How can I convert a file pointer ( FILE* fp ) to a file descriptor (int fd)?
CLinuxUnixFilePosixC Problem Overview
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?
C Solutions
Solution 1 - C
The proper function is int fileno(FILE *stream). It can be found in <stdio.h>, and is a POSIX standard but not standard C.
Solution 2 - C
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)
Solution 3 - C
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;