Printf a buffer of char with length in C

Question

I have a buffer which I receive through a serial port. When I receive a certain character, I know a full line has arrived, and I want to print it with printf method. But each line has a different length value, and when I just go with:

printf("%s", buffer);

I'm printing the line plus additional chars belonging to the former line (if it was longer than the current one).

I read here that it is possible, at least in C++, to tell how much chars you want to read given a %s, but it has no examples and I don't know how to do it in C. Any help?

I think I have three solutions:

printing char by char with a for loop
using the termination character
or using .*

QUESTION IS: Which one is faster? Because I'm working on a microchip PIC and I want it to happen as fast as possible

When you've received that *"certain character"* and have appended it to `buffer`, append a null character (`\0`) as well. This "tells" `sprintf()` when to stop printing characters. — Linus Kleen, Nov 17 '11 at 16:31

Soren · Answer 1 · 2011-11-17T16:53:40.050

62

You can either add a null character after your termination character, and your printf will work, or you can add a '.*' in your printf statement and provide the length

printf("%.*s",len,buf);

In C++ you would probably use the std::string and the std::cout instead, like this:

std::cout << std::string(buf,len);

If all you want is the fastest speed and no formatting -- then use

fwrite(buf,1,len,stdout);

edited Nov 17 '11 at 16:53

answered Nov 17 '11 at 16:32

Soren

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Perfect. So I have three solutions: printing char by char with a for loop, using the termination character, or using .* QUESTION IS: which one is faster? Because I'm working on a microchip PIC and I want it to happen fast – Roman Rdgz Nov 17 '11 at 16:42
@RomanRdgz: my guesses: `fwrite` or null-termination+`fputs` should be the fastest, followed by null-termination+`printf` (could be slower because it may have to parse the format string, unless the compiler is very smart) then `printf` with `.*` (surely needs to parse the format string) and last printing char by char (probably the slowest, because of all the function calls - that however could be inlined). – Matteo Italia Nov 17 '11 at 16:47
Also, your C++ version is quite inefficient, you are allocating/deallocating stuff on the heap (constructing an instance of `std::string`) when a simple call to `cout.write` would be enough. Not that it would matter once in a while on a normal machine, but with very limited resources or in a tight loop it may be noticeable. – Matteo Italia Nov 17 '11 at 16:48
I'm using CCS compiler, and I don't think it has cout.write. That's why I'm using fputs or printf. Thanks for your info @Matteo Italia – Roman Rdgz Nov 17 '11 at 16:50
@RomanRdgz: if you have `cout` you'll surely have `cout.write`, anyhow that was just a comment about the C++ solution of @Soren, for C solutions I already wrote my answer :) . – Matteo Italia Nov 17 '11 at 16:51
If all you want is the fastest speed and no formatting -- then use `fwrite(buf,1,len,stdout)` – Soren Nov 17 '11 at 16:52
PIC - ugh! Anyway, since you know you are at the end of the line, shove a null in and avoid the printf having to count, (if only to keep the format parsing to a minimum), - that's probably fastest. What are you printing to anyway? – Martin James Nov 17 '11 at 16:55
I am printting to a serial port, with fprintf/fputs/fputc – Roman Rdgz Nov 17 '11 at 16:57

Matteo Italia · Accepted Answer · 2011-11-17T16:42:14.890

The string you have is not null-terminated, so, printf (and any other C string function) cannot determine its length, thus it will continue to write the characters it finds there until it stumbles upon a null character that happens to be there.

To solve your problem you can either:

use fwrite over stdout:
```
fwrite(buffer, buffer_length, 1, stdout);
```
This works because fwrite is not thought for printing just strings, but any kind of data, so it doesn't look for a terminating null character, but accepts the length of the data to be written as a parameter;
null-terminate your buffer manually before printing:
```
buffer[buffer_length]=0;
printf("%s", buffer); /* or, slightly more efficient: fputs(buffer, stdout); */
```
This could be a better idea if you have to do any other string processing over buffer, that will now be null-terminated and so manageable by normal C string processing functions.

I am surprised that answers to duplicates of this SO question do not include this method, but only suggest `for` loops... — Alex, Jul 05 '17 at 13:45

score 3 · Answer 3 · answered Nov 17 '11 at 16:31

3

Once you've identified the end of the line, you must append a '\0' character to the end of the buffer before sending it to printf.

answered Nov 17 '11 at 16:31

Michael Price

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Anyone who has downvoted want to explain why so that I can improve my answer? – Michael Price Nov 22 '11 at 16:05
You should say what the OP did wrong, in addition to how to correct it. – Alex Jul 05 '17 at 13:46

score 3 · Answer 4 · answered Nov 17 '11 at 16:33

3

You can put a NUL (0x0) in the buffer after receiving the last character.

buffer[i] = 0;

answered Nov 17 '11 at 16:33

Gowtham

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Printf a buffer of char with length in C

4 Answers4