readline.c

一、第一版

#include <unistd.h>
#include <errno.h>

ssize_t readline(int fd, void *vptr, size_t maxlen) {
    ssize_t n, rc;
    char c, *ptr;

    ptr = vptr;
    for (n = 1; n <= maxlen; n++) {
        again:
        if ( (rc = read(fd, &c, 1)) == 1) {
            *ptr++ = c;
            if (c == '\n') {
                break;
            }
        } else if (rc == 0) {
            *ptr = 0;
            return (n - 1);
        } else {
            if (errno == EINTR) {
                goto again;
            } 
            return (-1);
        }
    }
    *ptr = 0;
    return ((maxlen == (n-1))?(n-1):n);
}

函数功能：从一个描述符读文本行，一次一个字节

二、改进版

#include <errno.h>
#include <unistd.h>

#define MAXLINE 4096

static int read_cnt = 0;
static char *read_ptr;
static char read_buf[MAXLINE];

static ssize_t my_read(int fd, char *ptr) {
    if (read_cnt <= 0) {
        again:
        if ( (read_cnt = read(fd, read_buf, sizeof(read_buf))) < 0) {
            if (errno == EINTR) {
                goto again;
            }
            return (-1);            
        } else if (read_cnt == 0) {
            return (0);
        }
        read_ptr = read_buf;
    }
    read_cnt--;
    *ptr = *read_ptr++;
    return (1);
}

ssize_t readline(int fd, void *vptr, size_t maxlen) {
    ssize_t n, rc;
    char c, *ptr;

    ptr = vptr;
    for (n = 1; n < maxlen; n++) {
        if ( (rc = my_read(fd, &c)) == 1) {
            *ptr++ = c;
            if (c == '\n') {
                break;
            } 
        } else if (c == 0) {
            *ptr = 0;
            return (n - 1);
        } else {
            return (-1);
        }
    }
    *ptr = 0;
    return (n);
}

ssize_t readlinebuf(void **vptrptr) {
    if (read_cnt) {
        *vptrptr = read_ptr;
    }
    return (read_cnt);
}

功能：readline的改进版，自带缓冲区
问题：使用静态变量实现跨域相继函数调用的状态信息维护，使函数变得不可重入或者说非线程安全