C struct problem

Question

I am trying to learn about structs in C, but i do not understand why i cannot assign the title as i my example:

#include <stdio.h>

struct book_information {
 char title[100];
 int year;
 int page_count;
}my_library;


main()
{

 my_library.title = "Book Title"; // Problem is here, but why?
 my_library.year = 2005;
 my_library.page_count = 944;

 printf("\nTitle: %s\nYear: %d\nPage count: %d\n", my_library.title, my_library.year, my_library.page_count);
 return 0;
}

Error message:

books.c: In function ‘main’:
books.c:13: error: incompatible types when assigning to type ‘char[100]’ from type ‘char *’

Steve Townsend · Accepted Answer · 2010-11-16 17:23:22Z

9

LHS is an array, RHS is a pointer. You need to use strcpy to put the pointed-to bytes into the array.

strcpy(my_library.title, "Book Title");

Take care that you do not copy source data > 99 bytes long here as you need space for a string-terminating null ('\0') character.

The compiler was trying to tell you what was wrong in some detail:

error: incompatible types when assigning to type ‘char[100]’ from type ‘char *’

Look at your original code again and see if this makes more sense now.

answered Nov 16, 2010 at 17:23

Steve Townsend

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5 Comments

peter_larsson80 Over a year ago

Thanks, after adding "#include <strings.h>" the program compiles and works as expected. All examples i have found, has been using scanf or gets to fill the strings, and this makes sense to me now.

Steve Townsend Over a year ago

Thanks Peter. I would consider the strncpy alternatives, unless you are sure your input data will always be small enough to fit.

Larry Engholm Over a year ago

Though including strings.h may happen to work on your implementation, you really should be including string.h to declare strcpy.

Alex Reynolds Over a year ago

Would recommend using strncpy over strcpy to make sure you're not going over array size limit.

Dust_In_The_Wind Over a year ago

I was going over structures today, and I came across the same problem. Of course, this solved my answer. But I don't see the logic behind it. Can someone explain why didn't C allow to equate a char array with a char pointer? After all, a pointer to an array is the first element of the array itself.

Steve Townsend · Accepted Answer · 2010-11-16 17:26:43Z

6

As the message says, the issue is you are trying to assign incompatible types: char* and char[100]. You need to use a function like strncpy to copy the data between the 2

strncpy(my_library.title, "Book Title", sizeof(my_library.title));

edited Nov 16, 2010 at 17:26

Steve Townsend

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answered Nov 16, 2010 at 17:23

JaredPar

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2 Comments

Oliver Charlesworth Over a year ago

sizeof(my_library.title)-1, no?

JaredPar Over a year ago

@Oli, indeed. Mentally smacked myself for the over site.

Nikolai Fetissov · Accepted Answer · 2010-11-16 17:24:21Z

3

title is a character array - these are not assignable in C. Use strcpy(3).

answered Nov 16, 2010 at 17:24

Nikolai Fetissov

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Comments

fabmilo · Accepted Answer · 2010-11-16 17:24:06Z

1

char* and char[100] are different types.

You want to copy those char elements inside the .title buffer.

strncpy(my_library.title, "Book Title", sizeof(my_library.title));

answered Nov 16, 2010 at 17:24

fabmilo

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