从stdin读取

Question

What are the possible ways for reading user input using read() system call in Unix. How can we read from stdin byte by byte using read() ?

Answer 1

You can do something like this to read 10 bytes:

char buffer[10];
read(STDIN_FILENO, buffer, 10);

remember read() doesn't add '\\0' to terminate to make it string (just gives raw buffer).

To read 1 byte at a time:

char ch;
while(read(STDIN_FILENO, &ch, 1) > 0)
{
 //do stuff
}

and don't forget to #include <unistd.h> , STDIN_FILENO defined as macro in this file.

There are three standard POSIX file descriptors, corresponding to the three standard streams, which presumably every process should expect to have:

Integer value   Name
       0        Standard input (stdin)
       1        Standard output (stdout)
       2        Standard error (stderr)

So instead STDIN_FILENO you can use 0.

Edit:
In Linux System you can find this using following command:

$ sudo grep 'STDIN_FILENO' /usr/include/* -R | grep 'define'
/usr/include/unistd.h:#define   STDIN_FILENO    0   /* Standard input.  */

Notice the comment /* Standard input. */ /* Standard input. */

Answer 2

From the man read :

#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);

Input parameters:

• int fd file descriptor is an integer and not a file pointer. The file descriptor for stdin is 0

• void *buf pointer to buffer to store characters read by the read function

• size_t count maximum number of characters to read

So you can read character by character with the following code:

char buf[1];

while(read(0, buf, sizeof(buf))>0) {
   // read() here read from stdin charachter by character
   // the buf[0] contains the character got by read()
   ....
}