Read a Line of Text from the User

[dcs]

Read a Line of Text from the User

Obtaining user input can be done in many surprisingly different ways. This code is somewhere in the middle: safer than gets(text) or scanf("%s", text), but not bulletproof. It is meant to be a simple but relatively safe demonstration.

The function mygetline reads user input from the stdin into a string using fgets. Then it attempts to remove a trailing newline that fgets may leave in the string. It returns a pointer to the destination string.

#include <stdio.h>
#include <string.h>

char *mygetline(char *line, int size)
{
   if ( fgets(line, size, stdin) )
   {
      char *newline = strchr(line, '\n'); /* check for trailing '\n' */
      if ( newline )
      {
         *newline =  '\0'; /* overwrite the '\n' with a terminating null */
      }
   }
   return line;
}

int main(void)
{
   char text[20] = "";
   fputs("prompt: ", stdout);
   fflush(stdout);
   printf("text = \"%s\"\n", mygetline(text, sizeof text));
   return 0;
}

/* my input/output
prompt: hello world
text = "hello world"
*/

[dcs]

Read a Line of Text from the User, Discard Excess Characters

If the user tries to put 80 characters in a 20-character buffer, you may have issues. This snippet shows one way to cap the input and discard excess input.

#include <stdio.h>

char *mygettext(char *text, size_t size)
{
   size_t i = 0;
   for ( ;; )
   {
      int ch = fgetc(stdin);
      if ( ch == '\n' || ch == EOF )
      {
         break;
      }
      if ( i < size - 1 )
      {
         text[i++] = ch;
      }
   }
   text[i] = '\0';
   return text;
}

int main(void)
{
   int i;
   for ( i = 0; i < 3; ++i )
   {
      char text[20];
      fputs("prompt: ", stdout);
      fflush(stdout);
      printf("text = \"%s\"\n", mygettext(text, sizeof text));
   }
   return 0;
}

/* my input/output
prompt: 1234567890123456789012345
text = "1234567890123456789"
prompt: hello world
text = "hello world"
prompt: goodbye
text = "goodbye"
*/