What is the difference between stdin and STDIN_FILENO?

Question

What is the practical difference, if any, between stdin and STDIN_FILENO in C?

Answer 1

stdin : 1. A file pointer (* FILE) 2. The file descriptor table holds its address when process is created. 3. present in /usr/include/stdio.h

STDIN_FILENO : 1. It is a macro 2. Its nothing but an array index of a file descriptor table (default 0). 3.present in /usr/include/unistd.h

Could be more clear by following code.

#include<stdio.h> 
#include<unistd.h> 
int main() 
{    
     printf("%d\t\t%p ----- ",STDIN_FILENO,stdin);

return 0; 
} 

Answer 2

stdin is a default FILE pointer used to get input from none other than standard in.

STDIN_FILENO is the default standard input file descriptor number which is 0. It is essentially a defined directive for general use.

Answer 3

From /usr/include/stdio.h,

/* Standard streams.  */
extern struct _IO_FILE *stdin;          /* Standard input stream.  */
extern struct _IO_FILE *stdout;         /* Standard output stream.  */
extern struct _IO_FILE *stderr;         /* Standard error output stream.  */
/* C89/C99 say they're macros.  Make them happy.  */
#define stdin stdin
#define stdout stdout
#define stderr stderr

From /usr/include/unistd.h

/* Standard file descriptors.  */
#define STDIN_FILENO    0       /* Standard input.  */
#define STDOUT_FILENO   1       /* Standard output.  */
#define STDERR_FILENO   2       /* Standard error output.  */

Ex, stdin (_IO_FILE defined in /usr/include/libio.h) is a structure data. STDIN_FILENO is a macro constant, which points to a file descriptor used by kernel.

#include <stdio.h>
#include <unistd.h>

void
stdin_VS_STDIN_FILENO(void)
{
    printf("stdin->_flags = %hd\n", stdin->_flags);
    printf("STDIN_FILENO  : %d\n", STDIN_FILENO);
}

int
main(void)
{
    stdin_VS_STDIN_FILENO();
    return 0;
}

Answer 4

The interface. Like everyone else has said, stdin is a FILE * as defined by the standard c library. You can use some of the higher level interfaces like fread, fwrite, and fprintf. On the other hand, STDIN_FILENO is just a file descriptor (almost certainly 0). This uses a slight lower level interface through the likes of read and write.