Passing Strings as Arguments to Functions in C

[cepheid]

Hey everyone!

If I have a function that takes a pointer to char (i.e. char array) as an argument, can I pass a "string literal" (I think that is the right term) as an argument to that function, even if that string hasn't previously been declared anywhere? For example:

void myfunc(char *message) 
{

  /* function definition goes in here */

}

/* 
.
.
.
and then some time later
*/


myfunc("this is my message");

Is that allowed? I'm wondering where the memory comes from for the string.

 

[Hurkyl]

Hey everyone!

If I have a function that takes a pointer to char (i.e. char array) as an argument, can I pass a "string literal" (I think that is the right term) as an argument to that function, even if that string hasn't previously been declared anywhere? For example:

void myfunc(char *message) 
{

  /* function definition goes in here */

}

/* 
.
.
.
and then some time later
*/


myfunc("this is my message");

This isn't really legal C -- this only works if your function takes const char* as its argument.

I'm wondering where the memory comes from for the string.

As far as the language is concerned, you're not supposed to know about "memory" -- you're only supposed to know about pointers and arrays. (and even then, you're only supposed to know about pointers that reference objects or pointers that point into dynamically allocated objects) The compiler can do whatever it wants, so long as your function receives a pointer which it can access as a string and see the contents "this is my message".

In reality, the string is probably placed in the data segment of your executable (in a constant section), and the string "this is my message" gets converted into a pointer constant that points into the right location in the data segment.

 

[cepheid]

In reality, the string is probably placed in the data segment of your executable (in a constant section), and the string "this is my message" gets converted into a pointer constant that points into the right location in the data segment.

Thanks for the explanation. Just to clarify, what you're saying above is what might happen in the case where I change the data type of the argument to const char*, which I need to do regardless in order to make this work, right?

 

[Hurkyl]

Thanks for the explanation. Just to clarify, what you're saying above is what might happen in the case where I change the data type of the argument to const char*, which I need to do regardless in order to make this work, right?

What I said is what (generally) happens to string literals.

The need to change the argument of your function is because string literals are arrays of const char, not arrays of char.

(For backwards compatibility, many compilers will let you pass them around as if they weren't const. Maybe the standard does too, I can't remember. But you still better not actually edit them)

 

[pheeesics]

It is my understanding that the string is allocated in a function 'frame' on the stack whenever you call myfunc("text"). Somebody PM me if I am mistaken.

Stick with char * str unless you want the string to be unedited. The const (basically) make the compiler alert you when you try to change the string.

 

[DrGreg]

It is my understanding that the string is allocated in a function 'frame' on the stack whenever you call myfunc("text").

The pointer to the string will be on the stack, but the string itself (array of chars) is likely to be in some other part of memory, as Hurkyl said, and it may well be in memory to be treated as read-only. (It is up to the compiler/linker to decide exactly where.)