How do I grow a dynamic buffer with read()? - c

Question

So I'm having a bit of trouble wrapping my head around growing a buffer, from what I'm seeing I'm definitely reading all of the bytes from the file descriptor, but it seems I'm not storing them in the buffer properly. Can someone help point me in the right direction?
char *read_all(int fd, int *nread){ //nread tracks total bytes read
int max_size = 1;
*nread = 0;
char *buf = malloc(max_size*sizeof(char));
int bytes_read = read(fd, buf, max_size*sizeof(char));
while(bytes_read > 0){
*nread+=bytes_read;
if(*nread >= max_size*sizeof(char)){
max_size*=2;
buf = realloc(buf, max_size*sizeof(char));
}
bytes_read = read(fd, buf, max_size*sizeof(char));
}
return buf;
}
==== EXPECT ==== ==== ACTUAL ====
{ {
// Tests the read_all() function to ensure that // Tests the read_all() function to ensure that
// it properly accumulates all data from an // it properly accumulates all data from an
// arbitrary input FD including allocating memory // arbitrary input FD including allocating memory
// for the data. // for the data.
int fd = open("test-data/quote.txt", O_RDONLY); int fd = open("test-data/quote.txt", O_RDONLY);
int bytes_read = -1; int bytes_read = -1;
char *actual_read = read_all(fd, &bytes_read); char *actual_read = read_all(fd, &bytes_read);
int result = close(fd); int result = close(fd);
printf("result: %d\n", result); printf("result: %d\n", result);
printf("bytes_read: %d\n", bytes_read); printf("bytes_read: %d\n", bytes_read);
actual_read[bytes_read] = '\0'; actual_read[bytes_read] = '\0';
printf("actual_read:\n" ); printf("actual_read:\n" );
printf("--------------------\n" ); printf("--------------------\n" );
printf("%s",actual_read); printf("%s",actual_read);
printf("--------------------\n" ); printf("--------------------\n" );
free(actual_read); free(actual_read);
} }
result: 0 result: 0
bytes_read: 125 bytes_read: 125
actual_read: actual_read:
-------------------- --------------------
Object-oriented programming is an exceptionally bad idea which could | could
only have originated in California. only have originated in California.
-- Edsger Dijkstra -- Edsger Dijkstra
-------------------- --------------------
ALERTS: ALERTS:
(

Answer 1

Somewhat bizzare...
I changed read_all() so it works more like expected: (full here)
char *read_all2(int fd, int *nread){ //nread tracks total bytes read
int max_size = 1;
*nread = 0;
char *buf = malloc(max_size*sizeof(char));
char *ptr = buf;
int bytes_read = 0;
//while(bytes_read > 0)
do
{
fprintf(stderr, "bytes_read=%d\n", bytes_read);
*nread += bytes_read;
ptr += bytes_read;
if(*nread >= max_size*1){
max_size *= 2;
fprintf(stderr, "realloc(buf=%08x, max_size=%d)\n", buf, max_size);
buf = realloc(buf, max_size*1);
}
} while((bytes_read = read(fd, ptr, max_size*1)) > 0);
return buf;
}
but when it realloc(buf, 16); it crashes.
$ gcc -o readynbuf2 readynbuf2.c ; ./readynbuf2
bytes_read=0
bytes_read=1
realloc(buf=016ba008, max_size=2)
bytes_read=2
realloc(buf=016ba008, max_size=4)
bytes_read=4
realloc(buf=016ba008, max_size=8)
bytes_read=8
realloc(buf=016ba008, max_size=16)
*** Error in `./readynbuf2': realloc(): invalid next size: 0x016ba008 ***
I'm out of ideas why would it happen.

Answer 2

Thanks guys, I managed to solve it, the key was passing &buf[*nread] into read().
char *read_all(int fd, int *nread){
int bytes_read = 0, size_mult = 1;
*nread = 0;
char * buf = malloc(1024);
bytes_read = read(fd, &buf[*nread], 256);
while(bytes_read > 0){
*nread += bytes_read;
if(*nread > size_mult*512){
size_mult*=2;
buf = realloc(buf, size_mult*1024);
}
bytes_read = read(fd, &buf[*nread], 256);
}
return buf;
}