Null issues raised by scanf

[Michael8808]

Hello everyone,

I am a beginer of C programming and have a short question here.
I declared my variables as bellow in the main method.

char a[10];
char b[25];
char c[25];
char d[5];
char e[3];

scanf("%s%s%s%s %s",a,b,c,d,e );

printf("%s\t%s\ t%s\t%s\t%s\t", a,b,c,d,e);


when I do the scanf, my inputs are
a
b
c
A80
P01

(I have them in the same line when I input the values, here just for make them more clearly to read.)


My problem is that after the printf() executed, the outputs are
a
b

A80
P01.

The variable c is supposed not to be null. It should have its own value, which is 'c' in this case. I am wondering what causes the null pointer and what is the solution?

Your help will be mostly gratitude. Thanks all in advance.

[donbock]

The e array is not large enough to hold the entered string. The resulting overflow leads to undefined behavior.

Follow this convention to prevent buffer overflow:

char e[3];
...
scanf("%2s",e);

Where the field width is one less than the size of the buffer (to allow for the terminating null character).

[Michael8808]

Thanks for your answer.
However, it doest not solve my problem. Actually, my array e works properly, i think, as it contains the string which i input. (in this case, it's P01) The issue is in array C, for nothing is in it. Any new ideas?

[donbock]

Suppose the compiler organizes memory such that the c array immediately follows the e array. Four characters ('P', '0', '1', and '\0') are written into the e array, but since it is only large enough to hold three characters, the last one overflows into the next variable. Thus, a null character is written into the first slot of the c array.

I don't know if that is what's happening, but it is plausible and it would explain your symptoms. Fix the overflows. If you still have problems then we'll keep investigating.

[Michael8808]

yeah.. i think u r definitely right. I fixed the problem by set all the sizes of array to 80, though i think this may be kind of a waste of memory.
By the way, array in c language, especially in char[] type is so different from other languages'. Sometimes, i am sure that there should be only 10 elements in the array but i have to initialize the size of array to be a very large number, for instance 30. Otherwise, fragmentation faults will be raised. Do u know exactly how to exactly set the array size according to a specific need rather than just giving a big number for considerations of safety.

[jkmyoung]

When reading in your input with scanf, set width modifiers for the maximum amount of input allowed in.

Reference: http://www.cplusplus.com/reference/c.../cstdio/scanf/
Example: http://www.tenouk.com/clabworksheet/labworksheet6.html
Garbage in, garbage out.