Re: Reading a string of unknown size


santosh wrote:
Santosh wrote:
I have to read characters from stdin and save them in a string. The
problem is that I don't know how much characters will be read.


First include necessary headers: stdio.h, stdlib.h

int main()

Better yet, replace above with int main(void)

{
char *str = NULL, ch ;
int i = 0 ;
str = (char*) malloc (2*sizeof(char)) ;

Don't cast return value of malloc() in C. It can hide the non-inclusion
of it's prototype, (by way of failure to include stdlib.h), and, on
some implementations, can result in nasty crashes during runtime.

Since sizeof(char) is by definition 1, you can omit that and instead do
'2 * sizeof *str'. This has the advantage of becoming automatically
updated when you later on happen to change the type of *str.

*str = '\0' ;

And now you're possibly writing to a random area of memory, since you
failed to check the return value of malloc() above for failure.

while( (ch=getchar()) != '\n' )

Check for EOF not newline. Moreover getchar() returns an int value
which you're storing in a char variable, probably getting a spurious
garbage return value when end-of-file is encountered.

{
*(str+i) = ch ;

You've overwritten your earlier nul character.

i++ ;
str = (char*) realloc(str, (2*sizeof(char)) + i ) ;

Again, _don't_ cast the return value of XXalloc() functions in C, and
check the call for failure before proceeding further. Also change
sizeof(char) to sizeof *str.

Anyway, your allocation strategy is very inefficient. Your calling
realloc() once every iteration of the loop. This could result in
fragmentation of the C library's memory pool. Why not allocate in terms
of fixed sized or dynamically growing blocks, say 128 bytes or so to
start with?

}
*(str+i) = '\0' ;

printf("\n\n %s ", str) ;

Unless you terminate the output with a newline character, it's not
guaranteed to show up on screen, or wherever stdout happens to be
directed to.

getch() ;

Non-standard, unportable and unnecessary function. Just get rid of it.


Can u please further explain the following statement....

"Don't cast return value of malloc() in C. It can hide the
non-inclusion
of it's prototype, (by way of failure to include stdlib.h), and, on
some implementations, can result in nasty crashes during runtime."

I am unable to understand the intricacies of the above statement.

.

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