来自fcntl.h的读取函数中的C ++字符串

Question

In my basic Linux Programming course at college, we use fcntl.h and unistd.h Using C++ strings, I get the following:

statusOfFunction = write(fileDescriptor, input.c_str(), input.length());

This line works. I get a file created, with the contents of input string. But, why doesn't any of these lines work:

statusOfFunction = read(fileDescriptor, reading.c_str(), 10);
Error: No matching function call to "read"

statusOfFunction = read(fileDescriptor, reading, 10);
Error: No matching function call to "read"

statusOfFunction = read(fileDescriptor, &reading, 10);
No error throws up, but does not get executed

statusOfFunction = read(fileDescriptor, &reading.c_str(), 10);
Error: No matching function call to "read"

https://www.dropbox.com/s/lnw208uo3xurqxf/Basic%20Unix%20Operations%20on%20Text%20Files.cpp?dl=0

Here is the program, for your reference. Thank you! :)

Answer 1

The problem with the first

statusOfFunction = read(fileDescriptor, reading.c_str(), 10);

is that c_str is declared to return a const pointer. Otherwise this is the closes to the correct method.

First of all you need to create a string that contains at least 10 characters:

std::string temp_string(10, ' ');  // Creates a string contains 10 spaces

Then you need to pass a non-constant pointer, which you can get by using the address-of operator & and the strings array-indexing operator [] :

statusOfFunction = read(fileDescriptor, &temp_string[0], temp_string.length());

And finally you assign it to the actual string:

reading = std::string(temp_string, 0, statusOfFunction);

Of course, you should check statusOfFunction so it's not an error (when it -1 ) or end of file (when its 0 ).

---

All the other methods you tried for reading are just very wrong.

Answer 2

read expects a buffer to fill the data read.

What you're doing is dangerous.

You should allocate a char buffer and make sure you add the NULL char after the string length as read will not do it for you. the c_str function exposes the internal buffer used by the string class. It is read only.

Overwriting the string object itself will cause a crash for sure when later used.

char buff[11];
size_t bytes_read = read(fd, buff, 10);
buff[bytes_read] = 0; // terminate string
string myString(buff);