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I have a simple program (which uses threads) called ./mpace that uses printf() in the thread function. When I ran it from the terminal, the output is printed as it should, but when I type:

**./mpace > text**

the file created is empty. The funny thing is that yesterday it worked perfectly. I typed:

echo "test" > text

to check if there is a serious problem with this function, but it worked. So, what could be the reason my program unexpectedly started failing to write to file?

Please note I'd prefer not to use fprintf() from my code, since time consumption is essential.

Thanks a lot, the problem was solved simply by using fflush(stdout). I thought printing in a new line would flush the channel, but as paxdiablo explains in a comment here it doesn't:

Why does printf not flush after the call unless a newline is in the format string?

You are right that one should offer as much as information as he can,but I thought that sharing 100 lines of code for a single printf() would be a drag for you to read. I see this was a bad decision, since not using fflush and the fact that the program is being terminated by a signal both contributed to the problem and should have been made known.

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blindeyes
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2 Answers2

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Could be a buffering issue. stdout is line buffered only if it goes to a character device. To check if this is the problem, call fflush(stdout) manually after every printf call. Make sure that the output buffer of stdout is flushed correctly before your program terminates.

fuz
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    Unless the process is terminated by a signal or calling `_exit()`, the buffers are guaranteed to be flushed. – EOF Aug 31 '15 at 09:18
  • @EOF This function is called `_Exit()` in ISO 9899:2011. `_exit()` is a POSIX function from `unistd.h` defined to be equal in behaviour to `_Exit()`. – fuz Sep 21 '15 at 17:18
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Well it could be because you are using > instead of >> and probably somehow your code (which by the way, you didn't share it) contains a newline (blank) and delete the old one. let me show you an Example.

Let say we have the following file called file1.txt which contains the following information:

Michael  
Jimmy
Dolores

And I wont to extract the name Jimmy and redirect the output to another file called file2.txt

cat file1.txt | grep "Jimmy" > file2.txt

Now the file2.txt contains:

Jimmy

Now lets run this command 3-4 times and check that file again:

cat file2.txt

Output:

Jimmy

Why is this ? it should be:

Jimmy
Jimmy
Jimmy
Jimmy

Well becouse i used > instead of >>. Now let's try again:

michi@michi-laptop:~$ cat file1.txt | grep "Jimmy" >> file2.txt 
michi@michi-laptop:~$ cat file1.txt | grep "Jimmy" >> file2.txt 
michi@michi-laptop:~$ cat file1.txt | grep "Jimmy" >> file2.txt 
michi@michi-laptop:~$ cat file1.txt | grep "Jimmy" >> file2.txt

Output:

Jimmy
Jimmy
Jimmy
Jimmy
Jimmy

This could be one of the reasons.

EDIT: I come with another short example, let us modify Jimmy with Timmy:

Michael
Timmy
Dolores

Remember the file2.txt looks like this:

Jimmy
Jimmy
Jimmy
Jimmy
Jimmy

Now if you run:

cat file1.txt | grep "Jimmy" > file2.txt

I just got myself with an empty file because of > instead of >> , there is no Jimmy in file1.txt.

Michi
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