Write to subdirectory using freopen

Question

I am trying to write out to files inside subdirectories using freopen:

freopen("output/output-1.txt", "w", stdout);

I have tried changing it to output to the current directory and it works. It terminates without errors when the target output file is in a subdirectory; however no files are created. Creating the required directory does not fix the issue.

void write_to(int subtask, int tc){
    string output = string("testcases/subtask-") + to_string(subtask) + "-tc-" + to_string(tc);
    freopen(output.c_str(), "w", stdout);
}

int main(){
    for(int i = 1; i <= 25; i++){
        write_to(1, i);
        // rest of code to generate and cout test cases
    }
}

Does anyone have a solution to this?

the path spec is relative to the current directory of the execution not to the directory the executable is in. — Jean-Baptiste Yunès, Dec 08 '16 at 13:46
Do not quote `stdout`. That gives you a string, whereas you need a `FILE *`. The resulting behavior is undefined. — John Bollinger, Dec 08 '16 at 14:00
@FanPu 1) show more relevant code, 2) show how you run your program. 3) What exactly do you mean by "execution directory" ? — Jabberwocky, Dec 08 '16 at 14:01
Do present a [mcve]. Without one we are left guessing at what the real problem might be, and you have already rejected all the most likely guesses. — John Bollinger, Dec 08 '16 at 14:02
Test the result of `freopen` because it can fail (and then gives `NULL`) — Basile Starynkevitch, Dec 08 '16 at 14:11
BTW, standard C99 or C11 does not know about directories. I guess that you are coding for POSIX standard, or even on some Linux system. Then you should tag your question appropriately (e.g. if on Linux, add the `Linux` tag). On Linux you might use [strace(1)](http://man7.org/linux/man-pages/man1/strace.1.html) to figure out what system calls are executed. — Basile Starynkevitch, Dec 08 '16 at 14:12
Use [`getcwd`](https://linux.die.net/man/3/getcwd) to printf the current directory right at the beginning of `main`, which may help to find out what is going on. — Jabberwocky, Dec 08 '16 at 14:13
Compile with all warnings & debug info (`gcc -Wall -Wextra -g`) and **use the debugger** (`gdb`) — Basile Starynkevitch, Dec 08 '16 at 14:15

Basile Starynkevitch · Accepted Answer · 2016-12-08T14:58:33.417

Read documentation of freopen(3). You should test and use its result:

The freopen() function opens the file whose name is the string pointed to by path and associates the stream pointed to by stream with it. The original stream (if it exists) is closed.

about its return value:

Upon successful completion fopen(), fdopen() and freopen() return a FILE pointer. Otherwise, NULL is returned and errno is set to indicate the error.

So you need to code at least (if on Linux or some POSIX system)

void write_to(int subtask, int tc){
   string output = 
     string("testcases/subtask-") + to_string(subtask) 
      + "-tc-" + to_string(tc);
   FILE*outf = freopen(output.c_str(), "w", stdout);
   if (!outf) {
     perror(output.c_str());
     char pwdbuf[128];
     memset (pwdbuf, 0, sizeof(pwdbuf));
     getcwd(pwdbuf, sizeof(pwdbuf)-1);
     fprintf(stderr, "failure in %s\n", pwdbuf);
     exit(EXIT_FAILURE);
   }
}

^{(the code above won't solve your issue, but will output a meaningful error message on error; perhaps you are not running your code in the appropriate current directory)}

I also recommend ending your for loop in main with fflush(stdout) or fflush(NULL).

If on Linux or POSIX, you might instead work at the file descriptor level (so code a redirection), and use open(2) & dup(2) (using STDOUT_FILENO as the second argument to dup2).

If testcases is a directory in your $HOME (that is, ~/testcases/ as expanded by your shell) you would want

string output =
  string (getenv("HOME")) + "/" 
  + string("testcases/subtask-") + to_string(subtask) 
  + "-tc-" + to_string(tc);

Thank you, this solved the issue. The error was Permission Denied - executing as root allowed the files to be created in subdirectories. — nerraruzi, Dec 08 '16 at 15:03
You should never execute as root a program that you wrote, unless you are sure to not have any security hole. — Basile Starynkevitch, Dec 08 '16 at 15:05

Write to subdirectory using freopen

1 Answers1