How to read a file passed through stdin in C

Question

I want to pass a text file to a C program like this ./program < text.txt . I found out on SO that arguments passed like that dont appear in argv[] but rather in stdin . How can I open the file in stdin and read it?

Answer 1

You can directly read the data without having to open the file. stdin is already open. Without special checks your program doesn't know if it is a file or input from a terminal or from a pipe.

You can access stdin by its file descriptor 0 using read or use functions from stdio.h . If the function requires a FILE * you can use the global stdin .

Example:

#include <stdio.h>
#define BUFFERSIZE (100) /* choose whatever size is necessary */

/* This code snippet should be in a function */

char buffer[BUFFERSIZE];

if( fgets(buffer, sizeof(buffer), stdin) != NULL )
{
    /* check and process data */
}
else
{
    /* handle EOF or error */
}

You could also use scanf to read and convert the input data. This function always reads from stdin (in contrast to fscanf ).