Linux内核中printf的实现（printk与vprintk）
大家应该都在找printf的实现，其实linux内核中就有，我本来想查一下，既然linux覆盖了中断向量表，那么怎么输出文字？就查printk的代码，发现原来printk就是printf。代码在printk.c
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#printk
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asmlinkage int printk( const char *fmt, ...)
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{
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va_list args;
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int r;
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va_start (args, fmt);
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r = vprintk(fmt, args);
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va_end (args);
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return r;
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}
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# vsprintk
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asmlinkage int vprintk( const char *fmt, va_list args)
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{
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int printed_len = 0;
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int current_log_level = default_message_loglevel;
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unsigned long flags;
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int this_cpu;
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char *p;
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boot_delay_msec();
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preempt_disable();
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/* This stops the holder of console_sem just where we want him */
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raw_local_irq_save(flags);
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this_cpu = smp_processor_id();
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/*
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* Ouch, printk recursed into itself!
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*/
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if (unlikely(printk_cpu == this_cpu)) {
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/*
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* If a crash is occurring during printk() on this CPU,
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* then try to get the crash message out but make sure
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* we can't deadlock. Otherwise just return to avoid the
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* recursion and return - but flag the recursion so that
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* it can be printed at the next appropriate moment:
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*/
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if (!oops_in_progress) {
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recursion_bug = 1;
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goto out_restore_irqs;
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}
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zap_locks();
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}
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lockdep_off();
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spin_lock(&logbuf_lock);
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printk_cpu = this_cpu;
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if (recursion_bug) {
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recursion_bug = 0;
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strcpy (printk_buf, recursion_bug_msg);
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printed_len = sizeof (recursion_bug_msg);
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}
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/* Emit the output into the temporary buffer */
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printed_len += vscnprintf(printk_buf + printed_len,
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sizeof (printk_buf) - printed_len, fmt, args);
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#ifdef CONFIG_DEBUG_LL
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printascii(printk_buf);
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#endif
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/*
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* Copy the output into log_buf. If the caller didn't provide
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* appropriate log level tags, we insert them here
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*/
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for (p = printk_buf; *p; p++) {
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if (new_text_line) {
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/* If a token, set current_log_level and skip over */
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if (p[0] == '<' && p[1] >= '0' && p[1] <= '7' &&
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p[2] == '>' ) {
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current_log_level = p[1] - '0' ;
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p += 3;
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printed_len -= 3;
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}
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/* Always output the token */
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emit_log_char( '<' );
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emit_log_char(current_log_level + '0' );
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emit_log_char( '>' );
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printed_len += 3;
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new_text_line = 0;
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if (printk_time) {
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/* Follow the token with the time */
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char tbuf[50], *tp;
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unsigned tlen;
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unsigned long long t;
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unsigned long nanosec_rem;
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t = cpu_clock(printk_cpu);
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nanosec_rem = do_div(t, 1000000000);
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tlen = sprintf (tbuf, "[%5lu.%06lu] " ,
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(unsigned long ) t,
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nanosec_rem / 1000);
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for (tp = tbuf; tp < tbuf + tlen; tp++)
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emit_log_char(*tp);
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printed_len += tlen;
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}
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if (!*p)
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break ;
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}
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emit_log_char(*p);
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if (*p == '/n' )
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new_text_line = 1;
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}
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/*
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* Try to acquire and then immediately release the
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* console semaphore. The release will do all the
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* actual magic (print out buffers, wake up klogd,
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* etc).
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*
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* The acquire_console_semaphore_for_printk() function
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* will release 'logbuf_lock' regardless of whether it
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* actually gets the semaphore or not.
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*/
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if (acquire_console_semaphore_for_printk(this_cpu))
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release_console_sem();
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lockdep_on();
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out_restore_irqs:
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raw_local_irq_restore(flags);
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preempt_enable();
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return printed_len;
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}
作者“[No]left的博客”
