Using Realloc in C
Solution 1
Yes is the short answer. Here's how it would look:
if ( i >= buffer_size )
{
temp = realloc(buffer, buffer_size*2);
if (!temp)
reportError();
buffer_size *= 2;
buffer = temp;
}
Note that you still need to use a temporary pointer to hold the result of realloc()
; if the allocation fails you still have the original buffer
pointer to the still-valid existing buffer.
Solution 2
Realloc is pretty much exactly what you're looking for - you can replace that entire block inside the if ( i >= buffer_size )
with something like:
buffer = (char*)realloc(buffer, buffer_size*2);
buffer_size *= 2;
Notice that this ignores the error condition (if the return from realloc
is NULL
); catching this condition is left to the reader.
Solution 3
Yes, realloc
could be used to slightly simplify your code. If you're not interested in error-handling, then this:
char *tmp = malloc(size*2);
memcpy(temp, buffer, size);
free(buffer);
buffer = tmp;
is essentially equivalent to this:
buffer = realloc(buffer, size*2);
If you are interested in error-handling (and you probably should be), then you will need to check for NULL
return values. This is true of your original code too.
Solution 4
Yes, to simplify your code, you can replace
if ( i >= buffer_size )
{
temp = (char*)malloc(buffer_size*2);
memcpy( temp, buffer, buffer_size );
free( buffer );
buffer_size *= 2;
buffer = temp;
}
with
if ( i >= buffer_size )
buffer = realloc(buffer, buffer_size *= 2);
This does not take into account error checking, so you will need to check to make sure realloc
doesn't return NULL
.
PnP
Updated on June 08, 2022Comments
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PnP about 2 years
Its really a post for some advice in terms of the use of realloc, more specifically, if I could make use of it to simplify my existing code. Essentially, what the below does, it dynamically allocate some memory, if i goes over 256, then the array needs to be increased in size, so I malloc a temp array, with 2x the size, memcpy etc. ( see below ).
I was just wondering if realloc could be used in the below code, to simplify it, any advice, sample code, or even hints on how to implement it is much appreciated!
Cheers.
void reverse(char *s) { char p; switch(toupper(s[0])) { case 'A': case 'E': case 'I': case 'O': case 'U': p = s[strlen(s)-1]; while( p >= s ) putchar( p-- ); putchar( '\n' ); break; default: printf("%s", s); break; } printf("\n"); } int main(void) { char c; int buffer_size = 256; char *buffer, *temp; int i=0; buffer = (char*)malloc(buffer_size); while (c=getchar(), c!=' ' && c!='\n' && c !='\t') { buffer[i++] = c; if ( i >= buffer_size ) { temp = (char*)malloc(buffer_size*2); memcpy( temp, buffer, buffer_size ); free( buffer ); buffer_size *= 2; buffer = temp; } } buffer[i] = '\0'; reverse(buffer); return 0;
}
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Kerrek SB over 12 years
realloc
might returnNULL
, giving you an instant unrecoverable leak. -
Graham Borland over 12 years-1. if the realloc fails (by returning NULL), you've lost the reference to the original block.
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Graham Borland over 12 years-1. if the realloc fails (by returning NULL), you've lost the reference to the original block.
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Oliver Charlesworth over 12 years@Graham: I explicitly said "if you're not interested in error-handling". The OP doesn't have any error-handling in his original code, either.
-
Kerrek SB over 12 yearsThis "error handling" isn't just some child's play like checking
std::cin >> n
; this one here may and will cause UB almost surely... -
Seth Carnegie over 12 years@GrahamBorland I said you could replace his original code with that code. He didn't check for
NULL
there, so neither did I. But I added a note to mention it. -
Salviati about 5 yearsCorrect me if I am wrong, but the
buffer=temp
, does that not copy over thetemp
memory address? So if I freetemp
afterwards, I also freebuffer
? If true, when do I free thetemp
variable in order to not waste memory of having two identical strings allocated? Sorry I am trying to learn how this works. -
Viaceslavus about 2 yearsbuffer = temp does copy the address of temp to buffer. But it does not allocate another string...