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Top Forums Programming Forcing a write to a file without newline? Post 302925068 by achenle on Thursday 13th of November 2014 06:25:36 PM
Old 11-13-2014
Quote:
Originally Posted by Don Cragun
... or flush STDOUT after adding the text to the buffer and before the execl().
The "cout" and "stdout" streams may not be the same.

If you want real control over IO streams, C++ "cout" style is NOT the way to do it.

Code:
setbuf( stdout, NULL );
printf( "String to stdout\n" );
fprintf( stderr, "String to stderr\n" );

And even then, if you're doing multithreaded/multiprocess writes to the same file, printf() and fprintf() are NOT atomic calls - the data from multiple calls to printf()/fprintf() can be interleaved. If you want atomic writes where data from a single call is guaranteed to be contiguous at the end of the file, such as for a log file, you need to open the file with standard C open() in O_APPEND mode, format your data yourself, and use write().

And if you're doing your own log files like that, now you have to handle the log file getting too big. It can't be deleted because you have an open file descriptor on it - if someone does try to delete it all they'll do is remove it from the directory and the file will stay on disk until your process closes its descriptor.

There's a reason why library calls such as "syslog()" already exist.

You're not going to come up with a better logging system. Redirecting stdout/stderr is a logging scheme just like a Lego house is a place to live. Sometimes that's OK. For important processes, it's not.
 

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STDIN(3)						     Linux Programmer's Manual							  STDIN(3)

NAME
stdin, stdout, stderr - standard I/O streams SYNOPSIS
#include <stdio.h> extern FILE *stdin; extern FILE *stdout; extern FILE *stderr; DESCRIPTION
Under normal circumstances every UNIX program has three streams opened for it when it starts up, one for input, one for output, and one for printing diagnostic or error messages. These are typically attached to the user's terminal (see tty(4) but might instead refer to files or other devices, depending on what the parent process chose to set up. (See also the "Redirection" section of sh(1).) The input stream is referred to as "standard input"; the output stream is referred to as "standard output"; and the error stream is referred to as "standard error". These terms are abbreviated to form the symbols used to refer to these files, namely stdin, stdout, and stderr. Each of these symbols is a stdio(3) macro of type pointer to FILE, and can be used with functions like fprintf(3) or fread(3). Since FILEs are a buffering wrapper around UNIX file descriptors, the same underlying files may also be accessed using the raw UNIX file interface, that is, the functions like read(2) and lseek(2). On program startup, the integer file descriptors associated with the streams stdin, stdout, and stderr are 0, 1, and 2, respectively. The preprocessor symbols STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO are defined with these values in <unistd.h>. (Applying freopen(3) to one of these streams can change the file descriptor number associated with the stream.) Note that mixing use of FILEs and raw file descriptors can produce unexpected results and should generally be avoided. (For the masochis- tic among you: POSIX.1, section 8.2.3, describes in detail how this interaction is supposed to work.) A general rule is that file descrip- tors are handled in the kernel, while stdio is just a library. This means for example, that after an exec(3), the child inherits all open file descriptors, but all old streams have become inaccessible. Since the symbols stdin, stdout, and stderr are specified to be macros, assigning to them is nonportable. The standard streams can be made to refer to different files with help of the library function freopen(3), specially introduced to make it possible to reassign stdin, std- out, and stderr. The standard streams are closed by a call to exit(3) and by normal program termination. CONFORMING TO
The stdin, stdout, and stderr macros conform to C89 and this standard also stipulates that these three streams shall be open at program startup. NOTES
The stream stderr is unbuffered. The stream stdout is line-buffered when it points to a terminal. Partial lines will not appear until fflush(3) or exit(3) is called, or a newline is printed. This can produce unexpected results, especially with debugging output. The buffering mode of the standard streams (or any other stream) can be changed using the setbuf(3) or setvbuf(3) call. Note that in case stdin is associated with a terminal, there may also be input buffering in the terminal driver, entirely unrelated to stdio buffering. (Indeed, normally terminal input is line buffered in the kernel.) This kernel input handling can be modified using calls like tcse- tattr(3); see also stty(1), and termios(3). SEE ALSO
csh(1), sh(1), open(2), fopen(3), stdio(3) COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2008-07-14 STDIN(3)
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