Thread: newbie question on strings & pointers

How can I create a variable for storing a string of indeterminate length where a pointer won't work? eg,

Code:

int main () {
     char *text;
     gets(text);
     puts(text);
}

will segfault. I'm asking because I want to read a file into one line with a loop, meaning this could be one very long line by the end. But string ops (strcpy, strcat) will not work with char pointers. Do I have to simply declare "text" as a gigantic array (char text[50000]) or is there a more dynamic means?

Short answer: You can't.

Longer answer: Allocate the memory dynamically. Use a buffer to read, and store into a dynamic buffer. Consistently allocate more memory for the dynamic buffer until you read an entire line. Then use the dynamic buffer. This is a complicated approach.

As for your statements that strcpy() and strcat() don't work with char pointers, you obviously are quite confused. They work perfectly fine with pointers. In your example with gets() (which shouldn't be used anyway), you have no memory allocated for your char *, so it points to some arbitrary memory location. gets() then attempts to trash wherever your char * points to. Not surprisingly, your OS slaps your program down to size and kills it for illegal memory access. You have to make your pointers point to valid locations. Pointers are just variables that contain addresses of other variables.

what would a "dynamic buffer" be?

Memory returned from one of the *alloc() functions. You probably want realloc() for such a case.

I had my fingers crossed that "dynamic" meant something more "automatic", but anyway, for posterity's sake, here's what I worked out re: reading a file into one character array:

Code:

char *func (char *filename) {
     size_t len, tmem = 0;
     char *cumul, *line;
     FILE *fst = fopen(filename, "r");
     if (fst == NULL) puts("!!fopen failed!");
     while ((line = linein(fst)) != NULL) {
           len = strlen(line);
           if (mem = 0) {
                 mem = len+1;
                 cumul = (char *)malloc(mem);
                 strcpy(cumul,line);
           }
           else {
                 mem += len;
                 cumul = (char *)realloc(mem);
                 strcat(cumul,line);
           }
      }
      return cumul;
}

"linein" is another function but I think the point is clear...thanx all (ps. is there a simpler way at all??)

Didn't test it, but it should work (but double-check it before usage

Code:

char *fReader(char *filename)
{
    int size=0;
    char *str = NULL;

    FILE *f = fopen(filename, "r");
    if (!f) { puts("Idiot..."); return(NULL); }

    // make sure that even on empty file we return valid string (ends with zero):
    // (but if file ends with byte 0, we'll have 2 zeros in the end ;)
    // (EDIT: remove this line if you want function to return NULL on both fopen() fail AND
    // empty file)
    str = calloc(1,1);

    for (;;)
    {
        int c = fgetc(f);
        if (c == EOF) break;

        size++;
        str = realloc(str, size+1);
        str[size-1] = c;
        str[size] = 0;
    }

    fclose(f);
    return(str);
}

Originally Posted by MK27

I had my fingers crossed that "dynamic" meant something more "automatic", but anyway, for posterity's sake, here's what I worked out re: reading a file into one character array:

Nope, sorry. It's the price you pay for working with such a low-level language as C.