How can I print a non-null-terminated string using printf, assuming that I know the length of the string at runtime?
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6If it's not null terminated, then by definition it's not a string.Keith Thompson– Keith Thompson2014-08-04 03:36:45 +00:00Commented Aug 4, 2014 at 3:36
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9it's only not a c string, as it would be a valid string in other languages.. and though a "non-null-terminated array of char" would be more accurate, I'm pretty sure it was universally understood.Wyllow Wulf– Wyllow Wulf2016-07-29 19:58:19 +00:00Commented Jul 29, 2016 at 19:58
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2Possible duplicate of Using printf with a non-null terminated stringflogram_dev– flogram_dev2017-04-01 19:06:36 +00:00Commented Apr 1, 2017 at 19:06
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2 Answers
printf("%.*s", length, string);
Use together with other args:
printf("integer=%d, string=%.*s, number=%f", integer, length, string, number);
// ^^^^ ^^^^^^^^^^^^^^
In C you could specify the maximum length to output with the %.123s format. This means the output length is at most 123 chars. The 123 could be replaced by *, so that the length will be taken from the argument of printf instead of hard-coded.
Note that this assumes the string does not contain any interior null bytes (\0), as %.123s only constrains the maximum length not the exact length, and strings are still treated as null-terminated.
If you want to print a non-null-terminated string with interior null, you cannot use a single printf. Use fwrite instead:
fwrite(string, 1, length, stdout);
See @M.S.Dousti's answer for detailed explanation.
1 Comment
M.M
note:
length must have type int here, after default argument promotions. If your length variable is something bigger (e.g. size_t) you should cast it to (int) else your code will break on a platform where size_t is wider than int.The answer provided by @KennyTM is great, but with a subtlety.
In general, if the string is non-null "terminated", but has a null character in the middle, printf("%.*s", length, string); does not work as expected. This is because the %.*s format string asks printf to print a maximum of length characters, not exactly length characters.
I'd rather use the more general solution pointed out by @William Pursell in a comment under the OP:
fwrite(string, sizeof(char), length, stdout);
Here's a sample code:
#include <stdio.h>
int main(void) {
size_t length = 5;
char string[length];
string[0] = 'A';
string[1] = 'B';
string[2] = 0; // null character in the middle
string[3] = 'C';
string[4] = 'D';
printf("With printf: %.*s\n", length, string);
printf("With fwrite: ");
fwrite(string, sizeof(char), length, stdout);
printf("\n");
return 0;
}
Output:
With printf: AB
With fwrite: AB CD
4 Comments
Keith Thompson
sizeof (char) is 1 by definition. Also, NULL is specifically a null pointer constant.
Ivan Black
@KeithThompson,
sizeof (char) is more semantic. Also, '\0' is more semantic than 0. But who cares ;-)
Matthew Moisen
My output for this is
With fwrite: AB - it seems to be stopping at the null terminator. Compiled with gcc 4.8.5
Sadeq Dousti
@MatthewMoisen: See Godbolt output for your compiler.