fgets读取的最大大小

Question

Using fgets to input a string, I have doubts related to length of string read.

For example, consider the following program.

char str[50];
int i;
int len;
printf("Enter name:\n");
fgets(str,11,stdin);
len = strlen(str);
printf("len :  %d\n",len);

1. If I enter 123456789 , strlen gives 10.

2. If I enter 1234567890 , strlen given is 10 again ??

I think strlen is considering newline also for string length. Am I correct? (I understand fgets using newline as part of string)

What's wrong with (2) where I enter exactly 10 characters, Here string length should be 11 right? 10 + 1 (for newline) = 11

Answer 1

fgets reads at most 1 fewer characters than the length argument given, and does retain the newline as part of the input - so long as the newline is part of the first (length - 1) characters.

So in your first case, assuming that 123456789 is followed by a newline, fgets has read 9 characters including the newline, yielding a string length of 10; in your second case, fgets will stop after reading the 10 characters 1234567890 , yielding a string length of 10.

Answer 2

Here is an example:

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

#define MAX_DIGITS 5

int
main ()
{
  char buf[80];
  char *s = NULL;
  printf ("\n>>Enter string, %d digits or less: ", MAX_DIGITS);
  s = fgets (buf, MAX_DIGITS+1, stdin);
  printf ("strlen(buf)=%d, buf=%s, strlen(s)=%d, s=%s\n",
    strlen(buf), buf, strlen(s), s);
  return 0;
}

Sample output, with "MAX_DIGITS" and "MAX_DIGITS + 1":

>>Enter string, 5 digits or less: 1
strlen(buf)=2, buf=1
, strlen(s)=2, s=1
.

>>Enter string, 5 digits or less: 12
strlen(buf)=3, buf=12
, strlen(s)=3, s=12
.

>>Enter string, 5 digits or less: 123
strlen(buf)=4, buf=123
, strlen(s)=4, s=123
.

>>Enter string, 5 digits or less: 1234
strlen(buf)=5, buf=1234
, strlen(s)=5, s=1234
.

>>Enter string, 5 digits or less: 12345
strlen(buf)=5, buf=12345, strlen(s)=5, s=12345.

>>Enter string, 5 digits or less: 123456
strlen(buf)=5, buf=12345, strlen(s)=5, s=12345.

You'll notice:

1. The return buffer retains the "\\n" as long as the #/digits are < MAX_DIGITS.

2. The "\\n" is REMOVED when #/digits >= MAX_DIGITS.

3. Your buffer must accomodate MAX_DIGITS+1

Answer 3

Actually fgets requires a size specification (in your case 11 ) account for the \\0 at the end of a string. From the fgets man page:

fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A terminating null byte ('\\0') is stored after the last character in the buffer.

So we know that reading stops at \\n , when you enter 123456789\\n . However when you enter 1234567890\\n , fgets() processes the input but it only takes the 10 characters and ignores everything else afterwards.

Any extra input of your string and your string will at size-1 of fget() with last character as \\0 so the output remains the same.