Reading and Writing in C - c

Question

so I was messing with the read functions fgets and scanf and with the printing functions write and printf with the following code:
#include <stdio.h>
#include <stdlib.h>
int main(int argc,char *argv[])
{
printf("Enter an integer: ");
int n = 0; scanf("%d",&n);
printf("The input number is: %d\n", n);
printf("Please enter a string now: ");
char buffer[200];
read(0,buffer,200);
printf("The input string is: %s", buffer);
printf("which is: %s\n", buffer);
printf("Now please enter another message: ");
fgets(buffer,200,stdin);
write(1,buffer,200);
return 0;
}
I would get these errors:
1-After the first scanf, it won't just show me the message to input the string.
2-What I write now it's what it's going to be saved in the string.
3-It will skip the last fgets...
An example of output:
Which doesn't make any sense at all; I would like to get an output like this:
Enter an integer: 15
The input number is: 15
Please enter a string now: This is the message1
The input string is: This is the message1 which is: This is the message1
Now please enter another message: This is the message2
This is the message2
Thanks for your help!

Answer 1

Interesting question. Mixing standard I/O (scanf(), fgets()) and file descriptor I/O (read()) on the same underlying file descriptor (0 aka standard input; stdin as a file stream) is at best problematic. You will get odd-ball effects.
At the file stream level, there is some synchronization between stdin and stdout when the input comes from a terminal; pending output on stdout is often flushed by the library. When you use read(), there is no such synchronization. That's why the prompt doesn't appear until after you hit return.
When you type 1 for the number, you also supply a newline. Standard I/O buffers the newline; it is kept so that the next file stream operation can read it. Then you read a line with read(). This does not know about the standard I/O buffer, so it waits for a new line of input from the terminal. You need to capture how much data was read because the input was not null terminated; that's a service provided by the standard I/O library, not the low-level read() function.
When you then call fgets(), it reads the newline that was buffered (not processed by scanf() when reading the integer) and returns with the empty line.
Note that if you had any buffered output waiting on standard output (e.g. you had used printf("Beginning of line: "); with no newline), then the output from write() would appear before the information buffered on stdout.
Using fread() and fwrite() would give you direct binary I/O (no null termination, for example), but would use the same I/O buffers as functions such as printf() and fgets(). You get hybrid behaviour when using these — it is normally best to use either fread()/fwrite() or the text I/O functions on a single file stream, and not both, but mixing them is permissible, relatively simple to understand, and occasionally useful.
So, what you see is all explainable, but it is hard work doing the explaining. It is also a clear indication of why you should not, in general, mix file stream and file descriptor I/O on the same underlying file descriptor — especially not standard input.

Answer 2

Why not use scanf with some better formatting:
#include <stdio.h>
#include <stdlib.h>
int main(int argc,char *argv[]) {
int n, i;
char buffer[200], ch;
printf("Enter an integer: ");
scanf("%d",&n);
printf("The input number is: %d\n", n);
printf("Please enter a string now: ");
scanf(" %[^\t\n]s",buffer);
printf("The input string is: %s", buffer);
printf("which is: %s\n", buffer);
printf("Now please enter another message: ");
scanf(" %[^\t\n]s",buffer);
printf("%s", buffer);
return 0;
}

Answer 3

read(0,buffer,200);
reads up to 200 characters but it won't add a terminating null character to buffer. Hence the following call to printf results in undefined behavior.
You need to add code to capture the return value of the function and make sure to null terminate the string.
int n = read(0, buffer, 199); // Leave one for the terminating null character
if ( n == -1 )
{
// Deal with the error.
}
else
{
// null terminate the string.
buffer[n] = '\0';
}