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?

Johannes Schaub - litb · Accepted Answer · 2010-06-03 18:49:49Z

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.

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.
ididak, it's not safe. the c-string pointed to is not writable :)
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
@X-Istence: Yeah, but be careful in GCC pre-5.1: stackoverflow.com/q/12199710/560648

f3lix · Accepted Answer · 2009-01-31 22:17:36Z

12

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.

edited Jan 31, 2009 at 22:17

answered Jan 31, 2009 at 21:50

f3lix

29.9k11 gold badges68 silver badges86 bronze badges

6 Comments

Frank Over a year ago

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

Frank Over a year ago

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.

R Samuel Klatchko Over a year ago

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

adl Over a year ago

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

dreamlax Over a year ago

You need to call free after using strdup as well.

|

WilliamKF · Accepted Answer · 2013-03-27 18:41:24Z

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.

jamadagni · Accepted Answer · 2017-10-31 04:17:22Z

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.

Collectives™ on Stack Overflow

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

4 Answers 4

4 Comments

6 Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

4 Comments

6 Comments

Comments

Comments

Linked

Related