Segmentation fault. Converting a string to an integer - c

Question

I have a problem where I'm trying to turn a value from an array of string pointers to an integer value: token[1]. However, I get a segmentation fault whenever I don't specify an integer at the first index which in some cases I won't need a number. For example what if I just wanted to type the command: list. I would get a segmentation fault after. How do I store the convert the string value at token[1] to an integer, if an integer is present or not?
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
int main(){
int ch, n = 1;
int i = 0;
int val = 0;
char str[512], *token[5], *act_token;
while(1){
printf("Enter text: ");
while((ch = getchar()) != '\n')
str[i++] = ch;
str[i] = '\0';
i = 0;
printf("string: %s\n", str);
int spaces = 0;
for(int counter = 0; counter < strlen(str) + 1; counter++){
if(str[counter] == ' '){
spaces++;
}
}
printf("Spaces: %d\n", spaces);
strtok(str, " ");
while(n <= spaces && (act_token = strtok(NULL, " "))){
token[n] = act_token;
n++;
}
token[n] = NULL;
n = 1;
// printf("token[1]: %s\n", token[1]);
for(int x = 1; x < spaces+1; x++){
printf("token[%d]: %s\n", x, token[x]);
}
if(isdigit(atoi(token[1])) != 0){
val = atoi(token[1]);
}
printf("value:%d\n", val);
}
return 0;
}

Answer 1

I don't know whether I understand you correctly. However, I have simply added some checks to prevent segfault that were occurring at varying points of the code. Tested with 'foo 3 33'. Formatting is poor.
int main(){
int ch, n = 1;
int i = 0;
int val = 0;
#define TOKEN_SZ 5
char str[512], *token[TOKEN_SZ+1], *act_token;
while(1){
printf("Enter text: ");
while((ch = getchar()) != '\n')
str[i++] = ch;
str[i] = '\0';
i = 0;
printf("string: %s\n", str);
int spaces = 0;
for(int counter = 0; counter < strlen(str) + 1; counter++){
if(str[counter] == ' '){
spaces++;
}
}
printf("Spaces: %d\n", spaces);
n=0;
strtok(str, " ");
while(n<TOKEN_SZ && n <= spaces && (act_token = strtok(NULL, " "))){
token[n] = act_token;
n++;
}
token[n] = NULL;
for(int i=0; token[i]; i++){
printf("%d token[%d]: %s\n", n,i, token[i]);
}
if(n>0 && (atoi(token[0])) != 0){
val = atoi(token[0]);
}
printf("value:%d\n", val);
}
return 0;
}
Update
bash> ./a.out
Enter text: list 2 4
string: list 2 4
Spaces: 2
2 token[0]: 2
2 token[1]: 4
value:2
Enter text:

Answer 2

Replace
if(isdigit(atoi(token[1])) != 0){
val = atoi(token[1]);
}
with
if(isdigit(token[1][0])) != 0){
val = atoi(token[1]);
}
The problem is that isdigit takes a character as its argument. Sure, the man page says it takes an integer, but that integer represents a character.
What your code is doing is:
convert token[1] to an integer (or 0 if it's not a valid integer)
determine whether that integer happens to match an ASCII digit
If so, convert it again, and save the value.
I doubt that's your intent.
My version checks whether the first character of token[1] is a digit, and converts the value if it is. Make sure you understand what token[1][0] means.
BTW, note that if you enter more than 5 space-separated words in your string, you'll store to tokens[6] and higher, which will produce undefined results (possibly crash.) Also, your program is wrong if the user enters more than two spaces between words.
Don't guess what strtok is going to do regarding how it detects and handles delimiters. Instead, let it do its job. Store the values as you get them. Either pick a limit value for the array where you're storing your results and exit the loop before exceeding it, or malloc space for more results as necessary. Here's an example:
char * tstr = str;
int tok_count = 0;
char *tok;
do {
tok = strtok(tstr, " ");
if (tok != NULL) {
token[tok_count++] = tok;
}
tstr = NULL;
} while (tok != NULL && tok_count < TOK_COUNT);
TOK_COUNT has to be at least 1, and should be the array size for tokens.