Why do I receive crash report with argv in this example?

Question

TL;DR

• 1. Why that code giving crash error as it start?

• 2. What argv[] do? Is it receive your cmd input? And how it differentiate from argv 1,2,3...so on?

• 3. Output not showing what is expected when I change argv[1] with VSF.txt

The following code gives an error:

File: minkernel\crts\ucrt\src\appcrt\stdio\fopen.cpp
Line: 30
Expression file_name != null_ptr

The idea is to print every file line with the respective number. I don't fully understand files yet and I believing argv[1] is what you write at your compiler prompt, is that right? How it differentiate with argv 1,2,3...and so on?

If I change argv[1] with VSF.txt (the file name) it shows strange chars (not what is inside VSF.txt )

The output is showing everything with just one line, no space and line variable not incrementing; why it isn't printing the content of VSF.txt ?

#define _CRT_SECURE_NO_WARNINGS

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

main(int argc, char *argv[])
{
    FILE *file = stdout;
    int lines = 1, start = 1;
    char ch;

    if ((file = fopen(argv[1], "r")) == NULL)
    {
        printf("Impossivel de abrir o arquivo :%s", argv[1]);
        exit(0);
    }
    while (ch = fgetc(file) != EOF)
    {
        if (ch == '\n')
        {
            lines++;
            start = 1;
        }
        else if (start == 1)
        {
            printf("%d: ",lines);
            putchar(ch);
            start = 0;
        }
        putchar(ch);
    }
    fclose(file);
}

Answer 1

The problem with the strange output is here:

while (ch = fgetc(file) != EOF)

The inequality operator != has higher precedence than the assignment operator = . So the expression above is the same as:

while (ch = (fgetc(file) != EOF))

This assigns ch a boolean value, either 0 or 1. That's why you get the output you get. Put parenthesis in the proper place:

while ((ch = (fgetc(file)) != EOF)

As for the crash, that is happening because you're not telling the program the name of the file you want to open when you run it. If you run it from a command prompt, do so like this:

archives VSF.txt

If you run it from inside Visual Studio, go to the project settings, and under Debugging set Command Arguments to VSF.txt.