C function argument char * vs char []

Question

I think this question is an extension of this SO answer. Say I have the following code:

#include <stdio.h>
#include <string.h>

void func(char *str)
{
    strcpy(str, "Test");
}

int main()
{
    char testStr[20] = "Original";
    func(testStr);
    printf("%s\n", testStr);  /* Prints "Test" followed by a new-line */
    return 0;
}

By my understanding, shouldn't func expect a pointer to a read-only literal as an argument? Whereas, what is being passed is a copy on the stack of a read-only literal.

Even though this yields correct results, is doing this 100% correct? Would it improve readability of code if func() accepted char [] instead of char *?

score 8 · Accepted Answer · answered Mar 21 '13 at 08:38

8

Your understanding is correct. The code is fine.

The following, on the other hand, isn't:

void func(char *str)
{
    strcpy(str, "Test");
}

int main()
{
    char* testStr = "Original";
    func(testStr);
}

This attempts to modify a string literal, resulting in undefined behaviour.

As to the question of readability, that's subjective.

answered Mar 21 '13 at 08:38

NPE

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@KierenJohnstone: Well, there's no user input ;) – NPE Mar 21 '13 at 08:39
@KierenJohnstone, Security flaws aside. I just whipped up a quick SSCCE. – Anish Ramaswamy Mar 21 '13 at 08:40
@NPE, Thanks for your answer. I currently don't have any Linux OS, so I can't test this with `gcc -Wall`. Would that compilation produce any warnings? – Anish Ramaswamy Mar 21 '13 at 08:59
@AnishRam: Don't think so. See http://stackoverflow.com/questions/4493139/are-string-literals-const – NPE Mar 21 '13 at 09:00
@KierenJohnstone, On a side note, what security flaws do you see? I see only `strcpy()` (instead of `strncpy()` maybe). – Anish Ramaswamy Mar 21 '13 at 09:08
@AnishRam Your use of `strcpy` is definitely not an issue here. – teppic Mar 21 '13 at 09:16

teppic · Answer 2 · 2013-03-21T08:59:06.133

All strings are pointers to arrays of char, so it makes no difference. There's no copy of the string allocated on the stack in the function, it's the same one as passed, and declaring the parameter as an array doesn't allocate any storage for the string in the function, it's just another way of writing the same thing.

However, if you initialise an array with a string literal, a copy of the literal is made into the array. It doesn't matter whether the string came from a literal, as it's entirely separate.

If you're expecting a literal you can declare the function parameter as const if you want to, but there's certainly no requirement. In C string literals are not read only, but it'll cause undefined behaviour to change them.

C function argument char * vs char []

2 Answers2