how to put different C strings in a pretty format? - c++

Question

I am writing a simple program that builds a directory index of the current directory.
Each file has two char* objects for file name and last-modified time, and one integer for the file size.
I want to put all these in one big string or char*.
#include <sys/types.h>
#include <sys/stat.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <stdio.h>
#include <string>
#include <vector>
#include <iostream>
#include <sstream>
using namespace std;
char* file_info(char*);
int main(void)
{
DIR *d;
struct dirent *dir;
d = opendir(".");
if (d)
{
while ((dir = readdir(d)) != NULL)
{
file_info(dir->d_name);
}
closedir(d);
}
return(0);
}
char* file_info(char* file) {
if(file[0] != '.') {
struct stat sb;
if (stat(file, &sb) == -1) {
perror("stat");
exit(EXIT_FAILURE);
}
char* lm = ctime(&sb.st_mtime);
*lm = '\0';
stringstream ss;
ss << file << " " << lm << " " << sb.st_size;
cout << ss.str() << endl;
}
return lm;
}
I want the returned char* to be an object that has content in this format:
homework-1.pdf 12-Sep-2013 10:57 123K
homework-2.pdf 03-Oct-2013 13:58 189K
hw1_soln.pdf 24-Sep-2013 10:36 178K
hw2_soln.pdf 14-Oct-2013 09:37 655K
The spacing is the major issue here.
How can I correct it easily?
My attempt so far was
const char* file_info(char* file) {
if(file[0] != '.') {
struct stat sb;
if (stat(file, &sb) == -1) {
perror("stat");
exit(EXIT_FAILURE);
}
char* lm = ctime(&sb.st_mtime);
string lastmod(lm);
lastmod.at(lastmod.size()-1) = '\0';
stringstream ss;
string spacing = " ";
ss << file << spacing.substr(0, spacing.size() - sizeof(file)) << lastmod << spacing.substr(0, spacing.size() - lastmod.size()) << sb.st_size;
cout << ss.str() << endl;
return ss.str().c_str();
}
else {
return NULL;
}
}
but it did not work, and I worked with strings so poorly.

Answer 1

Here is the problem:
// ...
stringstream ss;
// ...
return ss.str().c_str(); // woops! ss goes out of scope and string will be destroyed!
This can be easily solved by making your function return std::string instead of char const* and doing this:
return ss.str();
There is no reason to return char const* here. It complicates everything, requires manual memory management, will be exception-unsafe at some point, confuses people who call your function and makes your code utterly unmaintainable.

Answer 2

To answer your iostream formatting question, you want std::setw
std::cout << "'" << std::setw(16) << "Hello" << "'" << std::endl;
http://faculty.cs.niu.edu/~mcmahon/CS241/c241man/node83.html

Answer 3

There are two different problems. First you obviously can not return const char * from function which is stack-allocated. So you have to have it allocated at the heap. And this is the problem. It is problem of ownership. Where you have to delete this string? It can be easily solved by using std::string.
Second problem is yours question. How to have this well aligned. Using yours method you can not print filenames longer then preallocated string. There si simple solution. In header iomanip is defined function
/*unspecified*/ std::setw( int n );
which say "Hey, next thing you will be printing have to be n characters long". And this is what you want. When thing you will be printing is longer then this n it will be printed all. No cropping or something like this.

Answer 4

If you absolutly have to use null-terminated C-Strings, than rather use sprintf instead of std::stringstream. Mixing C and C++ like this is considered bad practice (like pointed out already: i.e. you have to manage memory manually). Also there are some other issues with your code: the sizeof() operator doesn't calculate the length of a string - rather the necessary memory-space (in bytes). Returning a reference to the ctime internal buffer isn't safe either:
The function also accesses and modifies a shared internal buffer,
which may cause data races on concurrent calls to asctime or ctime
Rather use Call-by-reference and don't return anything. Like this:
void file_info(char* file, char* buffer) {
if(file[0] != '.') {
struct stat sb;
if (stat(file, &sb) == -1) {
perror("stat");
exit(EXIT_FAILURE);
}
char* lm = ctime(&sb.st_mtime);
*lm = '\0';
sprintf(buffer, "%10s%10s%d", file, lm, sb.st_size);
}
}
To fix your formating problem, you could also use strlen() (but not sizeof()) and use whitespaces depending on the length of lm and file. But sprintf provides an fixed length Parameter with %"number of digits"s.
See also: printf reference
Minimum number of characters to be printed. If the value to be printed
is shorter than this number, the result is padded with blank spaces.
The value is not truncated even if the result is larger.
But you would allocate memory for char* buffer before you call this function and have to make sure, it's large enough for the sprintf string(!).
i.e.
char buffer[256];
file_info(file, buffer);
printf("%s\n", buffer);

Answer 5

Thank you all for your answers.
However, none of them worked as I intended.(especially, it's not for outputting, but for making a string object.)
I ended up achieving what I wanted, but it's by no means good.
I attach my program, though, below. Feel free to comment.
Thank you.
void file_info(char*, stringstream&);
int main(void)
{
DIR *d;
struct dirent *dir;
d = opendir(".");
stringstream ss;
if (d)
{
while ((dir = readdir(d)) != NULL)
{
file_info(dir->d_name, ss);
}
closedir(d);
}
cout << ss.str() << endl;
return(0);
}
void file_info(char* file, stringstream& ss) {
if(file[0] != '.') {
struct stat sb;
if (stat(file, &sb) == -1) {
perror("stat");
exit(EXIT_FAILURE);
}
char* lm = ctime(&sb.st_mtime);
string lastmod(lm);
lastmod.at(lastmod.size()-1) = '\0';
string spacing = " ";
ss << file << spacing.substr(0, spacing.size() - strlen(file)) << lastmod << spacing.substr(0, spacing.size() - lastmod.size()) << sb.st_size << '\n';
}
return;
}