Read from pipe line by line in C

Question

How can I separate the lines which are coming from a pipe. In the pipe there is this text:

HALLO:500\n
TEST:300\N
ADAD
ADAWFFA
AFAGAGAEG

I want to separate the lines from the pipe because i want to save the values in variables.

Here is my c code:

#include <stdio.h>
#include <stdlib.h>

#define BUFFERSIZE    1

int main(int argc, char **argv){
    unsigned char     buffer[BUFFERSIZE];
    FILE                         *instream;
    int                            bytes_read=0;
    int                            buffer_size=0;


    buffer_size=sizeof(unsigned char)*BUFFERSIZE;
    /* open stdin for reading */
    instream=fopen("/dev/stdin","r");

    /* did it open? */
    if(instream!=NULL){
        /* read from stdin until it's end */
        while((bytes_read=fread(&buffer, buffer_size, 1, instream))==buffer_size){
            fprintf(stdout, "%c", buffer[0]);
        }
    }
    /* if any error occured, exit with an error message */
    else{
        fprintf(stderr, "ERROR opening stdin. aborting.\n");
        exit(1);
    }

    return(0);
}

Is this the right way to read from pipe for the best line by line?

Answer 1

This is usually just called reading from stdin. The program shouldn't care whether the input is a pipe, a redirected file, or a keyboard.

fread will just read until the buffer is full. Use fgets to read a line.

Also the buffer size should be big enough to hold the line. For little one-off programs, you can just pick a number. Or there's a standard name BUFSIZ which gives you a pretty-big buffer. How big? Big enough. Really? Probably.

fgets will copy the newline character in the string unless the string fills up first. So you can test the last character to tell if the line was truncated or not. With reasonable inputs, that's not going to happen. But a more robust approach would allocate a larger buffer, copy the partial line, and call fgets again tp keep trying to get a complete line.

#include <stdio.h>

int main() {
    char buf[BUFSIZ];
    fgets(buf, sizeof buf, stdin);
    if (buf[strlen(buf)-1] == '\n') {
        // read full line
    } else {
        // line was truncated
    }
    return 0;
}

This gets you halfway to being protected from the dreaded buffer overflow problem. fgets will not write more than the size passed to it. The other half, as mentioned above, is doing something sensible with the possible partial lines that may result from unexpectedly long input lines.