写入文件

Question

I have a very simple question regarding file write.

I have this program:

char buf[20];
size_t nbytes;

strcpy(buf, "All that glitters is not gold\n");
fd= open("test_file.txt",O_WRONLY);
write(fd,buf,strlen(buf));
close(fd);

What am confused is when I open the file test_file.txt after running this program I see some characters like ^C^@^@^@^^^@ after the line "All that glitters is not": Notice that portion of the buf is not written and those characters appear instead. Why is that so?

Answer 1

You're writing more than 19 chars in that buffer. Once you've done that, the behavior of your program is undefined. It could do whatever it wants.

Allocate a large enough buffer. It has to be able to fit all the letters plus a terminating 0 if you need to be able to treat it as a C string.

Answer 2

The string "All that glitters is not gold\\n" is longer than 20 characters. I suggest you try it with a larger buffer.

Actually, if you're going to do any nontrivial work in CI suggest you never ever use strcpy , as a general habit. Use functions like strncpy which let you specify a buffer size so that it's clear you'll never overflow.

Answer 3

libgcc strcpy Manual says:

If the destination string of a strcpy() is not large enough (that is, if the programmer was stupid or lazy, and failed to check the size before copying) then anything might happen. Overflowing fixed length strings is a favorite cracker technique.

Also the strlen says

The strlen() function calculates the length of the string s, not including the terminating '\\0' character.

So i guess strlen () does not return what you expect it to return and as a result the extra characters are written

To make the thing work, you need to allocate a large enough buffer, which can hold the entire string.