; // Machine code
35 ; number of lines
12 ; [ 0 ] MOV R0 5
0 ; [ 1 ]
...

int main(){
    int size = 0;
    FILE *in;
    char filename[128] = "";

    printf("Enter the name of the file.n");
    scanf("%s", filename);
    in = fopen(filename, "r");
    size = findSize(filename);
    printf("size: %dn", size);

    return 0;
}

int findSize(FILE *in){
    char *textLine = NULL, *line_buf = NULL;
    size_t line_in_buf_size = 0;
    int number = 0, found = 0;

    while (getline(&line_buf, &line_in_buf_size, in) >= 0 && found == 0){ //keeps reading lines untile reaching EOF or finding the first valid number
        printf("analyzing %sn", line_buf);
        textLine = strtok(line_buf, ";");   //only considers characters before ";"
        if ((*textLine >= '0') && (*textLine <= '9')) {    //if the found characters are numbers, I'll convert them to integers and exit the loop
            number = atoi(textLine);
            found = 1;
        }
    }

    return number;
}

Enter the name of the file.
factorial.txt
=================================================================
==1480==ERROR: AddressSanitizer: attempting free on address which was not malloc()-ed: 0x7ffee8a649c0 in thread T0
    #0 0x10720c7f0 in wrap_realloc (libasan.5.dylib:x86_64+0x6c7f0)
    #1 0x7fff652faab1 in sappend (libsystem_c.dylib:x86_64+0x3dab1)
    #2 0x7fff652fa99f in getdelim (libsystem_c.dylib:x86_64+0x3d99f)
    #3 0x10719c19a in findSize (a.out:x86_64+0x10000119a)
    #4 0x7478742e6b  (<unknown module>)

Address 0x7ffee8a649c0 is located in stack of thread T0 at offset 112 in frame
    #0 0x10719c096 in findSize (a.out:x86_64+0x100001096)

  This frame has 2 object(s):
    [32, 40) 'line_in'
    [96, 104) 'line_in_buf_size' <== Memory access at offset 112 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: bad-free (libasan.5.dylib:x86_64+0x6c7f0) in wrap_realloc
==1480==ABORTING
Abort trap: 6

int findSize(const char *filename)

size = findSize("factorial.txt");