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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}

001
# 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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114
         lockdep_on();
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out_restore_irqs:
116
         raw_local_irq_restore(flags);
117
 
118
         preempt_enable();
119
         return printed_len;
120
}

作者“[No]left的博客”