Thread: Help with malloc and pointers.

Code:

int main(int argc, const char * argv[]) //remove the const key word

  if (scanf("%s", &in_name) != 1) //%s is used for strings, %c is used for char

Don't remove const from your main declaration. I doubt this is allowed anyway. The operating system gives you a const char * array it is not guaranteed to be writable.

The problem is, you declared your filesize function in this way:

Code:

int filesize(char * filename);

But when you say filename(argv[1]) you are giving it a const char *. The basic rule is to make your function take a const pointer if possible, or even better to make it take a const * restrict pointer.

See also: stat(3): file status - Linux man page

Notice stat accepts a const char * pointer, so if you make your filesize function take a const pointer, you can also use that with stat.

Originally Posted by c99tutorial

Don't remove const from your main declaration. I doubt this is allowed anyway. The operating system gives you a const char * array it is not guaranteed to be writable.

Actually, you have that completely backwards. The standard says const should not be used to qualify argv in the declaration of main, and argv and the strings pointed to must be writable:

Originally Posted by C99 5.1.2.2.1

1 The function called at program startup is named main. The implementation declares no
prototype for this function. It shall be defined with a return type of int and with no
parameters:
int main(void) { /* ... */ }
or with two parameters (referred to here as argc and argv, though any names may be
used, as they are local to the function in which they are declared):
int main(int argc, char *argv[]) { /* ... */ }
or equivalent;⁹⁾ or in some other implementation-defined manner.

2 If they are declared, the parameters to the main function shall obey the following
constraints:
— The value of argc shall be nonnegative.
— argv[argc] shall be a null pointer.
— If the value of argc is greater than zero, the array members argv[0] through
argv[argc-1] inclusive shall contain pointers to strings, which are given
implementation-defined values by the host environment prior to program startup. The
intent is to supply to the program information determined prior to program startup
from elsewhere in the hosted environment. If the host environment is not capable of
supplying strings with letters in both uppercase and lowercase, the implementation
shall ensure that the strings are received in lowercase.
— If the value of argc is greater than zero, the string pointed to by argv[0]
represents the program name; argv[0][0] shall be the null character if the
program name is not available from the host environment. If the value of argc is
greater than one, the strings pointed to by argv[1] through argv[argc-1]
represent the program parameters.
— The parameters argc and argv and the strings pointed to by the argv array shall
be modifiable by the program, and retain their last-stored values between program
startup and program termination.

⁹⁾ Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as
char ** argv, and so on.

@RElainaJ:
The problem is the const you put on argv. That declared argv as an array of pointers to const char, meaning the chars pointed to (the command line params) can't be modified. It's like a contract between you and the compiler. So you promised not to modify them, but then you passed them to a function (filesize()) that does not make such a promise, thus the compiler complains since you violate that contract. It's safe to pass non-const variables to const parameters but not the other way around. Since the teacher provided the filesize() function without the const qualifier, leave it that way. Remove const from argv in main.

So the const issue is fixed but I'm still getting a seg fault. I had a friend try the code on his Linux and he got this same thing. Specifically, mine says "Segmentation fault: 11" while his just says "segmentation fault".

Code:

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdlib.h>


int filesize(char * filename)
{
    struct stat status;
    if (stat(filename, &status) == -1)
    {
        return -1;
    }
    else
    {
        return (int) status.st_size;
    }
}


int main(int argc, char * argv[])
{
    FILE *read;
    char in_name;
    int n = 0;

    printf("What file should be read?\n");
    if (scanf("%s", &in_name) != 1)
    {
        printf("Invalid input.\n");
        return 1;
    }

    read = fopen(&in_name,"r");

    if (read == NULL)
    {
        printf("Cannot open input file.\n");
        return 1;
    }

    char * data = malloc(filesize(argv[1]));

    data[n] = fgetc(read);
    while (data[n] != EOF)
    {
        if (data[n] == '\n')
            data[n] = '\0';
        data[++n] = fgetc(read);
    }

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

    fclose(read);
    return 0;
}

You don't need to choose between fgets and scanf, just use both (sscanf)

Code:

char mybuf[10000], filename[10000];

fgets(mybuf, sizeof(mybuf), stdin);
sscanf(mybuf, " %s", filename);

Ultimately, the program is supposed to be able to sort a combination of integers and char arrays.
For example, my text file reads
"goat4
nine
1
2
pink"
and should, in the end, read
"1
2
4
goat
nine
pink"

I have to admit, using char arrays instead of strings really confuses me in C, but I thought in_name would work. In an earlier project, I did this.

Code:

FILE *read;
char in_name;

printf("What file should be read?\n");
   if (scanf("%s", &in_name) != 1)
   {
      printf("Invalid input.\n");
      return 1;
   }

read = fopen(&in_name,"r");
   if (read == NULL)
   {
      printf("Cannot open input file.\n");
      return 1;
   }

That worked just fine.

I just noticed I was trying to print decimal numbers. Whoops. Changed it. I understand that I'm not sorting anything yet, I'm just trying to allocate space and read the file at the moment.

The error seems to be occurring at "data[n] = fgetc(read);" as I finally tracked down the library in Xcode and was able to run it in the application. What's more, now it says "(lldb)" and clicking on that gives me the following.
"argv = (char **)0x7fff5fbff8f0n = (int)0
data = (char *)NULL
read = (FILE *) 0x7fff7d88d280"

I feel vaguely like I have more information now, though I'm not quite sure how to interpret it.

Originally Posted by RElainaJ

I have to admit, using char arrays instead of strings really confuses me in C, but I thought in_name would work. In an earlier project, I did this.
...
I feel vaguely like I have more information now, though I'm not quite sure how to interpret it.

The fact that you did it before and it happened to work doesn't mean anything. Welcome to Undefined Behaviour (UB). If you overwrite the bounds of your array there's a good chance that it will happen to work sometimes, because there are so many spots available in memory to your program, you might happen to hit one that will sort of work some of the time under certain conditions. Obviously however this is not a correct program.