Help - String input header

What I want to do is to get a string from input without having to guess at the size of the array needed beforehand, and be able to concatenate a new input string on the end of it, something like this:

Code:

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char * str = instring();  //get string from input allocating just enough space for it
incat(str);                    //concatenate a new string to the end of str

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My header file so far:

Code:

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char * instring(){
    char * str = (char *) malloc(sizeof(char));
    str[0] = '\0';

    int c;                //char to be put into
    int i = 0;                //index of string to store

    while((c = getchar()) != '\n'){
        str[i] = c;
        str = (char *)realloc(str, (i+2) * sizeof(char));
        str[i+1]='\0';

        i++;

    }
    return str;
}



void incat(char * str1){
        char * instr = instring();
        int newlength = strlen(str1) + strlen(instr)

        str1 = (char *)realloc(str1, (newlength + 1) * sizeof(char));

        strcat(str1, instr);
}

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instring seems to work all the time, but incat is not right. It seems to work for shorter strings, but strangness occurs on longer ones. Any help would be appreciated.

Of course it doesn't work.

Who said the pointer returned by realloc() is going to be what you passed it? No one.
Perhaps you wanted:

Code:

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void incat(char ** str1)
{
    /* ... */

  *str1 = realloc(*str1, ...);

  strcat(*str1, instr);
}

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Some tips:
1. sizeof(char) is always 1. So just omit it.
2. free() memory when you're done.
3. You should consider reading say 256 bytes, then realloc() another 256 if you need it (say there is still more data in the buffer). This is much faster than doing 256 realloc()'s. You could do this with fgets() and some error checking.
4. You may hit EOF before you hit a new line.
5. Don't cast malloc()/realloc()/calloc() in C.

Are you sure you've giving us the code you're working with? 2nd line in incat() is lacking a semicolon.

Quote:

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Originally Posted by MacGyver

Are you sure you've giving us the code you're working with? 2nd line in incat() is lacking a semicolon.

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After I posted the code I saw that a bunch of extra space was added so I edited it out, but I must have accidentally gotten a semicolon.

zacs7- thanks for the tips. I realize the pointer returned by realloc could point to a different location than str originally pointed to, but wouldn't the new address be assigned to the str pointer in the code I had? Maybe it's a stupid question but I don't quite get it.

Pointers store values.
You can't change the value of something you pass to a function, unless you instead pass its address.
Thus, to change the value of a pointer (read: what it's actually pointing at, not the value of what is being pointed at), you need to pass the address of that pointer.

Consider:

To change an int's value, you pass its address. This way you can make it keep the value changes you do inside the function.
To change a pointer's value, you pass its address.

So, to make a pointer point elsewhere, you need a pointer to a pointer.


Quzah.

No function can change the value of an input parameter for good. If you want to be able to change a value (such as str), you need to pass in a pointer to that value. (Yes, str is a pointer, but it is not a pointer to the thing you want to change.)

Quote:

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Originally Posted by quzah

Pointers store values.
You can't change the value of something you pass to a function, unless you instead pass its address.
Thus, to change the value of a pointer (read: what it's actually pointing at, not the value of what is being pointed at), you need to pass the address of that pointer.

Consider:

To change an int's value, you pass its address. This way you can make it keep the value changes you do inside the function.
To change a pointer's value, you pass its address.

So, to make a pointer point elsewhere, you need a pointer to a pointer.


Quzah.

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Ahh, thats right, I get it now. Thanks.

I changed incat() so that it returns char * so I can do something like

Code:

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str = incat(str);

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That works fine if str was initialized by instring(), but something like

Code:

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char * str = "Hello";
str = incat(str);

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won't work. Why is that?


And a couple quick questions about zacs7s' post.

Quote:

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Originally Posted by zacs7

Some tips:
1. sizeof(char) is always 1. So just omit it.

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It wouldn't be 2 if you're working with unicode?

Quote:

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Originally Posted by zacs7

5. Don't cast malloc()/realloc()/calloc() in C.

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The examples I've seen online and in books have used a cast, if I remember correctly the reason my c book gave was that it isn't required in c but is in c++ so you might as well put it in in case you bring it to c++ at some point. Why shouldn't you cast it in C?

Quote:

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Originally Posted by Nextstopearth

The examples I've seen online and in books have used a cast, if I remember correctly the reason my c book gave was that it isn't required in c but is in c++ so you might as well put it in in case you bring it to c++ at some point. Why shouldn't you cast it in C?

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Unlike C++, C allows you to call functions without declaring them, in which case their return types and parameter types are assumed to be int. So, if you did not #include <stdlib.h> and set a high enough warning level, you would be able to compile code that casts the return value of malloc(), even though what is returned is an int instead of a pointer.

If you don't include stdlib.h, wouldn't the code just refuse to compile and say something like

"undefined implicit reference to builtin function malloc()" ?

Nope, "implicit calls" are legal in C, as opposed to C++.
(Were they legal in C99, again? I can't remember.)
Some compilers will spit out a warning if you do this, but a lot of compiler will not, so it's usually good practice not to cast the return to give yourself (and the compiler) an extra opportunity to catch such mistakes!