I'm trying to understand how to solve this trivial problem in C, in the cleanest/safest way. Here's my example:
#include <stdio.h>
int main(int argc, char *argv[])
{
typedef struct
{
char name[20];
char surname[20];
int unsigned age;
} person;
//Here i can pass strings as values...how does it works?
person p = {"John", "Doe",30};
printf("Name: %s; Age: %d\n",p.name,p.age);
// This works as expected...
p.age = 25;
//...but the same approach doesn't work with a string
p.name = "Jane";
printf("Name: %s; Age: %d\n",p.name,p.age);
return 1;
}
The compiler's error is:
main.c: In function ‘main’: main.c:18: error: incompatible types when assigning to type ‘char[20]’ from type ‘char *’
I understand that C (not C++) has no String type and instead uses arrays of chars, so another way to do this was to alter the example struct to hold pointers of chars:
#include <stdio.h>
int main(int argc, char *argv[])
{
typedef struct
{
char *name;
char *surname;
int unsigned age;
} person;
person p = {"John", "Doe",30};
printf("Name: %s; Age: %d\n",p.name,p.age);
p.age = 25;
p.name = "Jane";
printf("Name: %s; Age: %d\n",p.name,p.age);
return 1;
}
This works as expected but I wonder if there a better way to do this. Thanks.
Think of strings as abstract objects, and char arrays as containers. The string can be any size but the container must be at least 1 more than the string length (to hold the null terminator).
C has very little syntactical support for strings. There are no string operators (only char-array and char-pointer operators). You can't assign strings.
But you can call functions to help achieve what you want.
The strncpy() function could be used here. For maximum safety I suggest following this pattern:
strncpy(p.name, "Jane", 19);
p.name[19] = '\0'; //add null terminator just in case
Also have a look at the strncat() and memcpy() functions.
The two structs are different. When you initialize the first struct, about 40 bytes of memory are allocated. When you initialize the second struct, about 10 bytesof memory are allocated. (Actual amount is architecture dependent)
You can use the string literals (string constants) to initalize character arrays. This is why
person p = {"John", "Doe",30};
works in the first example.
You cannot assign (in the conventional sense) a string in C.
The string literals you have ("John") are loaded into memory when your code executes. When you initialize an array with one of these literals, then the string is copied into a new memory location. In your second example, you are merely copying the pointer to (location of) the string literal. Doing something like:
char* string = "Hello";
*string = 'C'
might cause compile or runtime errors (I am not sure.) It is a bad idea because you are modifying the literal string "Hello" which, for example on a microcontroler, could be located in read-only memory.
Source: Stackoverflow.com