C : scanf skips the first iteration while taking char* inputs from user through scanf

Question

I am trying to get Subject names from user using dynamic memory allocation and char **. I am not sure its the best way to do so.

Problem : scanf is getting skipped for first iteration in for loop. Tried with putting " %s", "\n%s" as suggested from StackOverflow but still facing the same.

Here is my code :

int nSubjects;
char **subNames;

printf("\nEnter no. of Subjects : ");
scanf("%d",&nSubjects);

subNames = malloc(nSubjects *  sizeof(char*));

 for(i=0;i<nSubjects;i++){

    *(subNames+i)= (char*)malloc(sizeof(char*));
    printf("\nEnter Subject %d name : ",i+1);
    fflush(stdin);
    scanf("%s",subNames[i]);
}

Complete code (if required): http://pastebin.com/7Ncw0mWF

Please guide me where I am doing wrong in this code. Any help will be highly appreciated.

don't use `fflush(stdin);` it causes undefined behavior on other then Microsoft compilers, Also don't cast returned address of malloc and calloc in C — Grijesh Chauhan, Jan 30 '14 at 07:27
Instead of using `*(subNames+i)`, why not the more easy to understand `subNames[i]`? You do it later? — Some programmer dude, Jan 30 '14 at 07:29
To help avoid allocation size bugs, use GNU `getline()` (http://www.gnu.org/software/libc/manual/html_node/Line-Input.html) or something similar to read dynamically sized strings. If you're not using a GNU toolchain a public domain (if not particularly efficient) version can be found here: http://stackoverflow.com/a/12169132/12711 — Michael Burr, Jan 30 '14 at 07:37
@Kartik: give an example of your input - does it contain whitespace within the lines of data? — Michael Burr, Jan 30 '14 at 08:16
For no. of subject I am giving 5. Then loop starts : Enter Subject 1 name : Enter Subject 2 name : __(cmd pointer) — Kartik Domadiya, Jan 30 '14 at 08:18
malloc , fflush, scanf ,To work correctly if you modify correctly. And utilizing a different code otherwise. — BLUEPIXY, Jan 30 '14 at 08:18
@MichaelBurr here is the complete code http://pastebin.com/7Ncw0mWF — Kartik Domadiya, Jan 30 '14 at 08:27
i insert test print. it work fine. Do you or not enter a non-numeric in place of typing twice the number in the first place? — BLUEPIXY, Jan 30 '14 at 08:36
@BLUEPIXY I am entering number then press enter. Please have a look to complete code http://pastebin.com/7Ncw0mWF — Kartik Domadiya, Jan 30 '14 at 08:39
There is a problem with your code(other place). In order to inspect the part in question is now about after the input of the subject `for (i = 0; i — BLUEPIXY, Jan 30 '14 at 09:00

score 2 · Accepted Answer · edited May 23 '17 at 12:11

2

You allocation is wrong for string in side loop:

*(subNames+i)= (char*)malloc(sizeof(char*));

should be:

*(subNames+i)=  malloc(sizeof(char) * (string_lenght + 1));

Additionally, don't use fflush(stdin); it causes undefined behavior on other then Microsoft compilers, Also don't cast returned address of malloc() and calloc() in C

Read: Why fflush(stdin) is wrong?, and read: fflush @msdn.microsoft.

edited May 23 '17 at 12:11

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answered Jan 30 '14 at 07:29

Grijesh Chauhan

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Thanks for very quick response. I changed the allocation code as you said and removed fflush. But still facing the problem :( – Kartik Domadiya Jan 30 '14 at 07:35
Don't know whats the problem actually.. :( – Kartik Domadiya Jan 30 '14 at 08:15
@Kartik Try by replacing `scanf("%s",subNames[i]);` with `scanf(" %s",subNames[i]);` added a space before `%` in format string in scanf .. – Grijesh Chauhan Jan 30 '14 at 08:27
Tried with that as well. Couldnot help :( – Kartik Domadiya Jan 30 '14 at 08:29
2

@Kartik I checked you code you have many mistakes in format string in your code e.g. `scanf("%d",&p->name);` should be `scanf("%s",p->name);` Compile your code with `gcc -Wall` you will get to know. – Grijesh Chauhan Jan 30 '14 at 08:46

score 0 · Answer 2 · answered Jan 30 '14 at 07:37

Your malloc is wrong:

 *(subNames[i]) = (char*)malloc(sizeof(char*));

You are trying to allocate memory for a char** which is actually then being converted to a char*. You also need the specify the size of length, or this will allocate a single pointer. Note: subNames[i] doesn't need to be dereferenced.

The below should work:

  (subNames[i]) =  (char*)malloc(sizeof(char) * (string_length + 1));

Note: you will have to declare the string length variable, maybe perhaps as 1000, and then realloc it to the strlen of the string entered afterwards.

Tried it. But still the same problem. Here is the complete code http://pastebin.com/7Ncw0mWF — Kartik Domadiya, Jan 30 '14 at 08:25

score 0 · Answer 3 · edited May 23 '17 at 12:28

0

If your compiler is gcc and your gcc> 2.7, you can use "%ms". this will allow scanf to allocate memory for your pointer:

for(i=0;i<nSubjects;i++){
    printf("\nEnter Subject %d name : ",i+1);
    scanf("%ms",&subNames[i]);
}

How to store a character string into a character pointer declared in a structure

edited May 23 '17 at 12:28

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answered Jan 30 '14 at 07:40

MOHAMED

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Whether the completely non-portable %ms hack works is not a feature of the compiler, but of the standard IO library, of which scanf is a part of. So it's likely a glibc thing, not a gcc thing. – Jens Jan 30 '14 at 07:45

C : scanf skips the first iteration while taking char* inputs from user through scanf

3 Answers3