How to fdopen as open with the same mode and flags?

Question

I would like to have a FILE* type to use fprintf. I need to use fdopen to get a FILE* instead of open that returns an int. But can we do the same with fdopen and open? (I never used fdopen)

I would like to do a fdopen that does the same as :

open("my_file", 0_CREAT | O_RDWR | O_TRUNC, 0644);

Douglas

Answer 1

fdopen takes a file descriptor that could be previously returned by open, so that is not a problem.

Just open your file getting the descriptor, and then fdopen that descriptor.

fdopen simply creates a userspace-buffered stream, taking any kind of descriptor that supports the read and write operations.

Plank

Don't forget to that the modes to fdopen must be compatible with the file descriptor ones. – Entasis

Answer 2

fdopen()use file descriptor to file pointer:

The fdopen() function associates a stream with a file descriptor. File descriptors are obtained from open(), dup(), creat(), or pipe(), which open files but do not return pointers to a FILE structure stream. Streams are necessary input for almost all of the stdio library routines.

FILE* fp = fdopen(fd, "w");

this example code may help you more as you want to use fprintf():

int main(){
 int fd;
 FILE *fp;
 fd = open("my_file",O_WRONLY | O_CREAT | O_TRUNC);
  if(fd<0){
    printf("open call fail");
    return -1;
 }
 fp=fdopen(fd,"w");
 fprintf(fp,"we got file pointer fp bu using File descriptor fd");
 fclose(fp);
 return 0;
}

Notice:

• The mode of the stream must be compatible with the mode of the file descriptor.
• When the stream is closed via fclose(), fd is closed also. ref: The fclose() function performs a close() on the file descriptor that is associated with the stream pointed to by stream.

Oppression

You should not close the file descriptor after having used fdopen on it and then fclose. The fd is not dup'ed and calling fclose will close it. – Entasis

Quoting man page: The file descriptor is not dup'ed, and will be closed when the stream created by fdopen() is closed. Also see #8417623 – Entasis

That doesn't write anything in the file. – Douglas

@Douglas It should work, Try again also here is one more example – Oppression

Answer 3

If you don't have previously acquired a file-descriptor for your file, rather use fopen.

FILE* file = fopen("pathtoyourfile", "w+");

Consider, that fopen is using the stand-library-calls and not the system-calls (open). So you don't have that many options (like specifying the access-control-values).

See the man-page.

Subcutaneous