When I compile and run this the output is:
press n to continue
n
Enter the filename: [ �h�� ]
But, if I call the new(); directly it run perfectly. But when I call new(); in if statement or switch statement, it shows the above output.
I tried scanf, fgets and gets in the new() fucntion but still not working.
#include<stdio.h>
#include<stdlib.h>
int menu();
int new();
int main(){
menu();
return 0;
}
int menu(){
printf("press n to continue\n");
//char c = getc(stdin);
char c = getchar();
if(c=='n'){
new();
}
else if(c==27){
return 0;
}
}
int new(){
char filename[50];
printf("Enter the filename: ");
//fgets(filename, 50, stdin);
scanf("%[^\n]s", filename);
printf("[ %s ]\n\n", filename);
return 0;
}
getchar() will read one character from stdin and leave the \n. So when you call scanf - it stops immediately and you got nothing. To skip whitespaces and start reading from non-space character add space before format.
scanf(" %49[^\n]", filename);
Do not mix %[] and %s
Always specify max number of chars to read (leaving one additional char for nul-terminator)
And compile with highest warning level - so you do not leave menu function without return.
Oh. and check the return value of scanf
if(scanf(" %49[^\n]", filename) == 1)
printf("[ %s ]", filename);
Related
I just started programming in C and I was wondering why I can't store a string with multiple words with scanf().
For example, I enter: "That's an example" and it's stores only the first word "That's"
My code:
int main(void) {
char string[100];
printf("Please enter something: ");
scanf("%s", &string);
printf("You entered: %s", string);
return (0);
}
You can let scanf() read more than one word with the character class conversion specifier: %[^\n] will stop at the newline and leave it pending in the input stream. Note that you must tell scanf the maximum number of characters to store into the destination array to avoid undefined behavior on long input lines. When passing an array to scanf(), you should not pass its address as &string, but just pass string as arrays decays into a pointer to their first element when passed as a function argument.
Here is a modified version:
#include <stdio.h>
int main(void) {
char string[100];
int c;
for (;;) {
printf("Please enter something: ");
/* initialize `string` in case the `scanf()` conversion fails on an empty line */
*string = '\0';
if (scanf("%99[^\n]", string) == EOF)
break;
printf("You entered: %s\n", string);
/* read the next byte (should be the newline) */
c = getchar();
if (c == EOF) /* end of file */
break;
if (c != '\n')
ungetc(c, stdin); /* not a newline: push it back */
}
return 0;
}
Note however that it is much simpler to use fgets() for this task:
#include <stdio.h>
int main(void) {
char string[100];
for (;;) {
printf("Please enter something: ");
if (!fgets(string, sizeof string, stdin))
break;
/* strip the trailing newline, if any */
string[strcspn(string, "\n")] = '\0';
printf("You entered: %s\n", string);
}
return 0;
}
#include <stdio.h>
#define BUFF_SIZE 512
int main(void) {
char string[BUFF_SIZE];
printf("Enter something: ");
fgets(string, BUFF_SIZE, stdin);
printf("You entered: %s", string);
return (0);
}
fgets() is the best option
I think there's a problem in you scanf(); I recommend you to remove & from it. then your code should see like that:
int main(void) {
char string[100];
printf("Please enter something: ");
scanf("%s", string);
printf("You entered: %s", string);
return (0);
}
In the c language, there is no data type called a string.
A string is stored as an array of characters.
Moreover, the variable itself points to the first element of the array. Therefore, there is no need to use the '&' operator to pass the address.
So, all you have to do is the following:
int main(void) {
char string[100];
printf("Please enter something: ");
scanf("%s", string);
printf("You entered: %s", string);
return (0);
}
Don't use '&' in scanf function.
int main()
{
char string[100];
printf("Please enter something: ");
scanf("%[^\n]%*c",string);
printf("You entered: %s", string);
return 0;
}
According to https://man7.org/linux/man-pages/man3/scanf.3.html, %s will ignore white-space characters. To capture spaces you would have to use %c with the additional size of the input argument, or use %[ format. Check if scanf will add \0 byte to the end or not.
My code:
#include <stdio.h>
#include <math.h>
int main()
{
int t;
scanf("%d",&t);
while(t--)
{
char a[10],b[10];
puts("enter");
gets(a);
puts("enter");
gets(b);
puts("enter");
puts(a);
puts(b);
}
return 0;
}
Output:
1
enter
enter
surya (string entered by user)
enter
surya (last puts function worked)
How can I use “gets” function many times in C program?
You should never ever use gets() in your program. It is deprecated because it is dangerous for causing buffer overflow as it has no possibility to stop consuming at a specific amount of characters - f.e. and mainly important - the amount of characters the buffer, a or b with each 10 characters, is capable to hold.
Also explained here:
Why is the gets function so dangerous that it should not be used?
Specially, in this answer from Jonathan Leffler.
Use fgets() instead.
Also the defintion of a and b inside of the while loop doesn´t make any sense, even tough this is just a toy program and for learning purposes.
Furthermore note, that scanf() leaves the newline character, made by the press to return from the scanf() call in stdin. You have to catch this one, else the first fgets() thereafter will consume this character.
Here is the corrected program:
#include <stdio.h>
int main()
{
int t;
char a[10],b[10];
if(scanf("%d",&t) != 1)
{
printf("Error at scanning!");
return 1;
}
getchar(); // For catching the left newline from scanf().
while(t--)
{
puts("Enter string A: ");
fgets(a,sizeof a, stdin);
puts("Enter string B: ");
fgets(b,sizeof b, stdin);
printf("\n");
puts(a);
puts(b);
printf("\n\n");
}
return 0;
}
Execution:
$PATH/a.out
2
Enter string A:
hello
Enter string B:
world
hello
world
Enter string A:
apple
Enter string B:
banana
apple
banana
The most important message for you is:
Never use gets - it can't protect against buffer overflow. Your buffer can hold 9 characters and the termination character but gets will allow the user to typing in more characters and thereby overwrite other parts of the programs memory. Attackers can utilize that. So no gets in any program.
Use fgets instead!
That said - what goes wrong for you?
The scanf leaves a newline (aka a '\n') in the input stream. So the first gets simply reads an empty string. And the second gets then reads "surya".
Test it like this:
#include <stdio.h>
int main()
{
int t;
scanf("%d",&t);
while(t--)
{
char a[10],b[10];
puts("enter");
gets(a); // !!! Use fgets instead
puts("enter");
gets(b); // !!! Use fgets instead
puts("enter");
printf("|%s| %zu", a, strlen(a));
printf("|%s| %zu", b, strlen(b));
}
return 0;
}
Input:
1
surya
whatever
Output:
enter
enter
enter
|| 0|surya| 5
So here you see that a is just an empty string (length zero) and that b contains the word "surya" (length 5).
If you use fgets you can protect yourself against user-initiated buffer overflow - and that is important.
But fgets will not remove the '\n' left over from the scanf. You'll still have to get rid of that your self.
For that I recommend dropping scanf as well. Use fgets followed by sscanf. Like:
if (fgets(a,sizeof a, stdin) == NULL)
{
// Error
exit(1);
}
if (sscanf(a, "%d", &t) != 1)
{
// Error
exit(1);
}
So the above code will automatically remove '\n' from the input stream when inputtin t and the subsequent fgets will start with the next word.
Putting it all together:
#include <stdio.h>
int main()
{
int t;
char a[10],b[10];
if (fgets(a,sizeof a, stdin) == NULL)
{
// Error
exit(1);
}
if (sscanf(a, "%d", &t) != 1)
{
// Error
exit(1);
}
while(t--)
{
puts("enter");
if (fgets(a,sizeof a, stdin) == NULL)
{
// Error
exit(1);
}
puts("enter");
if (fgets(b,sizeof b, stdin) == NULL)
{
// Error
exit(1);
}
puts("enter");
printf("%s", a);
printf("%s", b);
}
return 0;
}
Input:
1
surya
whatever
Output:
enter
enter
enter
surya
whatever
Final note:
fgets will - unlike gets - also save the '\n' into the destination buffer. Depending on what you want to do, you may have to remove that '\n' from the buffer.
Given the following code:
#include <stdio.h>
int main()
{
int testcase;
char arr[30];
int f,F,m;
scanf("%d",&testcase);
while(testcase--)
{
printf("Enter the string\n");
fgets(arr,20,stdin);
printf("Enter a character\n");
F=getchar();
while((f=getchar())!=EOF && f!='\n')
;
putchar(F);
printf("\n");
printf("Enter a number\n");
scanf("%d",&m);
}
return 0;
}
I want a user to enter a string, a character and a number until the testcase becomes zero.
My doubts / questions:
1.User is unable to enter a string. It seems fgets is not working. Why?
2.If i use scanf instead of fgets,then getchar is not working properly, i.e whatever character I input in it just putchar as a new line. Why?
Thanks for the help.
Mixing functions like fgets(), scanf(), and getchar() is error-prone. The scanf() function usually leaves a \n character behind in the input stream, while fgets() usually does not, meaning that the next call to an I/O function may or may not need to cope with what the previous call has left in the input stream.
A better solution is to use one style of I/O function for all user input. fgets() used in conjunction with sscanf() works well for this. Return values from functions should be checked, and fgets() returns a null pointer in the event of an error; sscanf() returns the number of successful assignments made, which can be used to validate that input is as expected.
Here is a modified version of the posted code. fgets() stores input in a generously allocated buffer; note that this function stores input up to and including the \n character if there is enough room. If the input string is not expected to contain spaces, sscanf() can be used to extract the string, leaving no need to worry about the newline character; similarly, using sscanf() to extract character or numeric input relieves code of the burden of further handling of the \n.
#include <stdio.h>
int main(void)
{
int testcase;
char arr[30];
char F;
int m;
char buffer[1000];
do {
puts("Enter number of test cases:");
if (fgets(buffer, sizeof buffer, stdin) == NULL) {
/* handle error */
}
} while (sscanf(buffer, "%d", &testcase) != 1 || testcase < 0);
while(testcase--)
{
puts("Enter the string");
/* if string should not contain spaces... */
if (fgets(buffer, sizeof buffer, stdin) == NULL) {
/* handle error */
}
sscanf(buffer, "%29s", arr);
printf("You entered: %s\n", arr);
putchar('\n');
puts("Enter a character");
if (fgets(buffer, sizeof buffer, stdin) == NULL) {
/* handle error */
}
sscanf(buffer, "%c", &F);
printf("You entered: %c\n", F);
putchar('\n');
do {
puts("Enter a number");
if (fgets(buffer, sizeof buffer, stdin) == NULL) {
/* handle error */
}
} while (sscanf(buffer, "%d", &m) != 1);
printf("You entered: %d\n", m);
putchar('\n');
}
return 0;
}
On the other hand, if the input string may contain spaces, fgets() can read input directly into arr, but then the stored string will contain a \n character, which should probably be removed. One way of doing this is to use the strcspn() function to find the index of the \n:
#include <string.h> // for strcspn()
/* ... */
puts("Enter the string");
/* or, if string may contain spaces */
if (fgets(arr, sizeof arr, stdin) == NULL) {
/* handle error */
}
/* replace newline */
arr[strcspn(arr, "\r\n")] = '\0';
printf("You entered: %s\n", arr);
putchar('\n');
/* ... */
Note that a maximum width should always be specified when using %s with the scanf() functions to avoid buffer overflow. Here, it is %29s when reading into arr, since arr can hold 30 chars, and space must be reserved for the null terminator (\0). Return values from sscanf() are checked to see if user input is invalid, in which case the input is asked for again. If the number of test cases is less than 0, input must be entered again.
Finally got the solution how can we use scanf and fgets together safely.
#include <stdio.h>
int main()
{
int testcase,f,F,m;
char arr[30];
scanf("%d",&testcase);
while((f=getchar())!=EOF && f!='\n')
;
while(testcase--)
{
printf("Enter the string\n");
fgets(arr,30,stdin);
printf("Enter a character\n");
F=getchar();
while((f=getchar())!=EOF && f!='\n')
;
putchar(F);
printf("\n");
printf("Enter a number\n");
scanf("%d",&m);
while((f=getchar())!=EOF && f!='\n')
;
}
}
We need to make sure that before fgets read anything,flushout the buffer with simple while loop.
Thanks to all for the help.
A simple hack is to write a function to interpret the newline character. Call clear() after each scanf's
void clear (void){
int c = 0;
while ((c = getchar()) != '\n' && c != EOF);
}
Refer to this question for further explaination: C: Multiple scanf's, when I enter in a value for one scanf it skips the second scanf
I wrote a program that collects user data and saves it to a file. At the moment when he wants to view the file, the program loops and shows only the first record. I do not know what this error is caused.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
FILE *fptr;
struct notification {
char name[50];
char lastname[50];
char price[10];
char descreption[100];
}notification;
void insertRecord()
{
fptr=fopen("G:\\file.txt","a+");
fflush(stdin);
printf("Podaj imie: ");
gets(notification.name);
printf("Podaj nazwisko: ");
gets(notification.lastname);
printf("Podej cene: ");
gets(notification.price);
printf("Podaj opis usterki: ");
gets(notification.descreption);
strcat(notification.descreption,"\n");
if(fwrite(¬ification,sizeof(notification),1,fptr) != 1)
{
perror("Blad: ");
} else{
printf("Dane dodane poprawnie\n");
}
fclose(fptr);
}
void readDatabase()
{
struct notification *object2=malloc(sizeof(struct notification));
fptr=fopen("G:\\file.txt","rb");
fread(object2,sizeof(struct notification),1,fptr);
while(!feof(fptr))
{
printf("Imie: %s\n", object2->name);
printf("Nazwisko: %s\n", object2->lastname);
printf("Cena: %s\n", object2->price);
printf("Opis: %s\n", object2->descreption);
printf("==========\n");
}
fclose(fptr);
}
int main() {
int i,option=0,check=0;
do{
printf("1) Dodaj rekord do bazy \n");
printf("2) Odczytaj rekordy z bazy \n");
printf("0) Zakoncz program \n");
scanf("%d", &option);
switch (option)
{
case 1:
insertRecord();
break;
case 2:
readDatabase();
break;
default:
break;
}
}while(check == 0); //petla dziala dopóki zmienna check bedzie równa 0
}
EDIT:
Correct insertRecord function:
void insertRecord()
{
fptr=fopen("G:\\file.txt","a+");
fflush(stdin);
struct notification *obj = malloc(sizeof(struct notification));
printf("Podaj imie: ");
gets(obj->name);
printf("Podaj nazwisko: ");
gets(obj->lastname);
printf("Podej cene: ");
gets(obj->price);
printf("Podaj opis usterki: ");
gets(obj->descreption);
strcat(notification.descreption,"\n");
if(fwrite(obj,sizeof(struct notification),1,fptr) != 1)
{
perror("Blad: ");
} else{
printf("Dane dodane poprawnie\n");
}
free(obj);
fclose(fptr);
}
Now ALL display and insert OK, but in file.txt I see Chinese characters, why?
There are a variety of problems in the readDatabase function
while(!feof)-is-always-wrong
the fread needs to be in the loop.
you don't need to malloc the memory, but if you do malloc memory, you should free it when you're done with it
you always need to check the return value from fopen, because it can and does fail, e.g. because the file is not found
With all that in mind, the readDatabase function should look like this
void readDatabase( void )
{
struct notification object2;
if ( (fptr = fopen("G:\\file.txt","rb")) == NULL )
{
printf( "File not found\n" );
return;
}
while ( fread( &object2, sizeof(struct notification), 1, fptr ) == 1 )
{
printf("Imie: %s\n", object2.name);
printf("Nazwisko: %s\n", object2.lastname);
printf("Cena: %s\n", object2.price);
printf("Opis: %s\n", object2.descreption);
printf("==========\n");
}
fclose(fptr);
}
Move this line:
fread(object2,sizeof(struct notification),1,fptr);
inside your while loop.
scanf("%d", &option); followed by gets() leads to trouble. The first does not consume the '\n' after the number and the second only reads in the short line '\n'.
Do not use scanf(). Do not use gets(). Use fgets(), then parse the input.
scanf() will leave new line character in input stream by default. you can use getchar() function to clear this new line character or you can flush the input buffer like this.
while ((ch = getchar()) != '\n' && ch != EOF);
but don't use fflush(stdin) because if the file stream is for input use, as stdin is, the behaviour is undefined, therefore it is not acceptable to use fflush() for clearing keyboard input. As usual, there are some exceptions, check your compiler's documentation to see if it has a (non-portable) method for flushing input.
I am learning file handling in C.I have this code but it is not accepting string as an input to write it to a file.Any help will be appreciated.
#include<stdio.h>
#include<string.h>
#include <stdlib.h>
int main(void)
{
FILE * fp1;
fp1 = fopen("abc.txt","a+");
if(fp1==NULL)
{printf("An error occurred");
}
printf("Delete file?\n");
int a,c;
char name [20];
int flag=1;
int ch=1;
while(flag!=0)
{
printf("Enter id input \n");
scanf("%d",&a);
fprintf(fp1,"\n%d\t",a);
printf("Enter Name");
gets(name);
fputs(name, fp1);
printf("Enter No \n");
scanf("%d",&c);
fprintf(fp1,"\t%d\t",c);
printf("Write more then press 0 else 1");
scanf("%d",&ch);
if(ch==1)
{
flag=0;
}
}
fclose(fp1);
}
On running this code the code does not take an input after Enter Name and directly skips to Enter No.I want the output to be in a tabular form.
Use a getchar() after entering id because the \n of 1st scanf stays in buffer.
printf("Enter id input \n");
scanf("%d",&a);
getchar();
When you enter a number for scanf("%d",&a);, you type in a number and press the Enter key. The scanf consumes the number and leaves the newline character ('\n') in the standard input stream (stdin). When the execution of the program reaches gets(name);, gets sees the newline character and consumes it, storing it in name.
Firstly, never use gets as it is dangerous as it doesn't prevent buffer overflows. Use fgets instead:
fgets(name, sizeof(name), stdin);
Secondly, you have to get rid of the newline character. You can do this by flushing the stdin. Or you can simply scan and discard the newline character just after reading the number from scanf by changing
scanf("%d",&a);
to
scanf("%d%*c",&a);
%*c scans and discards a character.
gets() is deprecated, don't use it. you can still use scanf()...
as for the tabulation...think it through.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void)
{
FILE* fp1;
fp1 = fopen("abc.txt", "a+");
if (fp1 == NULL) {
printf("An error occurred");
}
int a, c;
char name [20];
int flag = 1;
int ch = 1;
while (flag != 0) {
printf("Enter id input:\n");
scanf("%d", &a);
fprintf(fp1, "%d\t", a);
printf("Enter Name:\n");
scanf("%s", name);
fprintf(fp1, "%s\t", name);
printf("Enter No:\n");
scanf("%d", &c);
fprintf(fp1, "%d\n", c);
printf("Again (0) or Exit(1) ?:\n");
scanf("%d", &ch);
if (ch == 1) {
flag = 0;
}
}
fclose(fp1);
return 0;
}