How to create a std::ofstream to a temp file?

Question

Okay, mkstemp is the preferred way to create a temp file in POSIX.

But it opens the file and returns an int, which is a file descriptor. From that I can only create a FILE*, but not an std::ofstream, which I would prefer in C++. (Apparently, on AIX and some other systems, you can create an std::ofstream from a file descriptor, but my compiler complains when I try that.)

I know I could get a temp file name with tmpnam and then open my own ofstream with it, but that's apparently unsafe due to race conditions, and results in a compiler warning (g++ v3.4. on Linux):

warning: the use of `tmpnam' is dangerous, better use `mkstemp'

So, is there any portable way to create an std::ofstream to a temp file?

Belgian

Answer 1

I think this should work:

    char *tmpname = strdup("/tmp/tmpfileXXXXXX");
    ofstream f;
    int fd = mkstemp(tmpname);
    f.attach(fd);

EDIT: Well, this might not be portable. If you can't use attach and can't create a ofstream directly from a file descriptor, then you have to do this:

char *tmpname = strdup("/tmp/tmpfileXXXXXX");
mkstemp(tmpname);
ofstream f(tmpname);

As mkstemp already creates the file for you, race condition should not be a problem here.

Fedak

That doesn't compile with my g++ v3.4.4 on Linux. Apparently only some platforms have that function. – Belgian

Thanks! For your second method (using mkstemp and then ofstream): Is that still efficient in terms of I/O? That will access the file system twice, right? Our file system is super slow and I'm worried that it will put an unnecessary burden on it. – Belgian

mkstemp can fail and return -1; your code should catch that case. – Venusberg

Also the second example is leaking the file descriptor returned by mkstemp. – Prussianism

You need to call free after using strdup as well. – Webber

mkstemp is not portable? std::tmpfile is portable, but doesn't seem very useful. – Antin

Answer 2

I've done this function:

#include <stdlib.h>
#include <fstream>
#include <iostream>
#include <vector>

std::string open_temp(std::string path, std::ofstream& f) {
    path += "/XXXXXX";
    std::vector<char> dst_path(path.begin(), path.end());
    dst_path.push_back('\0');

    int fd = mkstemp(&dst_path[0]);
    if(fd != -1) {
        path.assign(dst_path.begin(), dst_path.end() - 1);
        f.open(path.c_str(), 
               std::ios_base::trunc | std::ios_base::out);
        close(fd);
    }
    return path;
}

int main() {
    std::ofstream logfile;
    open_temp("/tmp", logfile);
    if(logfile.is_open()) {
        logfile << "hello, dude" << std::endl;
    }
}

You should probably make sure to call umask with a proper file creation mask (I would prefer 0600)- the manpage for mkstemp says that the file mode creation mask is not standardized. It uses the fact that mkstemp modifies its argument to the filename that it uses. So, we open it and close the file it opened (so, to not have it opened twice), being left with a ofstream that is connected to that file.

Gula

I wonder if it's safe to just use std::string as template and use (char *) dst.path.c_str(). Seems to be fine for most sensible implementations of std::string. – Laborious

ididak, it's not safe. the c-string pointed to is not writable :) – Gula

In C++11 a std::string can have it's first entry dereferenced. Thus you can pass &somestring[0]; to a function expecting a char*. See: en.cppreference.com/w/cpp/string/basic_string – Lateritious

@X-Istence: Yeah, but be careful in GCC pre-5.1: https://mcmap.net/q/17472/-legality-of-cow-std-string-implementation-in-c-11/560648 – Thetos

Answer 3

Maybe this will work:

char tmpname[256];
ofstream f;
sprintf (tmpname, "/tmp/tmpfileXXXXXX");
int fd = mkstemp(tmpname);
ofstream f(tmpname);

I haven't tried it, but you can check.

Madeline

Answer 4

char tempFileName[20]; // name only valid till next invocation of tempFileOpen
ofstream tempFile;
void tempFileOpen()
{
    strcpy(tempFileName, "/tmp/XXXXXX");
    mkstemp(tempFileName);
    tempFile.open(tempFileName);
}

This code works for me with GCC/libstdc++6 4.8.4 and Clang 3.9 as well. Thanks to the other replies as well which were helpful to me.

Phosphatase