Re: [Dev-C++] Malloc Problem

[Luigi S. <gi...@li...>]

Iuri-
Suppose that your problem is the following: you have a text file (say it=20
"script") with lines of different lengths, you wish to load a line and=20
print it on the screen until all the lines have been processed.

script
------
aaaaaa
bbbbb
iuri
------

1- assume that you know that no line has more than 10 chars
Here is how I would solve it.

#include <stdio.h>
#include <assert.h>

#define MAX_LEN 11
#define FILE_NAME "script"

int main()
{
    char buf[MAX_LEN];
    FILE *inp =3D fopen(FILE_NAME, "r");

    assert(inp !=3D 0); /* a quick way to report errors */

    while (fgets(buf, MAX_LEN, inp))
       printf("%s", buf);

    fclose(inp);

    return 0;
}

As you can see the second argument of fgets is the size of the array where=
=20
the line will be stored, not the size of the line that will be read. Also=20
keep in mind that fgets will read only MAX_LEN - 1 =3D 10 chars, the last=
 one=20
will be used by fgets as the place for the '\0'.
So when you write:

fgets(linha1_ptr, sizeof(linha1_ptr), script);

It is "strange" (because usually you already know how big a string is), but=
=20
OK, if by "sizeof(linha1_ptr)" you mean the length of the string linha1_ptr=
=20
before the call to fgets, it is not correct if you mean: "read the line and=
=20
get its length, return this length as the value of sizeof(linha1_ptr)".

Suppose you say: (assume linha1_ptr is an array of 31 chars)

fgets(linha1_ptr, 31, script); /* and load the string "123\n" */
/* now sizeof(linha1_ptr) is 4 */

fgets(linha1_ptr, sizeof(linha1_ptr), script); /* trying to load "iuri\n" */
/* fgets will read 3 chars, "iur", the fourth will be '\0' */


Of course when you say:

fgets(linha1_ptr, 31, script); /* and load the string "123\n" */
fgets(linha1_ptr, 31, script); /* trying to load "iuri\n" */
/* now "iuri" will be loaded as expected */

Hope this clearly explains what should be used as the second argument of=
 fgets.

2 - assume that you do not know the size of the biggest line
Here is how I would solve it.

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

#define FILE_NAME "script"
char *getline(FILE *);

int main(int argc, char *argv[])
{
    char *buf;
    FILE *inp =3D fopen(FILE_NAME, "r");

    assert(inp !=3D 0);

    while (buf =3D getline(inp))
    {
       printf("%s", buf);
       free(buf);
    }

    fclose(inp);
    return 0;
}

/*
getline =3D read a line from the file, return the line read.
The string that is returned is always terminated by a '\n'
*/

#define SIZE 6
#define GET_MEM (char *)calloc(len + 1, sizeof(char))

char *getline(FILE *file)
{
    char buf[SIZE];
    char *line =3D 0;
    int len =3D 0;

    while (fgets(buf, SIZE, file))
    {
       char *tmp =3D line;
       int n =3D strlen(buf);

       len +=3D n;
       line =3D GET_MEM;
       assert(line !=3D 0);

       if (tmp)
       {
          strcpy(line, tmp);
          free(tmp);
       }

       strcat(line, buf);

       if (buf[n - 1] =3D=3D '\n') /* finished reading line */
          break;
    }

    return line;
}

Hope this is of some help

At 14.46 18/07/01 -0300, you wrote:

>Dear Luigi,
>         Thank you for your help. I tried to use calloc because de string=
=20
> read from the
>file string has diferent sizes. I.e. the first line of the file script is=
=20
>"iuri", when I write:
>fgets (linha1_ptr ,sizeof(linha1_ptr), script);
>         It returns: iur
>If I write:
>fgets (linha1_ptr , 30 , script);
>         It returns: iuri
>
>With static memory the problem is that it reads the hole thing:
>So if my string is:
>         aaaaaaaaaa
>         bbbb
>With the fgets it will read:
>         aaaaaaaaaa
>         bbbb\0aaaa
>
>I think with sizeof it shouldn't happen, but it does. He reads the size of=
=20
>the 1=BA line and
>returns this same size for every line, even if it's bigger.
>
>                                 Thanks,
>                                                 Iuri

---
gisan