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);

Solution 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.

Solution 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.

Solution 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.

Comments

[Jite]

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

[Jite]

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.

[Jite]

Quoting man page: The file descriptor is not dup'ed, and will be closed when the stream created by fdopen() is closed. Also see stackoverflow.com/questions/8417123/…

[Elfayer]

That doesn't write anything in the file.

[Grijesh Chauhan]

@Elfayer It should work, Try again also here is one more example