antonpug antonpug - 1 year ago 64
C Question

What is the simplest way of getting user input in C?

There seem to be a LOT of ways you can get user input in C.

What is the easiest way that requires little code?

Basically I need to display this:

Enter a file name: apple.text

Basically I need to ask the user for a file name. So I need something that just gets that one word that the user will be inputting.

Answer Source

The simplest "correct" way is probably this one, taken from Bjarne Stroustrup's paper Learning Standard C++ As A New Language.

(Note: I changed Bjarne's code to check for isspace() instead of just end of line. Also, due to @matejkramny's comment, to use while(1) instead of while(true)...and so long as we're being heretical enough to edit Stroustrup's code, I've subbed in C89 comments instead of C++ style too. :-P)

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>

void quit() /* write error message and quit */
    fprintf(stderr, "memory exhausted\n");

int main()
    int max = 20;
    char* name = (char*) malloc(max); /* allocate buffer */
    if (name == 0) quit();

    printf("Enter a file name: ");

    while (1) { /* skip leading whitespace */
        int c = getchar();
        if (c == EOF) break; /* end of file */
        if (!isspace(c)) {
             ungetc(c, stdin);

    int i = 0;
    while (1) {
        int c = getchar();
        if (isspace(c) || c == EOF) { /* at end, add terminating zero */
            name[i] = 0;
        name[i] = c;
        if (i == max - 1) { /* buffer full */
            max += max;
            name = (char*) realloc(name, max); /* get a new and larger buffer */
            if (name == 0) quit();

    printf("The filename is %s\n", name);
    free(filename); /* release memory */
    return 0;

That covers:

  • skipping whitespace until you reach character input
  • expanding the string buffer dynamically to fit arbitrary size strings
  • handling conditions of when memory can't be allocated

Are there simpler but broken solutions, which might even run a bit faster? Absolutely!!

If you use scanf into a buffer with no limit on the read size, then your input exceeds the size of the buffer, it will create a security hole and/or crash.

Limiting the size of the reading to, say, only 100 unique characters of a filename might seem better than crashing. But it can be worse; for instance if the user meant (...)/dir/foo/bar.txt but you end up misinterpreting their input and overwriting a file called bar.t which perhaps they cared about.

It's best to get into good habits early in dealing with these issues. My opinion is that if your requirements justify something close-to-the-metal and "C-like", it's well worth it to consider the jump to C++. It was designed to manage precisely these concerns--with techniques that are robust and extensible, yet still perform well.

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