a problem with array of char

**SoFarAway** · 03-31-2005

Hi, all,

I have the following problem:

Code:

int read_line(char *buf) {
    int max_size = findmaxsize();
    char result[max_size];

    // put some char into result

    buf = result;
}

It seems that buf and result contain nothing. I must do it like the following to get correct output.

Code:

int read_line(char *buf) {
    //int max_size = findmaxsize();
    char result[1024];

    // put some char into result

    buf = result;
}

However i need the max_size, cos everytime this value changes. Can anyone help me to solve this problem? Thank you so much!

**joshdick** · 03-31-2005

Code:

   int max_size = findmaxsize();
    char result[max_size];

You can't do that in C. You're attempting to create a C-string of a certain length at runtime. In order to do that, you need to allocate memory for it. You can use malloc() to do that.

**quzah** · 03-31-2005

You've got multiple issues here, aside from what was already mentioned.

1) You're assigning a passed pointer a different value. This has absolutely no effect outside of the function. To change a value inside a function, and have it keep outside the function, you have to pass a pointer to that object. Therefore, if you want to change what a pointer outside a function points to inside a function call, you have to pass a pointer to a pointer.

2) You're assigning what would be a local variable to a pointer. The local instance is actually destroyed as soon as the function ends, leaving your pointer (assuming it would in fact update the pointer--which it won't, as listed at point 1) pointing to a non-existant place in memory. That's bad.

Quzah.

**SoFarAway** · 03-31-2005

But i also cannot code like

Code:

char *result[max_size];
int i;
for(i = 0; i < max_size; i++)
    result[i] = (char *)malloc(sizeof(char));

cos i also need max_size at run-time. right?
so how can i do?

**quzah** · 03-31-2005

What exactly are you trying to do? Just get a user defined string length?

Code:

int x;
char *mystr;

x = getsizefromuserplusoneforthenull();

mystr = malloc( x );

...do whatever it is with this string...

Quzah.

**SoFarAway** · 03-31-2005

Originally Posted by quzah

You've got multiple issues here, aside from what was already mentioned.

1) You're assigning a passed pointer a different value. This has absolutely no effect outside of the function. To change a value inside a function, and have it keep outside the function, you have to pass a pointer to that object. Therefore, if you want to change what a pointer outside a function points to inside a function call, you have to pass a pointer to a pointer.

2) You're assigning what would be a local variable to a pointer. The local instance is actually destroyed as soon as the function ends, leaving your pointer (assuming it would in fact update the pointer--which it won't, as listed at point 1) pointing to a non-existant place in memory. That's bad.

Quzah.

Yes, you are correct! buf does not point to a valid value. But i cannot change the header int read_line(char *buf) cos it is a fixed API given.

So how can i make buf points to an array of char whose lengh is max_size?

Thank you so much!!

**SoFarAway** · 03-31-2005

Originally Posted by quzah

What exactly are you trying to do? Just get a user defined string length?

Code:

int x;
char *mystr;

x = getsizefromuserplusoneforthenull();

mystr = malloc( x );

...do whatever it is with this string...

Quzah.

It works..

Thanx Quzah!! You are amazing!!

**SoFarAway** · 04-01-2005

Another problem is, when i use

Code:

int read_line(char *buf) {
    int max_size = findmaxsize();

    buf = malloc(max_size + 1);
    // do something about buf...

    printf("%s\n", buf);
}

buf is printed out correctly here. However, when i printf buf outside the function, it does not print correctly :

Code:

char *buf;
read_line(buf);
printf("%s\n", buf);

Is it that we malloc for buf inside the function, but outside the function the memory is freed, so buf is no longer point to the correct memory?

How to solve this problem?

And when i use malloc(0), or malloc(1).... they will be same as i use malloc(max_size + 1). Why is it like this?

Thank you...

**Dave_Sinkula** · 04-01-2005

You need to keep track of the result that malloc gives you. Discarding the location of the memory you received is a bad thing to do.

**SoFarAway** · 04-01-2005

How to keep track the result malloc gives me?

buf points to the location of malloc inside the function read_line(buf), but how come it does not point to the same location outside the function? or it points to the same location, but i dont have the priviledge to printf it? how can i do?

Thank you...

**0rion** · 04-01-2005

Well if you could change the header to:

Code:

int read_line(char **buf) {
    int max_size = findmaxsize();

    *buf = malloc(max_size + 1);
    // do something about buf...

    printf("%s\n", *buf);
}

But since you said you can't change it then I'm not sure

**Dave_Sinkula** · 04-01-2005

Or return buf.

Code:

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

char *foo(char *buf)
{
   static const char text[] = "hello world"; /* simulation of other stuff */
   buf = malloc(strlen(text) + 1);
   if ( buf )
   {
      strcpy(buf, text);
      printf("foo:  buf = \"%s\"\n", buf);
   }
   return buf;
}

int main(void)
{
   char *ptr = foo(ptr);
   if ( ptr )
   {
      printf("main: ptr = \"%s\"\n", ptr);
      free(ptr);
   }
   return 0;
}

/* my output
foo:  buf = "hello world"
main: ptr = "hello world"
*/

**SoFarAway** · 04-01-2005

Hi, Dave_Sinkula,

Your method works. But the header is a fixed API, i cannot change int read_line(char *buf) to char *read_line(char *buf)...

Is there another way to do it?

Thank you...

**quzah** · 04-01-2005

Pass an array to the function, and fill that inside the function. As stated, you cannot make a pointer outside a function point to something new inside a function if you don't pass a pointer to that pointer.

Since you can't change your API, you have no other choice but to pass to the function the memory you need to fill. That is to say, you have to write another function to take the size of the needed input, create a string, and then pass that string to the read line function, modifying its contents.

This however presents a problem for you, since inside the function, since you can't change your API, you will have no way of knowing how long the input is.

In other words, whoever designed your API was an idiot.

Quzah.

**Dave_Sinkula** · 04-01-2005

Originally Posted by SoFarAway

Is there another way to do it?

Global variables? Otherwise...

Nope. You're hosed. If you need to do anything with the string, it must be done in read_line. Throw the malloc problem onto the others quzah already listed.

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