C fopen vs open

Question

Is there any reason  other than syntactic ones  that you d want to use   FILE  fdopen int fd  const char  mode     or  FILE  fopen const char  path  const char  mode     instead of   int open const char  pathname  int flags  mode t mode     when using C in a Linux environment

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I changed to open   from fopen   for my application  because fopen was causing double reads every time I ran fopen fgetc    Double reads were disruptive of what I was trying to accomplish   open   just seems to do what you ask of it

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Unless you re part of the 0 1  of applications where using open is an actual performance benefit  there really is no good reason not to use fopen  As far as fdopen is concerned  if you aren t playing with file descriptors  you don t need that call   Stick with fopen and its family of methods  fwrite  fread  fprintf  et al  and you ll be very satisfied  Just as importantly  other programmers will be satisfied with your code

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open   is a system call and specific to Unix-based systems and it returns a file descriptor  You can write to a file descriptor using write   which is another system call  fopen   is an ANSI C function call which returns a file pointer and it is portable to other OSes  We can write to a file pointer using fprintf        In Unix  You can get a file pointer from the file descriptor using     fP   fdopen fD   a      You can get a file descriptor from the file pointer using     fD   fileno  fP

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Using open  read  write means you have to worry about signal interaptions   If the call was interrupted by a  signal  handler the functions will return -1 and set errno to EINTR   So the proper way to close a file would be  while  retval   close fd   retval    -1  amp  amp  ernno    EINTR

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open   is a low-level os call  fdopen   converts an os-level file descriptor to the higher-level FILE-abstraction of the C language  fopen   calls open   in the background and gives you a FILE-pointer directly    There are several advantages to using FILE-objects rather raw file descriptors  which includes greater ease of usage but also other technical advantages such as built-in buffering  Especially the buffering generally results in a sizeable performance advantage

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open   will be called at the end of each of the fopen   family functions  open   is a system call and fopen   are provided by libraries as a wrapper functions for user easy of use

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If you have a FILE    you can use functions like fscanf  fprintf and fgets etc  If you have just the file descriptor  you have limited  but likely faster  input and output routines read  write etc

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First  there is no particularly good reason to use fdopen if fopen is an option and open is the other possible choice  You shouldn t have used open to open the file in the first place if you want a FILE    So including fdopen in that list is incorrect and confusing because it isn t very much like the others  I will now proceed to ignore it because the important distinction here is between a C standard FILE   and an OS-specific file descriptor   There are four main reasons to use fopen instead of open    fopen provides you with buffering IO that may turn out to be a lot faster than what you re doing with open  fopen does line ending translation if the file is not opened in binary mode  which can be very helpful if your program is ever ported to a non-Unix environment  though the world appears to be converging on LF-only  except IETF text-based networking protocols like SMTP and HTTP and such    A FILE   gives you the ability to use fscanf and other stdio functions  Your code may someday need to be ported to some other platform that only supports ANSI C and does not support the open function    In my opinion the line ending translation more often gets in your way than helps you  and the parsing of fscanf is so weak that you inevitably end up tossing it out in favor of something more useful   And most platforms that support C have an open function   That leaves the buffering question   In places where you are mainly reading or writing a file sequentially  the buffering support is really helpful and a big speed improvement   But it can lead to some interesting problems in which data does not end up in the file when you expect it to be there   You have to remember to fclose or fflush at the appropriate times   If you re doing seeks  aka fsetpos or fseek the second of which is slightly trickier to use in a standards compliant way   the usefulness of buffering quickly goes down   Of course  my bias is that I tend to work with sockets a whole lot  and there the fact that you really want to be doing non-blocking IO  which FILE   totally fails to support in any reasonable way  with no buffering at all and often have complex parsing requirements really color my perceptions

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Depends also on what flags are required to open  With respect to usage for writing and reading  and portability  f  should be used  as argued above   But if basically want to specify more than standard flags  like rw and append flags   you will have to use a platform specific API  like POSIX open  or a library that abstracts these details  The C-standard does not have any such flags   For example you might want to open a file  only if it exits  If you don t specify the create flag the file must exist  If you add exclusive to create  it will only create the file if it does not exist  There are many more   For example on Linux systems there is a LED interface exposed through sysfs  It exposes the brightness of the led through  a file  Writing or reading a number as a string ranging from 0-255  Of course you don t want to create that file and only write to it if it exists  The cool thing now  Use fdopen to read write this file using the standard calls

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fopen vs open in C   1  fopen is a library function while open is a system call   2  fopen provides buffered IO which is faster compare to open which is non buffered   3  fopen is portable while open not portable  open is environment specific    4  fopen returns a pointer to a FILE structure FILE     open returns an integer that identifies the file     5  A FILE   gives you the ability to use fscanf and other stdio functions

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opening a file using fopen before we can read or write  information from  to  a file on a disk we must open the file  to open the file we have called the function fopen     1 firstly it searches on the disk the file to be opened  2 then it loads the file from the disk into a place in memory called buffer  3 it sets up a character pointer that points to the first character of the buffer    this the way of behaviour of fopen function there are some causes while buffering process it may timedout  so while comparing fopen high level i o  to open  low level i o  system call   and it is a faster more appropriate than fopen

[c] C fopen vs open

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