_size *= 2;
ipinfo = realloc(ipinfo, str_size * sizeof(*ipinfo));
exit_if_null(ipinfo, "Reallocation");
}
ipinfo[str_count] = malloc(strlen(sequence)+1);
exit_if_null(ipinfo[str_count], "Initial Allocation");
strcpy(ipinfo[str_count], sequence);
str_count++;
sequence = strtok(NULL, "-\n");
}
}
for (i = 0; i < str_count; i++) {
printf("%s\n", ipinfo[i]);
free(ipinfo[i]);
ipinfo[i] = NULL;
}
free(ipinfo);
ipinfo = NULL;
fclose(filename);
return 0;
}
void
exit_if_null(void *ptr, const char *msg) {
if (!ptr) {
printf("Unexpected null pointer: %s\n", msg);
exit(EXIT_FAILURE);
}
}

Answer 2

The key thing to understand is that char *strtok(char *str, const char *delim) internally modifies the string pointed to by str and uses that to store the result. So the returned pointer actually points to somewhere in str.
In your code, the content of str is refreshed each time when you parse a new line in the file, but the address remains the same. So after your while loop, the content of str is the last line of the file, somehow modified by strtok. At this time, result[0][0] and result[1][0] both points to the same address, which equals the beginning of str. So you print the same thing twice in the end.
This is further illustrated in the comments added to your code.
int main()
{
FILE *fp;
char *result[10][4];
int i=0;
const char s[2] = "-";
char *value,str[128];
fp = fopen("test.txt", "r");
if (fp == NULL)
printf("File doesn't exist\n");
else{
while(!feof(fp)){
if(fgets(str,sizeof(str),fp)){
/* get the first value */
value = strtok(str, s);
// ADDED: value now points to somewhere in str
result[i][0]=value;
// ADDED: result[i][0] points to the same address for i = 0 and 1
printf("IP : %s\n",result[i][0]); //to be removed after testing
/* get second value */
value = strtok(NULL, s);
// ADDED: value now points to somewhere in str
result[i][1]=value;
// ADDED: result[i][1] points to the same address for i = 0 and 1
printf("PORT : %s\n",result[i][1]); //to be removed after testing
i++;
}}
// ADDED: now result[0][0]==result[1][0], result[0][1]==result[1][1], you can test that
for (int k=0;k<2;k++){
for (int j=0;j<2;j++){
printf("\n%s\n",result[k][j]);
}
}
}
return(0);
}
To get the expected output, you should copy the string pointed by the pointer returned by strtok to somewhere else each time, rather than just copy the pointer itself.