typedef struct nsmail_inputstream {

   void *rock; 
   int (*read) (void *rock, 
            char *buf, unsigned size); 
   void (*rewind) (void *rock);
   void (*close) (void *rock);
} nsmail_inputstream_t; 

int Test_read(void *rock,char *buf, unsigned size)
{
     if(fgets(buf,size,(FILE *)rock) == NULL)
         return -1;
     return strlen(buf);
}

nsmail_inputstream_t l_inputStream;

l_inputStream.read = Test_read;

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

typedef struct nsmail_inputstream {
    void *rock;
    int (*read)( void *rock, char *buf, unsigned size );
    void (*rewind)( void *rock );
    void (*close)( void *rock );
} nsmail_inputstream_t;

int Test_read( void *rock, char *buf, unsigned size ) {
    if ( fgets( buf, size, (FILE *)rock ) == NULL )
        return -1;
    
    return strlen( buf );
}

int main( void ) {
    nsmail_inputstream_t inputStream;
    inputStream.read = Test_read;
    
    return 0;
}

$ gcc -std=c99 -Wall -Wextra -pedantic example.c -o example
$ ./example