Somewhere online I've seen a technique to immediately unlink a temporary file after opening it, since you will discard it anyway. As per my understanding of the man-page for unlink, the file will not be unlinked as long as it has an open file descriptor.

When I execute the following piece of code:

char *file, *command;
asprintf(&file, "/tmp/tempXXXXXX");
int fd = mkstemp(file);
unlink(file);

asprintf(&command, "some_command > %s", file); /*Writes n bytes in temp file*/
FILE *f = popen(command, "re");
pclose(f);

struct stat sbuf;
fstat(fd, &sbuf);
printf("%i\n", sbuf.st_size);

close(fd);
free(file);
free(command);
exit(0); 

It will print a size of 0. However, if I comment unlink(file), it will show the correct file size. I would expect both scenarios to show the correct size of the file, since unlink should wait till no processes have the file open anymore. What am I missing here?

CodePudding user response：

You're missing the fact that the file referred to by your fd is not the same file as that created by your call to popen().

In a POSIX-like shell, some_command > some_file will create some_file if it does not already exist, otherwise it will truncate some_file.

Your call to popen() invokes a shell, which in turn creates or truncates the output file before invoking some_command as per POSIX.

Since you have unlinked some_file before the call to popen(), the file is created anew: that is, the output file set up by your popen() shell is allocated under a different inode than the (now anonymous) file created by your previous call to mkstemp().

You can see that the files are different if you compare st_ino values from your fstat() (by fd) and a separate call to stat() (by name) after the popen().