Want to write in stdout after closing the file opened using freopen

Question

I'm using fork(). However, before executing fork(), I open a file (say a.txt) using freopen for writing. Now the child process redirects the output of execlp to a.txt. After terminating the child process, the parent process closes a.txt. Now how can the parent process read a.txt and show some information in stdout?

score 1 · Answer 1 · answered Jun 27 '11 at 23:13

1

If the parent process opened the file with freopen(3), then the rewind(3) library call can be used to re-wind the stream's pointer to the start of the file, for use with fread(3) or fgets(3) or whatever API you'd like to use.

answered Jun 27 '11 at 23:13

sarnold

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@ sarnold: But I can't write to `stdout`. – miraj Jun 27 '11 at 23:29
@miraj, aha; I think R.. understood your problem better than I did. Is your trouble writing to the file in the child? Or is the problem reading from the file in the parent? – sarnold Jun 28 '11 at 00:02
The child can write perfectly. The problem is in reading the file and writing to stdout by the parent. – miraj Jun 28 '11 at 00:41

score 1 · Answer 2 · answered Jun 27 '11 at 23:13

freopen does not belong in this code at all. Instead, you should do something like:

FILE *tmp = tmpfile();
if (!(pid=fork())) {
    dup2(fileno(tmp), 1);
    close(fileno(tmp));
    execlp(...);
    _exit(1);
}
wait(&status);
/* read from tmp */

However it would actually be a lot better to use a pipe if possible.

Want to write in stdout after closing the file opened using freopen

2 Answers2