I'm looking for a way to write an input loop that continues to print prompts until the user enters a blank line.
This is what I want to do:
Loop starts, prints prompt > to the command line. User enters a line that ends with '\n', my program does whatever with that line, and then prints > again. This continues until the user enters a blank line (\n), at this point the loop terminates.
Would something along these lines answer your needs?
int ret;
// Basically, in order, 1 to indicate the file descriptor of the standard output
// ">" as the string you want to print
// 1 as the number of characters you want printed.
write(1, ">", 1);
while ((ret = read(0, buff, SizeBuff)) > 0) {
buff[ret] = 0;
if (buff[0] == '\n' && strlen(buff) == 1)
return (0);
/*Do your stuff*/
write(1, ">", 1);
}
write when there is printf? (In this case, followed by fflush to ensure the prompt appears. Or doesn't write need flushing?)A very simple scanf version can also be used:
#include <stdio.h>
#define MAXL 64
int main (void) {
char str[MAXL] = {0};
printf ("\n Enter a string ([enter] alone to quit)\n");
while (printf (" > ") && scanf ("%63[^\n]%*c", str) == 1)
{
/* do whatever in your code */
printf (" result: %s\n", str);
}
return 0;
}
Use/Output
$ ./bin/scanf_enter_quits
Enter a string ([enter] alone to quit)
> string
result: string
> another
result: another
>
Note: MAXL-1 added as maximum width specifier for scanf to prevent write beyond end of array.
getline Examplegetline by dynamically allocating the line buffer allows you to accept a line as long as you want to give it. It can be billions of characters (up to the extent of your memory). This is a strength and a weakness. If you need to limit the amount of data you accept, it is up to you to check/validate/etc....
#include <stdio.h>
#include <stdlib.h>
int main (void) {
char *str = NULL;
size_t n = 0;
ssize_t nchr = 0;
printf ("\n Enter a string ([enter] alone to quit)\n");
while (printf (" > ") && (nchr = getline (&str, &n, stdin)) > 1)
{
str[--nchr] = 0; /* strip newline from input */
printf (" (str: %s)\n", str); /* do whatever in your code */
}
if (str) free (str);
return 0;
}
Use/Output
$ ./bin/getline_enter_quits
Enter a string ([enter] alone to quit)
> string one as long as you want
(str: string one as long as you want)
> string 2 make it 1000 chars.........................................
(str: string 2 make it 1000 chars.........................................)
>
scanf Dynamic AllocationYou can also have scanf dynamically allocate the space for you by using the m conversion specifier (older versions of scanf use the a conversion specifier for this purpose). You must also provide a pointer-to-pointer to accept the address in this case. (e.g. scanf ("%m[^\n]%*c", &str) ).
#include #include
int main (void) {
char *str = NULL;
printf ("\n Enter a string ([enter] alone to quit)\n");
while (printf (" > ") && scanf ("%m[^\n]%*c", &str) == 1)
{
printf (" (str: %s)\n", str); /* do whatever in your code */
if (str) free (str); /* you must free each loop */
str = NULL;
}
if (str) free (str);
return 0;
}
Use/Output
$ ./bin/scanf_dyn_enter_quits
Enter a string ([enter] alone to quit)
> some string as long as you want
(str: some string as long as you want)
> another string any length .......... ............. .............
(str: another string any length .......... ............. .............)
>
scanf, fgets, and getline. All have there pluses and minuses... scanf is by far the simplest for a one-off prompt situation.malloc str after the user enters their input?getline. You could give it billions of chars (until you run out of memory). I'll add an example.getline example and an example showing dynamic allocation with scanf. You will find competing view as to which one is better. scanf implementations can vary, but within the reasonable past, either should work for you.
whileloop would handle this case easily, while a recursive program could crash because the recursion would be too deep (leading to a stack overflow and a segmentation fault).