How to correctly assign a new string value

Question

I m trying to understand how to solve this trivial problem in C  in the cleanest safest way  Here s my example    include  lt stdio h gt   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  lt stdio h gt   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

User · Answer

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

User · Answer

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

User · Answer

The first example doesn t work because you can t assign values to arrays - arrays work  sort of  like const pointers in this respect  What you can do though is copy a new value into the array   strcpy p name   Jane      Char arrays are fine to use if you know the maximum size of the string in advance  e g  in the first example you are 100  sure that the name will fit into 19 characters  not 20 because one character is always needed to store the terminating zero value    Conversely  pointers are better if you don t know the possible maximum size of your string  and or you want to optimize your memory usage  e g  avoid reserving 512 characters for the name  John   However  with pointers you need to dynamically allocate the buffer they point to  and free it when not needed anymore  to avoid memory leaks   Update  example of dynamically allocated buffers  using the struct definition in your 2nd example    char  firstName    Johnnie   char  surname    B  Goode   person p   p name   malloc strlen firstName    1   p surname   malloc strlen surname    1    p age   25  strcpy p name  firstName   strcpy p surname  surname    printf  Name   s  Age   d n  p name p age    free p surname   free p name

[c] How to correctly assign a new string value?

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