A simple C langauage w/ the basic input/output

[Insu Kim]

I wrote a very simple C program for any convenience.
I used scanf to get a standard input and printf for standard output like this.

--- a.out ----
main()
{
int instance ;
printf("Input A = ") ;
scanf("%d", &instance) ;
printf(%d, instance) ;
}
-- end of a.out ----

I'd like to save the output to a file.
If I run "a.out" using redirection, the result goes to certain file named but I can't see a message "Input A=" at the shell prompt.

For instance,
# a.out > aaa
"Input A=" is not displayed.
So I have to know what question is in advance.

If I execute "a.out" without any parameters, I can enter a value for the question displayed at the shell prompt but I can't save the result to a file.

Any ideas would be helpful.

[Volker Borowski]

Hi,

if you redirect stdout, you need to prompt on stderr:

fprintf(stderr, "Input A = ") ;
scanf("%d", &instance) ;
printf("Value= %d \n", instance) ;


--- Result :

# a.out
Input A = 23
Value= 23
# a.out > weg
Input A = 23
# cat weg
Value= 23
#

Hope this helps
Volker

[A. Clay Stephenson · Accepted solution]

Hi,

You have 3 choices:

1) print your prompt to stderr via fprintf(stderr,
That is not considered good form because stderr should only be used for error messages (but you can use it)

2) If you know that you will always be run as interactive, you can prompt to "/dev/tty".

int main(int argc, char *argv[])
{
int cc = 0;
FILE *f = NULL;

f = fopen("/dev/tty","w+");
if (f != NULL)
{
int instance = 0;
(void) fprintf(f,"Input A: ");
(void) fflush(f);
(void) fclose(f)

scanf("%d", &instance) ;
printf(%d, instance) ;
}
else cc = 2;
return(cc);
}

3) You could pass a filename as an argument
int main(int argc, char *argv)
{
int cc = 0;
if (argc > 1)
{
FILE *f = NULL;
f = fopen(argv[1],"w");
if (f != NULL)
{
int instance ;
printf("Input A = ") ;
scanf("%d", &instance) ;
fprintf(f,"%d", instance) ;
fclose(f);
}
else cc = 2;
}
else cc = 1;
return(cc);
}

Clay