Confusing Output From C Program

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I have a C program that compiles to an executable called myprogram. This is its main function:

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 system("ls");

 return 0;

When I run myprogram > output.txt in a Linux shell and then examine output.txt, I see the output of ls listed above "this is a test message."

I feel like it should be the other way around. Why is this happening, and what can I do so that "this is a test message" appears at the top of output.txt?

If it matters, I'm new to both C and working in a command line.

asked 3 hours ago

Archr

1575

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up vote
6
down vote

favorite

I have a C program that compiles to an executable called myprogram. This is its main function:

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 system("ls");

 return 0;

When I run myprogram > output.txt in a Linux shell and then examine output.txt, I see the output of ls listed above "this is a test message."

I feel like it should be the other way around. Why is this happening, and what can I do so that "this is a test message" appears at the top of output.txt?

If it matters, I'm new to both C and working in a command line.

asked 3 hours ago

Archr

1575

add a commentÂ |Â

up vote
6
down vote

favorite

I have a C program that compiles to an executable called myprogram. This is its main function:

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 system("ls");

 return 0;

When I run myprogram > output.txt in a Linux shell and then examine output.txt, I see the output of ls listed above "this is a test message."

I feel like it should be the other way around. Why is this happening, and what can I do so that "this is a test message" appears at the top of output.txt?

If it matters, I'm new to both C and working in a command line.

asked 3 hours ago

Archr

1575

I have a C program that compiles to an executable called myprogram. This is its main function:

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 system("ls");

 return 0;

When I run myprogram > output.txt in a Linux shell and then examine output.txt, I see the output of ls listed above "this is a test message."

I feel like it should be the other way around. Why is this happening, and what can I do so that "this is a test message" appears at the top of output.txt?

If it matters, I'm new to both C and working in a command line.

c linux io-redirection

asked 3 hours ago

Archr

1575

asked 3 hours ago

Archr

1575

asked 3 hours ago

Archr

1575

asked 3 hours ago

Archr

1575

asked 3 hours ago

Archr

1575

add a commentÂ |Â

3 Answers
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up vote
9
down vote

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By default output to stdout is line-buffered. That is, the buffer is flushed when it's full or when you add a newline.

However that's the default, when stdout is connected to a terminal. If stdout is not connected to a terminal, like what happens when you redirect the output from your program, then stdout becomes fully buffered. That means the buffer will be flushed and actually written either when it's full or when explicitly flushed (which happens when the program exits).

This means that the output of a separate process started from your code (like what happens when you call system) will most likely be written first, since the buffer of that process will be flushed when that process ends, which is before your own process.

What happens when using redirection (or pipes for that matter):

Your printf call writes to the stdout buffer.

The system function starts a new process, which writes to its own buffer.

When the external process (started by your system call) exits, its buffer is flushed and written. Your own buffer in your own process, isn't touched.

Your own process ends, and your stdout buffer is flushed and written.

To get the output in the "correct" (or at least expected) order, call fflush before calling system, to explicitly flush stdout.

edited 2 hours ago

answered 3 hours ago

Some programmer dude

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up vote
3
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I suspect it's because of the order in which the stdout buffer gets flushed, which is not necessarily deterministic. It's possible that the parent spawns the ls process and doesn't flush its own stdout until after that returns. It may not actually flush stdout until the process exits.

Try adding fflush (stdout) after the printf statement and see if that forces the output to appear first.

answered 3 hours ago

ConcernedOfTunbridgeWells

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up vote
3
down vote

It is related to output buffering. I managed to reproduce the same behaviour. Forcing the flush did it for me.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 fflush(stdout);
 system("ls");

 return 0;

Before adding the fflush:

$ ./main > foo
$ cat foo
main
main.c
this is a test message.

and after:

$ ./main > foo
$ cat foo
this is a test message.
foo
main
main.c

answered 3 hours ago

Aif

8,74912335

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3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

up vote
9
down vote

accepted

By default output to stdout is line-buffered. That is, the buffer is flushed when it's full or when you add a newline.

What happens when using redirection (or pipes for that matter):

Your printf call writes to the stdout buffer.

The system function starts a new process, which writes to its own buffer.

When the external process (started by your system call) exits, its buffer is flushed and written. Your own buffer in your own process, isn't touched.

Your own process ends, and your stdout buffer is flushed and written.

To get the output in the "correct" (or at least expected) order, call fflush before calling system, to explicitly flush stdout.

edited 2 hours ago

answered 3 hours ago

Some programmer dude

283k23231387

add a commentÂ |Â

up vote
9
down vote

accepted

By default output to stdout is line-buffered. That is, the buffer is flushed when it's full or when you add a newline.

What happens when using redirection (or pipes for that matter):

Your printf call writes to the stdout buffer.

The system function starts a new process, which writes to its own buffer.

When the external process (started by your system call) exits, its buffer is flushed and written. Your own buffer in your own process, isn't touched.

Your own process ends, and your stdout buffer is flushed and written.

To get the output in the "correct" (or at least expected) order, call fflush before calling system, to explicitly flush stdout.

edited 2 hours ago

answered 3 hours ago

Some programmer dude

283k23231387

add a commentÂ |Â

up vote
9
down vote

accepted

By default output to stdout is line-buffered. That is, the buffer is flushed when it's full or when you add a newline.

What happens when using redirection (or pipes for that matter):

Your printf call writes to the stdout buffer.

The system function starts a new process, which writes to its own buffer.

When the external process (started by your system call) exits, its buffer is flushed and written. Your own buffer in your own process, isn't touched.

Your own process ends, and your stdout buffer is flushed and written.

To get the output in the "correct" (or at least expected) order, call fflush before calling system, to explicitly flush stdout.

edited 2 hours ago

answered 3 hours ago

Some programmer dude

283k23231387

By default output to stdout is line-buffered. That is, the buffer is flushed when it's full or when you add a newline.

What happens when using redirection (or pipes for that matter):

Your printf call writes to the stdout buffer.

The system function starts a new process, which writes to its own buffer.

When the external process (started by your system call) exits, its buffer is flushed and written. Your own buffer in your own process, isn't touched.

Your own process ends, and your stdout buffer is flushed and written.

To get the output in the "correct" (or at least expected) order, call fflush before calling system, to explicitly flush stdout.

edited 2 hours ago

answered 3 hours ago

Some programmer dude

283k23231387

edited 2 hours ago

answered 3 hours ago

Some programmer dude

283k23231387

answered 3 hours ago

Some programmer dude

283k23231387

answered 3 hours ago

Some programmer dude

283k23231387

add a commentÂ |Â

up vote
3
down vote

Try adding fflush (stdout) after the printf statement and see if that forces the output to appear first.

answered 3 hours ago

ConcernedOfTunbridgeWells

50.7k13123185

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up vote
3
down vote

Try adding fflush (stdout) after the printf statement and see if that forces the output to appear first.

answered 3 hours ago

ConcernedOfTunbridgeWells

50.7k13123185

add a commentÂ |Â

up vote
3
down vote

Try adding fflush (stdout) after the printf statement and see if that forces the output to appear first.

answered 3 hours ago

ConcernedOfTunbridgeWells

50.7k13123185

Try adding fflush (stdout) after the printf statement and see if that forces the output to appear first.

answered 3 hours ago

ConcernedOfTunbridgeWells

50.7k13123185

answered 3 hours ago

ConcernedOfTunbridgeWells

50.7k13123185

answered 3 hours ago

ConcernedOfTunbridgeWells

50.7k13123185

answered 3 hours ago

ConcernedOfTunbridgeWells

50.7k13123185

add a commentÂ |Â

up vote
3
down vote

It is related to output buffering. I managed to reproduce the same behaviour. Forcing the flush did it for me.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 fflush(stdout);
 system("ls");

 return 0;

Before adding the fflush:

$ ./main > foo
$ cat foo
main
main.c
this is a test message.

and after:

$ ./main > foo
$ cat foo
this is a test message.
foo
main
main.c

answered 3 hours ago

Aif

8,74912335

add a commentÂ |Â

up vote
3
down vote

It is related to output buffering. I managed to reproduce the same behaviour. Forcing the flush did it for me.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 fflush(stdout);
 system("ls");

 return 0;

Before adding the fflush:

$ ./main > foo
$ cat foo
main
main.c
this is a test message.

and after:

$ ./main > foo
$ cat foo
this is a test message.
foo
main
main.c

answered 3 hours ago

Aif

8,74912335

add a commentÂ |Â

up vote
3
down vote

It is related to output buffering. I managed to reproduce the same behaviour. Forcing the flush did it for me.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 fflush(stdout);
 system("ls");

 return 0;

Before adding the fflush:

$ ./main > foo
$ cat foo
main
main.c
this is a test message.

and after:

$ ./main > foo
$ cat foo
this is a test message.
foo
main
main.c

answered 3 hours ago

Aif

8,74912335

It is related to output buffering. I managed to reproduce the same behaviour. Forcing the flush did it for me.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char ** argv) 
 printf("this is a test message.n");
 fflush(stdout);
 system("ls");

 return 0;

Before adding the fflush:

$ ./main > foo
$ cat foo
main
main.c
this is a test message.

and after:

$ ./main > foo
$ cat foo
this is a test message.
foo
main
main.c

answered 3 hours ago

Aif

8,74912335

answered 3 hours ago

Aif

8,74912335

answered 3 hours ago

Aif

8,74912335

answered 3 hours ago

Aif

8,74912335

add a commentÂ |Â

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