fopen --> How to reads what a file contains?

[hakermania]

Hello all, I wanna check if a file contains the word "Permitted".
Unfortunately I don;t know very well how to use fopen. Can you help me a bit? Thank you! Let as take for example that the file is at
/home/alex/file.txt

if ( words_in_file.txt = "Permitted" )
cout << "Permitted!"
else
cout << "Not permitted!"

I've though of making something like that but I don;t know how to read the containers of a file....Thx in advance!

[hayuhi]

First, declare an input file stream and associate it with the file for example using "open". Check this http://cplusplus.com/reference/iostream/ifstream/open/.

Then read word by word from the file and compare it with "Permitted". Read words using for example operator >>. Like this:

ifstream fin;
string s;
fin >> s;

In the end close the file. Check this http://cplusplus.com/reference/iostream/ifstream/close/.

[hakermania]

Hm, till now I have gone as far as this:

1. #include <stdio.h>
2. #include <cerrno>
3. #include <sys/stat.h>
4. #include <sys/types.h>
5. #include <pwd.h>
6. #include <unistd.h>
7. #include <string>
8. #include <iostream>
9. using namespace std;
10. int main()
11. {
12.    struct passwd *passwd;
13.    passwd = getpwuid ( getuid());
14.    string user(passwd->pw_name);
15.    user = "/home/" + user + "/.config/Home/unkilled.txt";
16. ifstream file( user );
17. 
18. string temp;
19. 
20. //Get file contents line by line
21. while(getline(user,temp)){
22. //Check if "Permitted" is in current string
23.    if(temp.find("permitted",0) != string::npos){
24.       cout<<"Permitted!";
25.   }
26.   else{
27.      cout<<"Not Permitted!";
28.   }
29. }
30. file.close;
31.
32. return 0;
33. }

but I get 2 errors:

1) /home/alex/Qt/check/main.cpp:line 16: error: variable ‘std::ifstream file’ has initializer but incomplete type
2) /home/alex/Qt/check/main.cpp:line 21: error: cannot convert ‘std::string’ to ‘char**’ for argument ‘1’ to ‘__ssize_t getline(char**, size_t*, FILE*)’

[Null]

To fix the first error, You need to #include <fstream>
Line 21: You are trying to read contents of string, not a file. So line 21 must be like this while(getline(file,temp)){
Hope this helps.

[athachil]

Hi,

ifstream constructor takes a const char* as the first argument. So change line no: 16 as follows.

ifstream file(user.c_str());

getline() takes an address of a char* buffer as the first argument and the size of that buffer as the second argument.

char buf[1024];
while(file.getline(&buf, 1024)){
.....

its good to refer any library reference document first which will help you to understand things more clearly than firing such a question directly to a forum and get answer to a particular problem.

[Null]

ifstream constructor takes a const char* as the first argument

Oops I forgot that (BTW in C++0x standard fstream constructor will take both const char* and std::string)
One more error: you forgot brackets in line 30 --> file.close();

[Galik]

athachil wrote:

getline() takes an address of a char* buffer as the first argument and the size of that buffer as the second argument.

There are two versions of getline(). The one he is using takes a string rather than a char array.

[athachil]

Hi

There are two versions of getline(). The one he is using takes a string rather than a char array.

Thanks for the information. I was unaware about the getline() defined in string.h .

istream has following versions of getline().

istream& getline (char* s, streamsize n );
istream& getline (char* s, streamsize n, char delim );

and string.h has following versions

istream& getline ( istream& is, string& str, char delim );
istream& getline ( istream& is, string& str );

If you are intended to use the version which uses a string instead of char array, then the first argument should be the input stream. Here it should be "file".

So line number 21 should be

while(getline(file,temp)){

I came to know that another version of getline() is also available which is defined in stdio.h as part of GNU extensions and are available in libc 4.6.27 onwards. Its signature is

ssize_t getline(char **lineptr, size_t *n, FILE *stream);

In the program the compiler tries to match the call in line number 21 to this version and thats why you are getting an error like this.

/home/alex/Qt/check/main.cpp:line 21: error: cannot convert ‘std::string’ to ‘char**’ for argument ‘1’ to ‘__ssize_t getline(char**, size_t*, FILE*)’

Anyway change your program as this and it should work fine.