problem using malloc function

[Gamma_123]

I'm new with c and it's my first time using the malloc funtion to reserve memory and it's causing me some problems.
I want to read a file with 3 values, all separated by a space bar.
So this is my code:

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


int main()
{
char *filename;
FILE *in;
    int i,q;
    int *reserve;
    int size;
    
    in = fopen(filename,"r");
    if(in==NULL)
    {
    perror("Datei konnte nicht geöffnet werden");
    return 0;
    }
    for(i=0;;i++) //count the entrys of the file
    {
    if(EOF==fscanf(in,"%i",&q)) break;
    }
     size = i;


    reserve =  malloc(size *sizeof(int));
     if (reserve==NULL)return 0;
    rewind(in);


    for(i=0;i<size;i++) fscanf(in,"%i",&reserve[i]);
    fclose(in);




return 0;
}

However, I get the error: invalid operands of types 'int*' and 'unsigned int' to binary 'operator*' reserve = malloc(size *sizeof(int));

I don't quite understand the malloc function so I don't know how to get rid of this error. I know the malloc function returns a pointer to the memory but I gave a wrong input.
Can somebody pleas help me?

[christop]

Where does "size" point to?

[Gamma_123]

Excuse me this was my fault. size points nowhere. I first wanted to write a function where size would have been a pointer but then I dicided to write the code in the main function.

[Gamma_123]

When I change size to a non pointer, I recive the error: invalid conversion from 'void*' to 'int*' [-fpermissive] reserve = malloc(size *sizeof(int));

[userxbw]

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
 
 
int main()
{
char *filename;
FILE *in;
    int i, q;
    int *reserve;
    // took away pointer to it
    int size;
     // no assingnet of value (actual filename) to variabe filename. 
     // how will it actually know what file to open? 
 
    in = fopen(filename,"r");
    if(in==NULL)
    {
        perror("Datei konnte nicht geöffnet werden");
        return 0; // 0 means no error gotten. fyi
    }
    for(i=0;;i++) //count the entrys of the file
    
    {     // logic is backwards here
        // what is going to happen first,
        // EOF or reading of the file?
        
        // a while loop might be a better
        // choice to use here as well. 
        if(EOF==fscanf(in,"%i",&q)) break;
         
    }
    size = i;
 
    // reserve = (int *)malloc(sizeof(int));
     // 'size' is the size you want, not int in this case.
     // because size has a size value within itself.  
    
    // reserve = (int *)malloc(sizeof(size));
    // gets amount of value inside of size. 
     reserve = (int *)malloc(sizeof(int) * size);

    // reserve =  malloc(size * sizeof(int));
     if (reserve==NULL)return 0;
    rewind(in);
 
 
    for(i=0;i<size;i++) fscanf(in,"%i",&reserve[i]); 
    fclose(in);
 
 
 
 
return 0;
}

Link tells a good bit of what you need to know about your malloc'ing

comments are for some more issues you are going to have
after you figure out your malloc. You might need to check fscanf return value as well.


C - Malloc - Learn C Programming

[Gamma_123]

I can compile the programm now. Thank you! The filname should later be written in the terminal. I just don't quite understand the malloc function! What does this function expect as an input??

[userxbw]

I can compile the programm now. Thank you! The filname should later be written in the terminal. I just don't quite understand the malloc function! What does this function expect as an input??

again

C - Malloc - Learn C Programming

[OldGuy2]

Why do you want to use malloc at all if you know beforehand how many numbers you want to read?
Better to follow to KISS priciple

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

#define MAX_NUMBERS 3

int main()
{
  char filename[] = "somename.txt";
  FILE *in;
  int data[MAX_NUMBERS] = {0}; 
  int num_read = 0;
  int i = 0;

  in = fopen(filename, "r");
  if (in == NULL)
  {
    perror("Datei konnte nicht geöffnet werden");
    return errno;
  }
  while (num_read < MAX_NUMBERS && fscanf(in, "%d", &data[num_read]))
    num_read++;

  printf ("%d numbers read\n\n", num_read);
  for (; i < num_read; i++)
    printf("Number %d: %d\n", i, data[i]);

  fclose(in);
  return 0;
}

[userxbw]

for the unknown you can read ahead to get bytes of data, figure out how to find the spaces using fgets output to 'c' then do the math to figure out what amount of data is minus spaces.

you can use 'c' to get words, spaces, lines count then work with which ever to get what you want done done.

    int size = 0, count = 1;
    char c = '\0';
    // seek to eof
    while ( (c = fgetc(in)) != EOF) {
           size = ftell(in);
           
           if ( c == ' ' )
                count++;
       printf("count = %d : size = %d = %c\n",count, size, c);
}
printf("words read = %d\n", count);

file contains

$ ./mallocing
count = 1 : size = 1 = 2
count = 1 : size = 2 = 3
count = 1 : size = 3 = 4
count = 2 : size = 4 =  
count = 2 : size = 5 = 2
count = 2 : size = 6 = 3
count = 2 : size = 7 = 

words read = 2
str = 234 23

[christop]

When I change size to a non pointer, I recive the error: invalid conversion from 'void*' to 'int*' [-fpermissive] reserve = malloc(size *sizeof(int));

It builds and runs fine for me. Are you trying to compile the program as C++? C++ is more strict about converting void* to other types.

Edit to add: in general, you shouldn't cast the return value from malloc. See FAQ > Casting malloc.

[Gamma_123]

Than maybe I use a different compiler. I use a mingw compiler but I don't know which version. Is there a way to check this with cmd? Is there also a different when I compile c codes than c++ codes? I compile(windows10) like:g++ -o programm programm.c?
The main reason why I wan't to use the malloc function is becaus it's asked for in my homework but thanks for all the advice!

[Guest]

Is there a way to check this with cmd?

Invoking gcc --version from the command line should work.

I compile(windows10) like:g++ -o programm programm.c?

g++ is for when you want to compile as C++. You need to type gcc instead.

[Gamma_123]

Thank yo very much. I have version 5.3.0. I now want to read a file with cmd like: program file.dat and get the perror message and "Invalid argument". Does somebody now why this is happening?

[Guest]

Maybe you forgot to add arguments to main, like so?

int main(int argc, char* argv[])
{
    // process argv here
    return 0;
}

You can find examples on parameter passing online.

[userxbw]

does main look like this,

int main(void)

or this

int main ( int argc, const char **argv)

?

whence you get that figured out, then you need to learn zero based counting and how argc works with argv.

0 = program name, then count up from there left to right on the command-line interface ( cli ) for whatever is written on it.