Storing inputs from stdin in an array in C

Question

I'm using the following code to store stdin into an array. I end up with a host of warnings and a segmentation fault.

while(fgets(str, 256, stdin)){

        size_t count = 0;
        char** arr = NULL;
        char* token = NULL;
        char* delim = ' ';

        for(int i=0; i < strlen(str); i++) 
            if (isspace(str[i])) count++;

        count++;
        arr = malloc(sizeof(char*) * (count));

        for(int i=0; i <= count; i++){

            token = strtok(str, delim);
            arr[i] = token;
        }

        for (int i = 0; i < count; ++i)
        {
            printf("%s\n", arr[i]);
        }

        //if(strncasecmp(str, "quit", 4) == 0) break;


        free(arr);

    }

I'm a little confused about this warning in compilation

c:34:12: warning: incompatible integer to pointer conversion initializing 'char *' with an expression of type 'int'
      [-Wint-conversion]
        char* delim = ' ';
              ^       ~~~

And ofcourse finally when I run it I end up with a seg fault.

admin 123 tyu
Segmentation fault: 11

What am I doing wrong here.

Answer 1

I noticed a first problem here. This is not good.

   for(int i=0; i <= count; i++){

        token = strtok(str, delim);
        arr[i] = token;
    }

Replace it with the following:

    token = strtok(str, delim);
    while(token){
        arr[i] = token;
        token = strtok(NULL, delim);
    }

The reference manual of strtok says that you use it on str once and for the rest use NULL to continuously split the string. Look at the strtok() manual page and the given example for a simple usage.

The second point is that delim has to be a C string so a char* type in your case, will be delim = " " not delim = ' '. Ideally, it should be const char *delim = " "; too.

Answer 2

The warning is about this line:

char* delim = ' ';

By using single quotes, you have created a char not a string. The compiler converts the space to an integer and tries to assign that to the pointer. When you then try to use the pointer, you get a segmentation fault.

To fix it you can simply change the single quotes to double quotes.

Answer 3

Single quotes are used for something called character constants, they are the ASCII value of the enclosed character.

What you have to pass to strtok() is a pointer to a string of delimiter characters, which could be more than one.

To generate a pointer to a string you can use a string literal, but they are not writeable, so you must be careful to prevent writing to them, one way to prevent that is to use the const qualifier, that way your compiler will warn about that, so

const char *delim = " ";

is more likely what you are looking for.

Since the single quotes produce an integer, your program is assigning an integer to a pointer type, which will almost surely cause undefined behavior.

That means that you did not enable compiler warnings, or else, the compiler will tell you that you are making a pointer from an incompatible integer type.