visual studio c++ how to properly use fopen() to open txt file - c

Question

I am learning c++ and the excersise involves opening a .txt file and reading from it.
The project structure is as following:
int main()
{
static const char* full_name = "C:\Users\Lukas\Desktop\Programming\file_system_test\file_system_test\items.txt";
FILE* my_file = fopen(full_name , "r");
if (my_file == NULL) {
perror("error opening file");
}
}
I am trying to fopen items.txt but with no luck yet..
I am not exactly sure what is the problem since I provide the full path..
METHODS TRIED SO FAR:
Using double backslash in full path
static const char* full_name = "C:\\Users\\Lukas\Desktop\\Programming\\file_system_test\\file_system_test\\items.txt";
Error still persists with:
error opening file: No such file or directory
SOLVED
The only thing that seems to fix this code is to use raw string literal as following:
static const char* full_name3 = R"(C:/Users/Lukas/Desktop/Programming/file_system_test/file_system_test/items.txt)";
FILE* my_file3 = fopen(full_name3, "r");
if (my_file3 == NULL) {
perror("error opening file");
}
Does not return any errors anymore.

Answer 1

Note escape sequences of string literals, so your path:
static const char* full_name = "C:\Users\Lukas\Desktop\Programming\file_system_test\file_system_test\items.txt";
contains \f escape sequence which is interpreted as form feed - new page byte 0x0c in ASCII encoding. This character can't be part of of a path so Invalid argument error is reported.
Also compilers complain that other escape sequences are unknown.
There are three ways to fix it.
As Luka Rahne suggested by using back slash escape sequence \\
Or by using forward slashes (since C suppose to be portable, standard library is able to convert Unix path separator to platform specific path separators).
static const char* full_name = "C:/Users/Lukas/Desktop/Programming/file_system_test/file_system_test/items.txt";
If you are using C++11 or newer (your code is C not C++, but tag says C++), you can leverage raw string literal:
static const char* full_name = R"(C:\Users\Lukas\Desktop\Programming\file_system_test\file_system_test\items.txt)";
Here I did some live testing with msvc (file named: open.c):
#include <stdlib.h>
#include <stdio.h>
int main(int argc, const char argv[])
{
#if VERSION == 0
// here '\f' is used to reproduce error "invalid argument":
static const char name[] = "C:\fUsers\\User\\Downloads\\open.c";
#elif VERSION == 1
static const char name[] = "C:\\Users\\User\\Downloads\\open.c";
#elif VERSION == 2
static const char name[] = "C:/Users/User/Downloads/open.c";
#elif VERSION == 3
static const char name[] = R"(C:\Users\User\Downloads\open.c)";
#endif
FILE* f = fopen(name, "r");
if (!f) {
perror("fopen");
return 1;
}
char buf[256] = "";
fgets(buf, sizeof(buf), f);
printf("%s\n", buf);
fclose(f);
return 0;
}
Here is result of compiling and running from cmd.exe:
C:\Users\User\Downloads>cl open.c /D VERSION=0 && open.exe
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29336 for x86
Copyright (C) Microsoft Corporation. All rights reserved.
open.c
Microsoft (R) Incremental Linker Version 14.28.29336.0
Copyright (C) Microsoft Corporation. All rights reserved.
/out:open.exe
open.obj
fopen: Invalid argument
C:\Users\User\Downloads>cl open.c /D VERSION=1 && open.exe
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29336 for x86
Copyright (C) Microsoft Corporation. All rights reserved.
open.c
Microsoft (R) Incremental Linker Version 14.28.29336.0
Copyright (C) Microsoft Corporation. All rights reserved.
/out:open.exe
open.obj
#include <stdlib.h>
C:\Users\User\Downloads>cl open.c /D VERSION=2 && open.exe
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29336 for x86
Copyright (C) Microsoft Corporation. All rights reserved.
open.c
Microsoft (R) Incremental Linker Version 14.28.29336.0
Copyright (C) Microsoft Corporation. All rights reserved.
/out:open.exe
open.obj
#include <stdlib.h>
C:\Users\User\Downloads>cl open.c /D VERSION=3 && open.exe
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29336 for x86
Copyright (C) Microsoft Corporation. All rights reserved.
open.c
open.c(11): warning C4129: 'm': unrecognized character escape sequence
open.c(11): warning C4129: 'D': unrecognized character escape sequence
open.c(11): warning C4129: 'o': unrecognized character escape sequence
open.c(11): error C2065: 'R': undeclared identifier
open.c(11): error C2143: syntax error: missing ';' before 'string'
open.c(11): error C2099: initializer is not a constant
So everything works as I described and last version 3 fails since I compile code as C.

Answer 2

I believe your issue is that \ in filename is not properly escaped.
Your filename string should have double backslash characters.
static const char* full_name = "C:\\Users\\Lukas\\Desktop\\Programming\\file_system_test\\file_system_test\\items.txt";
You can test this, by sending this string trough std::cout for debug.