How to work with string fields in a C struct

Question

I m having trouble making a database based on a singly-linked list in C  not because of the linked list concept but rather the string fields in the struct themselves   This is an assignment in C and as far as I know  I m a newbie   C doesn t recognize  string  as a data type    This is what my struct code looks like   typedef struct      int number    string name    string address    string birthdate    char gender    patient   typedef struct llist     patient num    struct llist  next    list    I was thinking of making a struct for the strings themselves so that I can use them in the struct  like this   typedef struct string       char  text     string    Then I will malloc   each one of them when it is required to make new data of the string type  array of char    typedef struct string     char  text     string   int main         int length   50      string s    string  malloc sizeof string       s- gt text    char    malloc len   sizeof char       strcpy s- gt text  patient name- gt text       Can someone help me figure this out  Thank you

User · Answer

You could just use an even simpler typedef   typedef char  string    Then  your malloc would look like a usual malloc   string s   malloc maxStringLength

User · Answer

On strings and memory allocation   A string in C is just a sequence of chars  so you can use char   or a char array wherever you want to use a string data type   typedef struct         int number    char  name    char  address    char  birthdate    char gender    patient    Then you need to allocate memory for the structure itself  and for each of the strings   patient  createPatient int number  char  name     char  addr  char  bd  char sex          Allocate memory for the pointers themselves and other elements      in the struct    patient  p   malloc sizeof struct patient       p- gt number   number     Scalars  int  char  etc  can simply be copied       Must allocate memory for contents of pointers   Here  strdup        creates a new copy of name   Another option       p- gt name   malloc strlen name  1        strcpy p- gt name  name     p- gt name   strdup name     p- gt address   strdup addr     p- gt birthdate   strdup bd     p- gt gender   sex    return p      If you ll only need a few patients  you can avoid the memory management at the expense of allocating more memory than you really need   typedef struct         int number    char name 50            Declaring an array will allocate the specified   char address 200        amount of memory when the struct is created    char birthdate 50       but pre-determines the max length and may   char gender             allocate more than you need    patient    On linked lists   In general  the purpose of a linked list is to prove quick access to an ordered collection of elements   If your llist contains an element called num  which presumably contains the patient number   you need an additional data structure to hold the actual patients themselves  and you ll need to look up the patient number every time   Instead  if you declare   typedef struct llist     patient  p    struct llist  next    list    then each element contains a direct pointer to a patient structure  and you can access the data like this   patient  getPatient list  patients  int num      list  l   patients    while  l    NULL        if  l- gt p- gt num    num          return l- gt p            l   l- gt next        return NULL

User · Answer

This does not work   string s    string malloc sizeof string      string refers to a pointer  you need the size of the structure itself   string s   malloc sizeof   string       Note the lack of cast as well  conversion from void   malloc s return type  is implicitly performed    Also  in your main  you have a globally delcared patient  but that is uninitialized   Try    patient number   3        patient name    John         patient address    Baker street         patient birthdate    4 15 2012         patient gender    M          before you read-access any of its members  Also  strcpy is inherently unsafe as it does not have boundary checking  will copy until the first   0  is encountered  writing past allocated memory if the source is too long    Use strncpy instead  where you can at least specify the maximum number of characters copied -- read the documentation to ensure you pass the correct value  it is easy to make an off-by-one error

User · Answer

I think this solution uses less code and is easy to understand even for newbie  For string field in struct  you can use pointer and reassigning the string to that pointer will be straightforward and simpler  Define definition of struct  typedef struct     int number    char  name    char  address    char  birthdate    char gender    Patient   Initialize variable with type of that struct  Patient patient  patient number   12345  patient address    quot 123 123 some road Rd  quot   patient birthdate    quot 2020 12 12 quot   patient gender    quot M quot    It is that simple  Hope this answer helps many developers

User · Answer

While Richard s is what you want if you do want to go with a typedef  I d suggest that it s probably not a particularly good idea in this instance  as you lose sight of it being a pointer  while not gaining anything     If you were treating it a a counted string  or something with additional functionality  that might be different  but I d really recommend that in this instance  you just get familiar with the  standard  C string implementation being a  char

[c] How to work with string fields in a C struct?

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