C + + stderr/stdout Redirect to file

Typically, stderr and stdout are used to display output content to the screen, but sometimes we need to write this information to the specified file for easy access at any time. The simplest way to achieve this is to put the output of the Stderr/stdout redirectto the file.
stderr/stdout Redirect to fileThis is illustrated by the stderr code.

#include <stdio.h> #include <stdlib.h>int main (void) {FILE *stream = freopen ("Freopen.out", "W", stderr); if ( stream = = NULL) fprintf (stdout, "error on freopen\n"), else{fprintf (stdout, "successfully reassigned\n"); Fflush (stdo UT); fprintf (Stream, "This would go to the file ' freopen.out ' \ n"); fprintf (stderr, "Also you can do it like this!\n"); FC Lose (stream);} Windwos Read the file Freopen.outsystem ("type Freopen.out"); GetChar (); return 0;}

Execution results are as follows,

the difference between stderr and stdout stdout (standard output), the output mode is row buffering. The output character is stored first in the buffer, and then the actual I/O operation is performed when the ENTER key is pressed. stderr (standard error)is not buffered, which allows the error message to be displayed directly as soon as possible.
description of the buffer:

Full buffer	I/O operations do not occur until the buffer is filled	1. Buffer Full 2. Brush out data (fflush) 3. Close File (fclose)
Row buffers	The actual I/O operation is usually performed only when a newline character is encountered, but the buffer full is also enforced	1. Line breaks encountered 2. Buffer Full 3. Brush out data (fflush) 4. Close file (fclose)
No buffering	Do not cache, direct I/O operation	Direct output

However, as far as buffering is concerned, there is no absolute difference between stdout and stderr because the buffer type can be set. Use the setvbuf () or setbuf () function here.

#include <stdio.h>  #include <stdlib.h>    int main (void)  {      char buf[512] = {0};setbuf ( stderr, buf), fprintf (stderr, "It is Error 1\n");p rintf ("Echo 1\n"), fprintf (stderr, "It is Error 2\n");p rintf ("Echo 2\n"); fprintf (stderr, "It is Error 3\n"); Fflush (stderr); GetChar (); return 0;}  

The results of the operation are as follows:This allows us to define the buffer size. The default size of the buffer is defined by the macro bufsiz in the stdio.h header file, which is 512 bytes. Also, check some information that the minimum can not be less than 256 bytes, but the test example does not have this problem (not yet).


setvbuf () and setbuf ()The setvbuf () function is prototyped as follows:

int setvbuf (FILE * stream, char * buffer, int mode, size_t size);

Setbuf () can be used as a call to Setvbuf (stream, buf, buf? _IOFBF: _IONBF, BUFSIZE);
Where mode is the type of the declaration buffer, as follows:

_IOFBF
_IOLBF	row buffer
_IONBF	No buffering

Size is the buffer size, in bytes.

/* SETVBUF Example */#include <stdio.h>int main () {  FILE *pfile=fopen ("MyFile.txt", "w");  Setvbuf (PFile, NULL, _IOFBF, 1024x768);  File operations here  fclose (pFile);  return 0;}

Reference: http://blog.csdn.net/mycwq/article/details/46554805

C + + stderr/stdout Redirect to file