Confusion about strings in C

Question

Char arrays have continuously confused me in C.

Here is the following code:

char tcp_port[100], udp_port[6];
tcp_port[99] = '\0'; udp_port[5] = '\0';
fscanf(fp, " tcp_port=%s", tcp_port);
fscanf(fp, " udp_port=%s", udp_port);
printf("%s\n", tcp_port); printf("%s\n", udp_port);

This works and prints out the right number. However, since tcp_port has 100 elements, how do those just disappear when printing? The port is only 5 characters long and the last element is null terminated. Does printf just ignore those unintialized elements, and do those uninitialized elements contain random data?

`do those uninitialized elements contain random data` yes, these elements contain random data — Iłya Bursov, Mar 20 '15 at 21:54
I explain what C strings are in [Arbitrary length string in C](http://stackoverflow.com/a/20918363/2157640). — Palec, Mar 20 '15 at 21:58
@Lashane: Strictly speaking, they don't contain *random* data, they contain *arbitrary* data. — Keith Thompson, Mar 20 '15 at 22:00
@KeithThompson agree, very important difference, especially for this question. poteito potato — Iłya Bursov, Mar 20 '15 at 22:05
The C standard defines a *string* as "a contiguous sequence of characters terminated by and including the first null character." — Keith Thompson, Mar 20 '15 at 22:12

Vilx- · Accepted Answer · 2015-03-20T21:58:47.357

4

Yes, printf() only prints the characters up to the first \0 character. All C string functions do this. They also automatically append that \0 character when necessary, like the scanf() function there. That's why it's called a "0-terminated string".

The other elements can contain anything and they will be completely ignored. In practice, they usually contain random junk, but it depends on a variety of factors.

Note that when you allocate memory you must keep that \0 character in mind. Your tcp_port string can only at most 99 characters, because the last one must be 0.

edited Mar 20 '15 at 21:58

answered Mar 20 '15 at 21:53

Vilx-

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Another point here is that `fscanf(3)` puts `\0` to the end of the scanned string. – holgac Mar 20 '15 at 21:56

Confusion about strings in C

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