CODEWITHSUNDEEP
clinuxunixfileposix
Phil Miller
Phil Miller

Reputation: 38138

How can I convert a file pointer ( FILE* fp ) to a file descriptor (int fd)?

I have a FILE *, returned by a call to fopen(). I need to get a file descriptor from it, to make calls like fsync(fd) on it. What's the function to get a file descriptor from a file pointer?

Upvotes: 220

Views: 166881

Answers (3)

Jim Hawkins
Jim Hawkins

Reputation: 51

  fd = _fileno(fp);  // Probably the best way
       fd = fp->_file;     // direct from the FILE structure, member 

   
    typedef struct _iobuf  {
       char*   _ptr;
       int     _cnt;
       char*   _base;
       int     _flag;
       int     _file;
       int     _charbuf;
       int     _bufsiz;
       char*   _tmpfname; } FILE;

Upvotes: 5

Phil Miller
Phil Miller

Reputation: 38138

The proper function is int fileno(FILE *stream). It can be found in <stdio.h>, and is a POSIX standard but not standard C.

Upvotes: 274

Mark Gerolimatos
Mark Gerolimatos

Reputation: 2584

Even if fileno(FILE *) may return a file descriptor, be VERY careful not to bypass stdio's buffer. If there is buffer data (either read or unflushed write), reads/writes from the file descriptor might give you unexpected results.

To answer one of the side questions, to convert a file descriptor to a FILE pointer, use fdopen(3)

Upvotes: 54

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