These days I was learning the "apue", a result of an typical case confused me. The following are the sample codes of "sample.c":
#include "apue.h"
#include <stdio.h>
#define BUFF_SZ 4096
int main()
{
int n = 0;
char buff[BUFF_SZ] = {'\0'};
while ((n = read(STDIN_FILENO, buff, BUFF_SZ)) > 0) {
printf("read %d bytes\n", n);
if (write(STDOUT_FILENO, buff, n) != n) {
err_sys("write error");
}
}
if (n < 0) {
err_sys("read error");
}
return 0;
}
After compilation gcc sample.c, you can use this command echo Hello | ./a.out and get the following std output on terminal:
read 6 bytesHello
However, if you redirect the output to a file echo Hello | ./a.out > outfile, then use cat outfile to see the content:
Helloread 6 bytes
The ouput changes order after redirection! I wonder if some one could tell me the reason?
For the standard I/O function printf, when you output to a terminal, the standard output is by default line buffered.
printf("read %d bytes\n", n);
\n here cause the output to flush.
However, when you output to a file, it's by default fully buffered. The output won't flush unless the buffer is full, or you explicitly flush it.
The low level system call write, on the other hand, is unbuffered.
In general, intermixing standard I/O calls with system calls is not advised.
printf(), by default, buffers its output, while write() does not, and there is no synchronisation between then.
So, in your code, it is possible that printf() stores its data in a buffer and returns, then write() is called, and - as main() returns, printf()s buffer is flushed so that buffered output appears. From your description, that is happening when output is redirected.
It is also possible that printf() writes data immediately, then write() is called. From your description, that happens when output is not redirected.
Typically, one part of redirection of a stream is changing the buffer - and therefore the behaviour when buffering - for streams like stdout and stdin. The precise change depends on what type of redirection is happening (e.g. to a file, to a pipe, to a different display device, etc).
Imagine that printf() writes data to a buffer and, when flushing that buffer, uses write() to produce output. That means all overt calls of write() will have their output produced immediately, but data that is buffered may be printed out of order.
The problem is that the writes are handled by write(2) call, so you effectively lose control of what happens.
If we look at the documentation for write(2) we can see that the writes are not guaranteed to be actually written until a read() occurs. More specifically:
A successful return from write() does not make any guarantee that data has
been committed to disk. In fact, on some buggy implementations, it does not even
guarantee that space has successfully been reserved for the data. The only way to
be sure is to call fsync(2) after you are done writing all your data.
This means that depending on the implementation and buffering of the write(2) (which may differ even between redirects and printing to screen), you can get different results.
Related
#include <stdio.h>
#define MAXLEN 256
int main() {
int n;
char buf[MAXLEN];
while((n = read(0,buf,sizeof(buf))) != 0){
printf("n: %d:",n);
write(1,buf,n);
}
return 1;
}
The output of the program (where the first read and first write is typed by the user and echoed by the terminal) is:
read
read
write
write
n: 5:n: 6:
The output of printf comes after pressing Ctrl+D at the standard input and not along with the subsequent reads. Why does this happen?
Printf is buffered.
You can force printf to 'flush' its buffer using the fflush call:
#include <stdio.h>
#define MAXLEN 256
int main() {
int n;
char buf[MAXLEN];
while((n = read(0,buf,sizeof(buf))) != 0){
printf("n: %d:",n);
fflush(stdout); /* force it to go out */
write(1,buf,n);
}
return 1;
}
In general, printf() being buffered is a good thing. Unbuffered output, particularly to visible consoles that require screen updates and such, is slow. Slow enough that an application that is printf'ing a lot can be directly slowed down by it (especially on the Windows platform; Linux and unixes are typically impacted less).
However, printf() being buffered does bite you if you also fprintf(stderr,) - stderr is deliberately unbuffered. As a consequence, you may get your messages with some printf() missing; if you write to another FILE handle that is also associated with the terminal, and might be unbuffered, make sure you first explicitly fflush(stdout).
The manpage for fgets tells me:
It is not advisable to mix calls to input functions from the stdio
library with low-level calls to read(2) for the file descriptor associ‐
ated with the input stream; the results will be undefined and very
probably not what you want.
So the best solution would be not to to use write and printf on the same descriptor.
Printf is using stdio and it is buffered.
Push it out by sending a changing to "n: %d:\n"
You can use the std fflush() function to flush the std out buffer or you can make use of an additional \n at the end of the control string inside the printf. Something like this
printf("\n :%d:\n",n);
Its always better to use the write() & read() functions in C instead of printf() and scanf(). Printf and scanf have got some problems like printf stores the string parameter in stdout buffer. So a manual flush is required which is done through fflush function or by means of \n. In a small hello world printing program you will not find such a problem as the stdout buffer is flushed at the end of the program execution. Better use write() which works fine. scanf also have the problem of reading spaces and a lot of other problems related to stdin buffer.
For example in the code below:
main() { char a; int i=0,c; for(;i<2;i++) { scanf("%d",&c); scanf("%c",&a);} }
The above program as got the problem of reading \n into stdin on pressing enter. We could resolve this but not flushing the stdin buffer or making use of \n character. Always better to use the read() and write() functions.
Hope that helps....
Use fwrite (streams version) rather than write.
Note that, while is associated with file number 1, it isn't the same thing.
I was writing a small program that had various console output strings depending upon different events. As I was looking up the best way to send these messages I came across something that was a bit confusing.
I have read that stderr is used to shoot messages directly to the console - not buffered. While, in contrast, I read that stdout is buffered and is typically used to redirect messages to various streams?, that may or may not be error messages, to an output file or some other medium.
What is the difference when something is said to be buffered and not buffered? It made sense when I was reading that the message is shot directly to the output and is not buffered .. but at the same time I realized that I was not entirely sure what it meant to be buffered.
Typically, stdout is line buffered, meaning that characters sent to stdout "stack up" until a newline character arrives, at which point that are all outputted.
A buffered stream is one in which you keep writing until a certain threshold. This threshold could be a specific character, as Konrad mentions for line buffering, or another threshold, such as a specific count of characters written.
Buffering is intended to speed up input/output operations. One of the slowest things a computer does is write to a stream (whether a console or a file). When things don't need to be immediately seen, it saves time to store it up for a while.
You are right, stderr is typically an unbuffered stream while stdout typically is buffered. So there can be times when you output things to stdout then output to stderr and stderr appears first on the console. If you want to make stdout behave similarly, you would have to flush it after every write.
When an output stream is buffered, it means that the stream doesn't necessarily output data the moment you tell it to. There can be significant overhead per IO operation, so lots and lots of little IO operations can create a bottleneck. By buffering IO operations and then flushing many at once, this overhead is reduced.
While stdout and stderr may behave differently regarding buffering, that is generally not the deciding factor between them and shouldn't be relied on. If you absolutely need the output immediately, always manually flush the stream.
Assume
int main(void)
{
printf("foo\n");
sleep(10);
printf("bar\n");
}
when executing it on the console
$ ./a.out
you will see foo line and 10 seconds later bar line (--> line buffered). When redirecting the output into a file or a pipe
$ ./a.out > /tmp/file
the file stays empty (--> buffered) until the program terminates (--> implicit fflush() at exit).
When lines above do not contain a \n, you won't see anything on the console either until program terminates.
Internally, printf() adds a character to a buffer. To make things more easy, let me describe fputs(char const *s, FILE *f) instead of. FILE might be defined as
struct FILE {
int fd; /* is 0 for stdin, 1 for stdout, 2 for stderr (usually) */
enum buffer_mode mode;
char buf[4096];
size_t count; /* number of chars in buf[] */
};
typedef struct FILE *FILE;
int fflush(FILE *f)
{
write(f->fd, f->buf, f->count);
f->count = 0;
}
int fputc(int c, FILE *f)
{
if (f->count >= ARRAY_SIZE(f->buf))
fflush(f);
f->buf[f->count++] = c;
}
int fputs(char const *s, FILE *f)
{
while (*s) {
char c = *s++;
fputc(c, f);
if (f->mode == LINE_BUFFERED && c == '\n')
fflush(f);
}
if (f->mode == UNBUFFERED)
fflush(f);
}
#include<stdio.h>
#include <unistd.h>
int main(){
while(1)
{
fprintf(stdout,"hello-out");
fprintf(stderr,"hello-err");
sleep(1);
}
return 0;
}
On compiling this programme in gcc and on executing it only prints hello-err and not hello-out.Why is that so?Can someone please explain the reason behind it?
If you add a '\n' to your message it will (or should), ie. "hello-out\n".
The reason being is that stdout is buffered in order to be more efficient, whereas stderr doesn't buffer it's output and is more appropriate for error messages and things that need to be printed immediately.
stdout will usually be flushed when:
A newline (\n) is to be printed
You read in from stdin
fflush() is called on it
EDIT: The other thing I wanted to add before my computer crashed...twice...was that you can also use setbuf(stdout, NULL); to disable buffering of stdout. I've done that before when I've had to use write() (Unix) and didn't want my output to be buffered.
It doesn't always print out the output to stdout because by design stdout is BUFFERED output, and stderr is unbuffered. In general, the for a buffered output stream, the stream is not dumped until the system is "free" to do so. So data can continue buffering for a long while, before it gets flushed. If you want to force the buffer to flush you can do so by hand using fflush
#include<stdio.h>
#include <unistd.h>
int main(){
while(1)
{
fprintf(stdout,"hello-out");
fflush(stdout); /* force flush */
fprintf(stderr,"hello-err");
sleep(1);
}
return 0;
}
Update: stdout is linebuffered when connected to a terminal, and simply buffered otherwise (e.g. a redirect or a pipe)
You forgot newlines (noted \n) in your strings. Or you need to call fflush(NULL); or at least fflush(stdout); before sleep(1);
And fprintf(stdout, ...) is the same as printf(...)
You need to output newlines or to call fflush because (at least on Linux) the stdout FILE buffer is line-buffered. This means that the C library is buffering data, and will really output it (using the write Linux system call) when the buffer is full enough, or when you flush it either with a new line, or by calling fflush. Buffering is needed because system calls are costly (calling write for every byte to be output is really too slow). Read also the man page of setbuf
have a look at this code:
#include<stdio.h>
#include <unistd.h>
int main()
{
int pipefd[2],n;
char buf[100];
if(pipe(pipefd)<0)
printf("Pipe error");
printf("\nRead fd:%d write fd:%d\n",pipefd[0],pipefd[1]);
if(write(pipefd[1],"Hello Dude!\n",12)!=12)
printf("Write error");
if((n=read(pipefd[0],buf,sizeof(buf)))<=0)
printf("Read error");
write(1,buf,n);
return 0;
}
I expect the printf to print Read fd and write fd before Hello Dude is read from the pipe. But thats not the case... see here. When i tried the same program in our college computer lab my output was
Read fd:3 write fd:4
Hello Dude!
also few of our friends observed that, changing the printf statement to contain more number of \n characters changed the output order... for example..printf("\nRead fd:%d\n write fd:%d\n",pipefd[0],pipefd[1]); meant that Read fd is printed then the message Hello Dude! then the write fd is printed. What is this behaviour??
Note: Out lab uses a linux server on which we run terminals, i don't remember the compiler version though.
It's because printf to the standard output stream is buffered but write to the standard output file descriptor is not.
That means the behaviour can change based on what sort of buffering you have. In C, standard output is line buffered if it can be determined to be connected to an interactive device. Otherwise it's fully buffered (see here for a treatise on why this is so).
Line buffered means it will flush to the file descriptor when it sees a newline. Fully buffered means it will only flush when the buffer fills (for example, 4K worth of data), or when the stream is closed (or when you fflush).
When you run it interactively, the flush happens before the write because printf encounters the \n and flushes automatically.
However, when you run it otherwise (such as by redirecting output to a file or in an online compiler/executor where it would probably do the very same thing to capture data for presentation), the flush happens after the write (because printf is not flushing after every line).
In fact, you don't need all that pipe stuff in there to see this in action, as per the following program:
#include <stdio.h>
#include <unistd.h>
int main (void) {
printf ("Hello\n");
write (1, "Goodbye\n", 8);
return 0;
}
When I execute myprog ; echo === ; myprog >myprog.out ; cat myprog.out, I get:
Hello
Goodbye
===
Goodbye
Hello
and you can see the difference that the different types of buffering makes.
If you want line buffering regardless of redirection, you can try:
setvbuf (stdin, NULL, _IOLBF, BUFSIZ);
early on in your program - it's implementation defined whether an implementation supports this so it may have no effect but I've not seen many where it doesn't work.
You shouldn't mix calls to write and printf on single file descriptor. Change write to fwrite.
Functions which use FILE are buffered. Functions which use file descriptors are not. This is why you may get mixed order.
You can also try calling fflush before write.
When you write onto the same file, or pipe, or whatever by two means at once (direct IO and output stream) you can get this behaviour. The reason is that the output stream is buffered.
With fflush() you can control that behaviour.
What is happening is that printf writes to stdout in a buffered way -- the string is kept in a buffer before being output -- while the 'write' later on writes to stdout unbuffered. This can have the effect that the output from 'write' appears first if the buffer from the printf is only flushed later on.
You can explicitly flush using fflush() -- but even better would be not to mix buffered and non-buffered writes to the same output. Type man printf, man fflush, man fwrite etc. on your terminal to learn more about what these commands do exactly.
#include <stdio.h>
#define MAXLEN 256
int main() {
int n;
char buf[MAXLEN];
while((n = read(0,buf,sizeof(buf))) != 0){
printf("n: %d:",n);
write(1,buf,n);
}
return 1;
}
The output of the program (where the first read and first write is typed by the user and echoed by the terminal) is:
read
read
write
write
n: 5:n: 6:
The output of printf comes after pressing Ctrl+D at the standard input and not along with the subsequent reads. Why does this happen?
Printf is buffered.
You can force printf to 'flush' its buffer using the fflush call:
#include <stdio.h>
#define MAXLEN 256
int main() {
int n;
char buf[MAXLEN];
while((n = read(0,buf,sizeof(buf))) != 0){
printf("n: %d:",n);
fflush(stdout); /* force it to go out */
write(1,buf,n);
}
return 1;
}
In general, printf() being buffered is a good thing. Unbuffered output, particularly to visible consoles that require screen updates and such, is slow. Slow enough that an application that is printf'ing a lot can be directly slowed down by it (especially on the Windows platform; Linux and unixes are typically impacted less).
However, printf() being buffered does bite you if you also fprintf(stderr,) - stderr is deliberately unbuffered. As a consequence, you may get your messages with some printf() missing; if you write to another FILE handle that is also associated with the terminal, and might be unbuffered, make sure you first explicitly fflush(stdout).
The manpage for fgets tells me:
It is not advisable to mix calls to input functions from the stdio
library with low-level calls to read(2) for the file descriptor associ‐
ated with the input stream; the results will be undefined and very
probably not what you want.
So the best solution would be not to to use write and printf on the same descriptor.
Printf is using stdio and it is buffered.
Push it out by sending a changing to "n: %d:\n"
You can use the std fflush() function to flush the std out buffer or you can make use of an additional \n at the end of the control string inside the printf. Something like this
printf("\n :%d:\n",n);
Its always better to use the write() & read() functions in C instead of printf() and scanf(). Printf and scanf have got some problems like printf stores the string parameter in stdout buffer. So a manual flush is required which is done through fflush function or by means of \n. In a small hello world printing program you will not find such a problem as the stdout buffer is flushed at the end of the program execution. Better use write() which works fine. scanf also have the problem of reading spaces and a lot of other problems related to stdin buffer.
For example in the code below:
main() { char a; int i=0,c; for(;i<2;i++) { scanf("%d",&c); scanf("%c",&a);} }
The above program as got the problem of reading \n into stdin on pressing enter. We could resolve this but not flushing the stdin buffer or making use of \n character. Always better to use the read() and write() functions.
Hope that helps....
Use fwrite (streams version) rather than write.
Note that, while is associated with file number 1, it isn't the same thing.