System functions call in C

Question

I can't understand why this piece of code does not work. Read() returns -1. I opened file like this:

int descr = open(filepath, O_RDWR);

then I tried this:

void* bufi = malloc(blockSize);
void* bufj = malloc(blockSize);


//Loading to buffs
descr = lseek(descr, blockSize*i, SEEK_SET);

printf("%zd\n", read(descr, bufi, blockSize));

descr = lseek(descr, blockSize*j, SEEK_SET);

read(descr, bufj, blockSize);


//Writing from bufs to apropriate places in file
descr = lseek(descr, blockSize*j, SEEK_SET);

write(descr, bufi, blockSize);

descr = lseek(descr, blockSize*i, SEEK_SET);

write(descr, bufj, blockSize);

File I tried to open was treated with "chmod 777" in case that was a problem. Thanks in advance.

Answer 1

int descr = open(filepath, O_RDWR);

descr = lseek(descr, blockSize*i, SEEK_SET);

printf("%zd\n", read(descr, bufi, blockSize));

By the time you get to read , you've changed the value of descr to be whatever lseek returned, which is very unlikely to be a valid file descriptor. Don't use descr to hold the value returned by lseek .