CODEWITHSUNDEEP
cio
cameron
cameron

Reputation: 33

fgets() input overflow to stderr

I have two inputs using fgets() function. Obviously, I declare size of input hoping it will truncate the input. It does truncate the input, but the remaining characters overflow into the following fgets(). I thought flushing the stdin would be the fix, but it doesn't seem to be working. Any help? Example below.

char a[3];
char b[3];
fgets(a, sizeof(a), stdin);
fflush(stdin);
fgets(b, sizeof(b), stdin);
fprintf(stderr, "%s%s", a, b);

an input of "help" followed by "ok" would produce the following:

hel
pok

Upvotes: 0

Views: 574

Answers (2)

user3629249
user3629249

Reputation: 16540

how to flush stdin

Do NOT use: fflush( stdin )

The function: fflush() is ONLY for output streams and the C programming language specifically states that calling fflush() with an input stream results in undefined behavior. (regardless of what visual studio might allow)

Suggest:

int ch;
while( (ch = getchar() ) != EOF && ch != '\n' ){;}

regarding the function: fgets()

That function always appends a NUL byte to the input and always stops 1 short of the length it is given, so there will be room for the NUL byte

regarding:

char a[3];
char b[3];
fgets(a, sizeof(a), stdin);
fflush(stdin);
fgets(b, sizeof(b), stdin);
fprintf(stderr, "%s%s", a, b)

replace:

fflush(stdin);

with:

int ch;
while( ( ch = getchar() ) != EOF && ch != '\n' ){;}

Upvotes: 0

user3121023
user3121023

Reputation: 8286

strchr could be used to look for a newline. If found, set the newline to zero to terminate the string. Call fgets to fill the remainder of the array. The loops continue until both arrays are filled.
The array with size [4] can store three characters and a terminating zero.

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

int main ( void) {
    char a[4];
    char b[4];
    char *newline = NULL;
    size_t len = 0;

    fgets(a, sizeof(a), stdin);
    while ( ( newline = strchr ( a, '\n'))) {//find a newline
        *newline = 0;//set terminating zero
        len = strlen ( a);
        if ( len < sizeof a - 1) {//space available in array
            fgets ( newline, sizeof a - len, stdin);//more input into array
        }
        else {
            break;
        }
    }

    fgets(b, sizeof(b), stdin);
    while ( ( newline = strchr ( b, '\n'))) {//find a newline
        *newline = 0;//set terminating zero
        len = strlen ( b);
        if ( len < sizeof b - 1) {//space available in array
            fgets ( newline, sizeof b - len, stdin);//more input into array
        }
        else {
            break;
        }
    }

    fprintf(stderr, "%s%s\n", a, b);
    return 0;
}

getchar could be used to clear the input stream if the first array does not contain a newline.

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

int main ( void) {
    char a[4];
    char b[4];

    fgets(a, sizeof(a), stdin);
    if ( ! strchr ( a, '\n')) {//no newline
        while ( '\n' != getchar ( )) {//clear the input stream
            if ( EOF == getchar ( )) {
                fprintf ( stderr, "problem EOF\n");
                return 1;
            }
        }
    }

    fgets(b, sizeof(b), stdin);

    fprintf(stderr, "%s%s\n", a, b);
    return 0;
}

Upvotes: 1

Related Questions