void someFunction(){
    char *buffer;
    size_t bufsize = 32;
    int bytes_read;

    for (;;) {
        buffer = (char *) malloc(bufsize * sizeof(char));
        if (buffer == NULL) {
            perror("Unable to allocate buffer");
            exit(1);
        }

        FILE *ptr;
        ptr = fopen("sample.txt", "a");
        printf("Enter Stuff to write down:\n");
        //getline(&buffer,&bufsize,stdin);
        //fgets(buffer, 30, stdin);
        //scanf("%[^\n]%*c", buffer);
        //scanf("%s", buffer);

        if (buffer[0] == '0') {
            break;
        }

        WriteWithFprintf(ptr, buffer);
        free(buffer);
        fclose(ptr);
    }
}

The problem is: if I use

    getline(&buffer,&bufsize,stdin);

or

    fgets(buffer, 30, stdin);

then it escapes the first like so:

    Enter Stuff to write down:
    Enter Stuff to write down:
    0

If I use:

    scanf("%[^\n]%*c", buffer);

then I get an infinite loop.

It does work with:

   scanf("%s", buffer);

but I want input with space so this is not an option for me.

CodePudding user response：

All of the behaviors described for different variations on your code are consistent with the next character available to be read from stdin being a newline, presumably from a preceding line of input.

In that case,

• the getline() and fgets() alternatives will read the newline (and any preceding characters) as a line, and then loop to read the line you actually want on the second pass.
• the first scanf() variation will read nothing on account of a matching failure for the %[^\n] field (leading whitespace is not skipped for %[ directives). Not having matched anything to that, there will be no attempt to match anything to the %*c.
• the second scanf() alternative will work as you describe, because scanf will automatically consume leading whitespace when processing a %s directive, including any newline.

There is a variety of things you could do, depending on exactly how want to handle input. Here is one:

    int c = fgetc(stdin);
    if (c == EOF) {
        // handle eof ...
    } else if (c != '\n') {
        ungetc(c, stdin);
    }
    // your choice for reading the wanted data ...

That will consume up to one leading newline from stdin to get it out of your way.

CodePudding user response：

getline and fgets failed to get input on first attempt

Neither failed. Both simply read a '\n' and immediately returned. This '\n' was left-over from a previous input function like scanf("%s", ...), that did not consume the entire line. getline() is not part of the standard C library.

scanf("%[^\n]%*c", buffer); fails to read anything when the first available character is '\n'.

scanf("%s", buffer); consumes all optional leading white space like '\n'. This may appear to work for OP.

scanf("%[^\n]%*c", buffer); and scanf("%s", buffer); are both poor code as they do not have a width limit, risking buffer overflow.

I recommend for a learner to not use scanf() at all and perform all input with fgets(), including the part of code not posted.