How to accept string input only if it of certain length in C else ask user to input the string again

Question

How to accept set of strings as input in C and prompt the user again to re-enter the string if it exceeds certain length. I tried as below

#include<stdio.h>
int main()
{
    char arr[10][25]; //maximum 10 strings can be taken as input of max length 25
    for(int i=0;i<10;i=i+1)
    {
        printf("Enter string %d:",i+1);
        fgets(arr[i],25,stdin);
    }
}

But here fgets accepts the strings greater than that length too. If the user hits return, the second string must be taken as input. I'm new to C

Answer 1

How to accept string input only if it of certain length

Form a helper function to handle the various edge cases. Use fgets(), then drop the potential '\n' (which fgets() retains) and detect long inputs.

Some untested code to give OP an idea:

#include <assert.h>
#include <stdio.h>

// Pass in the max string _size_.
// Return NULL on end-of-file without input.
// Return NULL on input error.
// Otherwise return the buffer pointer.
char* getsizedline(size_t sz, char *buf, const char *reprompt) {
  assert(sz > 0 && sz <= INT_MAX && buf != NULL); // #1
  while (fgets(buf, (int) sz, stdin)) {
    size_t len = strlen(buf);
    // Lop off potential \n
    if (len > 0 && buf[--len] == '\n') {   // #2
      buf[len] = '\0';
      return buf;
    }
    // OK if next ends the line
    int ch = fgetc(stdin);
    if (ch == '\n' || feof(stdin)) {       // #3 
      return buf;
    }

    // Consume rest of line;
    while (ch != '\n' && ch != EOF) {      // #4
      ch = fgetc(stdin);
    }
    if (ch == EOF) {                       // #5
      return NULL;
    }

    if (reprompt) {
      fputs(reprompt, stdout);
    }
  }
  return NULL;
}

Uncommon: reading null characters remains a TBD issue.

Details for OP who is a learner.

1. Some tests for sane input parameters. A size of zero does not allow for any input saved as a null character terminated string. Buffers could be larger than INT_MAX, but fgets() cannot directly handle that. Code could be amended to handle 0 and huge buffers, yet leave that for another day.

2. fgets() does not always read a '\n'. The buffer might get full first or the last line before end-of-file might lack a '\n'. Uncommonly a null character might be read - even the first character hence the len > 0 test, rendering strlen() insufficient to determine length of characters read. Code would need significant changes to accommodate determining the size if null character input needs detailed support.

3. If the prior fgets() filled its buffer and the next read character attempt resulted in an end-of-file or '\n', this test is true and is OK, so return success.

4. If the prior fgetc() resulted in an input error, this loops exits immediately. Otherwise, we need to consume the rest of the line looking for a '\n' or EOF (which might be due to an end-of-file or input error.)

5. If EOF returned (due to an end-of-file or input error), no reason to continue. Return NULL.

Usage

// fgets(arr[i],25,stdin);
if (getsizedline(arr[i], sizeof(arr[i]), "Too long, try again.\n") == NULL) {
  break;
}