trouble in passing structure literal as function parameter

Question

Please have a look at this program

#include<stdio.h>
#include<string.h>

typedef struct hs_ims_msrp_authority
{
   int     host_type;
   char    buf[50];
   int     port;
}hs_i;

int main()
{
 char dom[50];
 int i = 10, j = 20;
 strcpy(dom, "ine");
 fun((hs_i){i, dom, j});   // doesnt work
 fun((hs_i){i, "dom", j}); // this works
}

int fun(hs_i c)
{
 printf("%d %s %d\n", c.host_type, c.buf, c.port);
}

In call to fun function in main; how come function call work when string literal ("dom") is passed where as when array variable (dom) is passed it doesnt work?

To make variable work should it be typecasted in a specific way? or is there any other way?

Answer 1

The presence of the compound literal is distracting and the cause of the error is the attempt to initialize a char[] with another char[] array. The following is illegal:

char dom[50] = "test";
char dom1[50] = dom;  /* Line 16, the cause of the error. */

and clang reports the following error:

main.c:16:10: error: array initializer must be an initializer list or string literal

Point 14 in section 6.7.8 Initialization of the C99 standard states:

An array of character type may be initialized by a character string literal, optionally enclosed in braces. Successive characters of the character string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.

So the call with the string literal "dom" is permitted as it is legal to initialize an array with a string literal, but the call with the char[] is not permitted.

Possible solutions:

• change the type of buf to be a const char*

• wrap the buf member in a struct which would enable it to be copied. For example:

  struct char_array
  {
      char data[50];
  };
  
  typedef struct hs_ims_msrp_authority
  {
      int     host_type;
      struct char_array buf;
      int     port;
  } hs_i;
  
  struct char_array dom = { "ine" };
  int i = 10, j = 20;
  
  fun((hs_i){i, dom, j});
  fun((hs_i){i, { "dom" }, j});
        /* Note ^       ^ */
  

Answer 2

In this case,

fun((hs_i){i, dom, j});

you are just passing the pointer to the string. In other words, you are just passing the

&"ine"[0]