Unable to read file contents into buffer using read()

Question

Following is a sample code compiled using GNU compiler (g++ command) on an Ubuntu OS 16.04:

#include<iostream>
#include<unistd.h>
#include<fcntl.h>
#include <errno.h>
int main()
{           char* pBuffer;          

            char* storedfilepath = "/home/rtpl/Desktop/ts.mp4";

            std::cout<<"\n Opening file at "<<storedfilepath<<"\n";

            int NumBytesToRead = 1000 ;
            int filedes = open(storedfilepath,O_RDONLY);

            std::cout<<"\n value of error is "<<errno<<"\n";

            std::cout<<"\n value of filedes is "<<filedes;

            if (filedes==0)
            std::cout<<"\n File cannot be opened";
            else
            {
            std::cout<<"\n File opened successfully";
            std::cout<<"\n Now reading file\n"; 

            }

            //if(
            int ret = read(filedes,pBuffer,NumBytesToRead);

            std::cout<<"\n value of error is "<<errno<<"\n";

            if(ret!= -1)
            std::cout<<"\n File read successfully";
            else
            std::cout<<"\n File contents cannot be read";   

            std::cout<<"\nEnd.\n";  

            close(filedes);
            return 0;

}

When compiled; I get this message:

rtpl@rtpl-desktop:~/Desktop$ g++ -g checkts.cpp
checkts.cpp: In function ‘int main()’:
checkts.cpp:8:27: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
    char* storedfilepath = "/home/rtpl/Desktop/ts.mp4";

Upon execution:

rtpl@rtpl-desktop:~/Desktop$ ./a.out

 Opening file at /home/rtpl/Desktop/ts.mp4

 value of error is 0

 value of filedes is 3
 File opened successfully
 Now reading file

 value of error is 14

 File contents cannot be read
End.

Entire gdb debug can be found here.

Question : Why won't the file contents be read when the file is legit and the compiler throws no error?

You have lot of different and unrelated questions. Please only one question per question. — Some programmer dude, Nov 08 '17 at 09:05
oh but I have only one question why am I unable to open the file — vedant gala, Nov 08 '17 at 09:09
Then it's a lot of unrelated information that we don't really need. And to answer your question: The file you look for is part of the system C library, and you should not have those sources unless you're a developer of the library itself. The "missing" files are irrelevant for any normal debugging. Go `up` in the callstack until you come to your code instead. — Some programmer dude, Nov 08 '17 at 09:13
Oh and listen to your compiler and its warnings. Don't just dismiss them. — Some programmer dude, Nov 08 '17 at 09:14
To execute the file, such as mp4 either you need to use a system call within your code or have a library that can execute (read, parse...) it. However, you should be able to `read` with `fread` I presume. — macroland, Nov 08 '17 at 09:19
@Someprogrammerdude Hi, I just edited my question and removed certain parts that might have misled you into assuming that there was more than one question asked. I request you to have a look again? And yes, will listen to compiler and its warnings hereafter. Thanks for that. — vedant gala, Nov 08 '17 at 09:20
@macroland actually I would like to read into a buffer and then operate on those Mpeg packets further - I narrowed down the problem with my bigger code into an MCVE ( I hope this falls under the cateogory of an MCVE) — vedant gala, Nov 08 '17 at 09:21
Use std::string, std::vector, std::array, fstream not char pointer and open. — , Nov 08 '17 at 09:22
`if (filedes==0) std::cout<<"\n File cannot be opened";` is wrong. `open()` returns `-1` on failure, not zero. `int ret = read(...); is also incorrect. `read()` returns `ssize_t` and not `int` - although that's probably not causing any problems with your code, it can cause problems in certain circumstances. You can use `perror( "read()" )` to translate the `errno` value into a string that's output to `stderr`. — Andrew Henle, Nov 08 '17 at 12:28
@manni66 *Use std::string, std::vector, std::array, fstream not char pointer and open.* Under all those layers of code, what do you think `std::vector`, `std::array`, and `fstream` use to actually read data into a process's address space? Perhaps the OP is trying to learn how things actually work and *wants* to use `read()`. — Andrew Henle, Nov 08 '17 at 12:43
@AndrewHenle perhaps, but most likely the OP is learning C with classes. — , Nov 08 '17 at 12:46

score 5 · Accepted Answer · answered Nov 08 '17 at 12:38

Assuming you're running Linux, an errno value of 14 is EFAULT, or "bad address".

Given the code

char* pBuffer;
  .
  .
  .
int ret = read(filedes,pBuffer,NumBytesToRead);

pBuffer is not initialized or otherwise set, so the value in pBuffer is indeterminate and it certainly doesn't point to a valid address.

You need to actually provide a buffer where read() can place the data read:

char buffer[ 1024 ]
   .
   .
   .
ssize_t ret = read(filedes,buffer,NumBytesToRead);

would work, as long as NumBytesToRead does not exceed the number of bytes in buffer. Note also that ret is now the proper ssize_t instead of int.

Unable to read file contents into buffer using read()

1 Answers1