C语言中带有换行符的printf的怪异行为

Question

This could be simple or lame question. I have been playing with printf function in C++. When I execute following code on Debian Wheezy with any 9 character argument (eg:"argument1") ;

#include<stdio.h>

int main(int argc, char** argv){

    printf(argv[1]+'\n');

}

the out put was;

SSH_AGENT_PID=4375

Then I checked the environment variable $SSH_AGENT_PID and its value is 4375.

Could you please tell me what is going on with my code ? (I tried to examine the disassembled code in gdb. But my assembly knowledge is poor to understand exactly whats going on)

Answer 1

Simple fix:

printf("%s\n", argv[1]);

This is an improvement on your code for two reasons: First, it's generally a good idea to have the first argument to printf be a string constant, as this prevents printf code injection. Second, it fixes the bug because as @Captain Oblivious pointed out, the code you wrote doesn't do what you think it does.

Answer 2

char* strings and string literals and character literals can't be added together. You need to create a new string and use strcat to concatenate one to the other.

What you're actually getting is undefined behavior, since you're adding some constant to a pointer and not checking that you've passed the end of the string. It's only a coincidence that the output was readable at all and that your program didn't crash.

If C++ is an option you can use std::string instead, which does allow + to concatenate two strings.

Answer 3

To explain what you are seeing from argv[1] + '\\n' . This is pointer plus integer (character constants have type int in C).

The definition of that in C is that the pointer is advanced by as many units as are in the integer. \\n is 10 in ASCII so this will advance the pointer by 10 characters. If your string is shorter than 10 then you're now reading whatever is in memory beyond the end of that string.

The fix suggested by Ben Braun is a good one; another option is puts( argv[1] ); which will output the string without doing any printf-like translations, and output a newline.