Read from stdin write to stdout in C

问题:

I am trying to write a cat clone to exercise C, I have this code:

#include <stdio.h>
#define BLOCK_SIZE 512
int main(int argc, const char *argv[])
{
    if (argc == 1) { // copy stdin to stdout
        char buffer[BLOCK_SIZE];
        while(!feof(stdin)) {
            size_t bytes = fread(buffer, BLOCK_SIZE, sizeof(char),stdin);
            fwrite(buffer, bytes, sizeof(char),stdout);
        }
    }
    else printf("Not implemented.\n");
    return 0;
}

I tried echo "1..2..3.." | ./cat and ./cat < garbage.txt but I don't see any output on terminal. What I am doing wrong here?

Edit: According to comments and answers, I ended up doing this:

void copy_stdin2stdout()
{
    char buffer[BLOCK_SIZE];
    for(;;) {
        size_t bytes = fread(buffer,  sizeof(char),BLOCK_SIZE,stdin);
        fwrite(buffer, sizeof(char), bytes, stdout);
        fflush(stdout);
        if (bytes < BLOCK_SIZE)
            if (feof(stdin))
                break;
    }

}

回答1:

i can quote an answer by me: https://stackoverflow.com/a/296018/27800

fread(buffer, sizeof(char), block_size, stdin);

回答2:

Your problem appears to be the return value of fread. I modified your code to print out the value bytes, and I get 0 every time. The man page for fread makes it clear that the return value of fread is NOT the number of characters. If EOF is encountered the return value could be zero (which it is in this case). This is because you are attempting to read in 1 thing that is size BLOCK_SIZE rather than BLOCK_SIZE things that are size 1.

回答3:

Try calling fflush(stdout) after the fwrite()

回答4:

Ignore my comment about fflush; that's not the issue.

Swap the order of the block size and the element size in the fread call. You want to read up to BLOCK_SIZE elements of size 1 (sizeof (char) is 1 by definition); what you're doing is trying to read 1 element of size BLOCK_SIZE, so unless you type in at least BLOCK_SIZE characters, fread will return 0. IOW, your fread call needs to be

size_t bytes = fread(buffer, 1, sizeof buffer, stdin);

Make a similar change to the fwrite call.