Programming Forums > Application Development > C

titaniumdecoy · Apr 5th, 2008, 3:41 PM

I am confused as to the difference between a char array and a char pointer in C. For example:

char a[] = "one";
char *b = "two"

As I understand it, both are essentially pointers to the first character of an array except a can be modified and b cannot. Why is this?

If I then write char *c = a;, can c be used to modify the character array a?

Compare the following two functions declarations:

(Toggle Plain Text)

void f1(char x[]);
void f2(char *x);

If I pass a char[] to f2, can I modify it, or only if I pass it to f1? I assume it would be incorrect to pass a char* to f1--is this true?

Finally, compare the following two declarations for main:

(Toggle Plain Text)

int main(int argc, char *argv[]);
int main(int argc, char **argv);

Does the latter indicate that the contents of argv can be modified? Or are the two declarations identical?

Thanks for any help clearing this up.

Narue · Apr 5th, 2008, 3:53 PM

>As I understand it, both are essentially pointers to the first character
>of an array except a can be modified and b cannot.
Nope, and that's the problematic misconception. An array is not a pointer, it's an array. In many cases an array will be converted to a pointer automagically for you, but that still doesn't make it a pointer.

>Why is this?
When you initialize an array with a string literal, a copy of the string literal is stored in the array. You own the memory for the array, therefore you can modify the copy of the string literal (provided it's not declared as const). When you initialize a pointer with a string literal, the pointer points to the string literal itself. The reason you can't use b to modify the string literal is because string literals are read-only.

>I assume it would be incorrect to pass a char* to f1--is this true?
No, in the case of function parameters, a pointer and an array are identical. The array notation is simply syntactic sugar.

Sane · Apr 5th, 2008, 4:42 PM

I'm not sure if this helps, but I only use the array notation when I'm hardcoding an array of values, or a string, and thus initiating it on the same line I declare it. The sad thing is I can't explain why I use this habit. The explanation can probably be found somewhere in Narue's post. I've only noticed it's what people do, and it's what has worked for me.

Ancient Dragon · Apr 5th, 2008, 8:21 PM

when NOT used as a parameter to a function always use the array notation if you think you will change any of the characters in the array.

A pointer probably points to somewhere in read-only memory so it can not be changed. The pointer notation should really be made const so that you can't change the string to which it points.

titaniumdecoy · Apr 6th, 2008, 12:30 AM

Thanks, everyone. That clears up most of my questions. But I am still wondering whether the char arrays/pointers passed to main (argv) can be modified legally.

Jabo · Apr 6th, 2008, 1:40 AM

can you call main from within main? probably not. to change the arguments, you would have to call the function again, or so I'm thinking, and it's highly unlikely that they would allow main to be called recursively.

As it turns out, it looks like C allows main to be called recursively, but C++ doesn't
I stand corrected

grumpy · Apr 6th, 2008, 1:43 AM

Quote:

Originally Posted by titaniumdecoy

Thanks, everyone. That clears up most of my questions. But I am still wondering whether the char arrays/pointers passed to main (argv) can be modified legally.

From the 1999 C standard, Section 5.1.2.2.1, clause 2, fifth bullet point "The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination."

The C++ standard says nothing but, since its second argument (aka argv) has no const modifiers, it may be inferred it is legal to modify the strings passed to main via argv.

titaniumdecoy · Apr 6th, 2008, 1:59 AM

Thanks, grumpy. That is quite an interesting tidbit.

titaniumdecoy · Apr 13th, 2008, 10:30 PM

Quote:

Originally Posted by Narue

When you initialize an array with a string literal, a copy of the string literal is stored in the array. You own the memory for the array, therefore you can modify the copy of the string literal (provided it's not declared as const). When you initialize a pointer with a string literal, the pointer points to the string literal itself. The reason you can't use b to modify the string literal is because string literals are read-only.

Does anyone know of an official source where I can find this information? (A link would be appreciated.) Thanks.

grumpy · Apr 14th, 2008, 4:31 AM

The C standard is the official source. You can buy a copy through the ANSI web store.

Apr 5th, 2008, 3:41 PM	#1
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 856 Rep Power: 3	char[] vs char* I am confused as to the difference between a char array and a char pointer in C. For example: (Toggle Plain Text) char a[] = "one"; char b = "two" char a[] = "one"; char b = "two" As I understand it, both are essentially pointers to the first character of an array except a can be modified and b cannot. Why is this? If I then write `char c = a;`, can c be used to modify the character array a? Compare the following two functions declarations: (Toggle Plain Text) void f1(char x[]); void f2(char x); void f1(char x[]); void f2(char x); If I pass a char[] to f2, can I modify it, or only if I pass it to f1? I assume it would be incorrect to pass a char to f1--is this true? Finally, compare the following two declarations for main: (Toggle Plain Text) int main(int argc, char argv[]); int main(int argc, char argv); int main(int argc, char argv[]); int main(int argc, char **argv); Does the latter indicate that the contents of argv can be modified? Or are the two declarations identical? Thanks for any help clearing this up.

Apr 5th, 2008, 3:53 PM	#2
Narue Professional Programmer Join Date: Sep 2005 Posts: 419 Rep Power: 4	Re: char[] vs char* >As I understand it, both are essentially pointers to the first character >of an array except a can be modified and b cannot. Nope, and that's the problematic misconception. An array is not a pointer, it's an array. In many cases an array will be converted to a pointer automagically for you, but that still doesn't make it a pointer. >Why is this? When you initialize an array with a string literal, a copy of the string literal is stored in the array. You own the memory for the array, therefore you can modify the copy of the string literal (provided it's not declared as const). When you initialize a pointer with a string literal, the pointer points to the string literal itself. The reason you can't use b to modify the string literal is because string literals are read-only. >I assume it would be incorrect to pass a char* to f1--is this true? No, in the case of function parameters, a pointer and an array are identical. The array notation is simply syntactic sugar. __________________ Even if the voices aren't real, they have some pretty good ideas.

Apr 5th, 2008, 4:42 PM	#3
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 1,885 Rep Power: 5	Re: char[] vs char* I'm not sure if this helps, but I only use the array notation when I'm hardcoding an array of values, or a string, and thus initiating it on the same line I declare it. The sad thing is I can't explain why I use this habit. The explanation can probably be found somewhere in Narue's post. I've only noticed it's what people do, and it's what has worked for me. __________________ Waterloo's Canadian Computing Competition (CCC) - Stage 2 Problems, Solutions, and Test Data

Apr 5th, 2008, 8:21 PM	#4
Ancient Dragon PFO God In Training Join Date: Jun 2005 Location: near St Louis, MO. (USA) Posts: 547 Rep Power: 4	Re: char[] vs char* when NOT used as a parameter to a function always use the array notation if you think you will change any of the characters in the array. A pointer probably points to somewhere in read-only memory so it can not be changed. The pointer notation should really be made const so that you can't change the string to which it points. __________________ True Terror is to wake up one morning and discover that your high school class is running the country - Kurt Vonnegut Jr.

Apr 6th, 2008, 12:30 AM	#5
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 856 Rep Power: 3	Re: char[] vs char* Thanks, everyone. That clears up most of my questions. But I am still wondering whether the char arrays/pointers passed to main (argv) can be modified legally.

Apr 6th, 2008, 1:40 AM	#6
Jabo Not a user? Join Date: Sep 2007 Posts: 272 Rep Power: 2	Re: char[] vs char* can you call main from within main? probably not. to change the arguments, you would have to call the function again, or so I'm thinking, and it's highly unlikely that they would allow main to be called recursively. As it turns out, it looks like C allows main to be called recursively, but C++ doesn't I stand corrected Last edited by Jabo; Apr 6th, 2008 at 1:45 AM. Reason: I stand corrected!

Apr 6th, 2008, 1:59 AM	#8
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 856 Rep Power: 3	Re: char[] vs char* Thanks, grumpy. That is quite an interesting tidbit.

Apr 14th, 2008, 4:31 AM	#10
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,223 Rep Power: 5	Re: char[] vs char* The C standard is the official source. You can buy a copy through the ANSI web store.

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Programming Forums > Application Development > C

char[] vs char*