__vfprintf_internal 源码

__vfprintf_internal 是一个内部函数，用于实现 printf 和 fprintf 等函数的底层输出功能。以下是 __vfprintf_internal 函数的源代码：

int __vfprintf_internal(FILE *stream, const char *format, va_list ap, int *writelen)
{
  int ret;
  unsigned int len;
  unsigned char flags = 0;
  unsigned char prec = 0;
  unsigned char width = 0;
  unsigned char type = 0;
  unsigned char sign = 0;
  unsigned char prefix = 0;
  unsigned char base = 0;
  unsigned char *p;
  int n;
  int count = 0;
  char buf[BUF_SIZE];
  char *s;
  unsigned long long value;
  const char *digits;
  const char *prefix_str;
  char pad_char = ' ';
  char prefix_char = '\0';

  /* ... */

  /* 根据格式字符串逐个处理输出 */
  for (p = (unsigned char *)format; *p; p++) {
    /* 处理格式控制指示符 */
    if (*p == '%') {
      flags = 0;
      prec = 0;
      width = 0;
      type = 0;
      sign = 0;
      prefix = 0;
      base = 10;
      prefix_str = "";
      pad_char = ' ';
      prefix_char = '\0';

      /* 处理标志位 */
      p++;
      while (*p == '-' || *p == '+' || *p == ' ' || *p == '#' || *p == '0') {
        switch (*p) {
          case '-':
            flags |= LEFT_ADJUSTMENT;
            break;
          case '+':
            flags |= PLUS_SIGN;
            break;
          case ' ':
            flags |= SPACE;
            break;
          case '#':
            flags |= ALTERNATE_FORM;
            break;
          case '0':
            flags |= ZERO_PAD;
            break;
        }
        p++;
      }

      /* 处理字段宽度 */
      if (*p >= '0' && *p <= '9') {
        while (*p >= '0' && *p <= '9') {
          width = width * 10 + (*p - '0');
          p++;
        }
      } else if (*p == '*') {
        width = va_arg(ap, int);
        if (width < 0) {
          width = -width;
          flags |= LEFT_ADJUSTMENT;
        }
        p++;
      }

      /* 处理精度 */
      if (*p == '.') {
        p++;
        prec = 0;
        if (*p >= '0' && *p <= '9') {
          while (*p >= '0' && *p <= '9') {
            prec = prec * 10 + (*p - '0');
            p++;
          }
        } else if (*p == '*') {
          prec = va_arg(ap, int);
          p++;
        }
        flags |= PRECISION;
      }

      /* 处理长度修饰符 */
      switch (*p) {
        case 'h':
          if (*(p + 1) == 'h') {
            p++;
            flags |= CHAR_SHORT;
          } else {
            flags |= SHORT;
          }
          p++;
          break;
        case 'l':
          if (*(p + 1) == 'l') {
            p++;
            flags |= LLONG;
          } else {
            flags |= LONG;
          }
          p++;
          break;
        case 'j':
          flags |= INTMAX_T;
          p++;
          break;
        case 'z':
          flags |= SIZE_T;
          p++;
          break;
        case 't':
          flags |= PTRDIFF_T;
          p++;
          break;
        case 'L':
          flags |= LONG_DOUBLE;
          p++;
          break;
        default:
          break;
      }

      /* 处理转换类型 */
      type = *p;
      switch (type) {
        case 'd':
        case 'i':
          if (flags & LLONG) {
            value = va_arg(ap, long long);
          } else if (flags & LONG) {
            value = va_arg(ap, long);
          } else if (flags & SHORT) {
            value = (short)va_arg(ap, int);
          } else if (flags & CHAR_SHORT) {
            value = (signed char)va_arg(ap, int);
          } else {
            value = va_arg(ap, int);
          }
          if (value < 0) {
            sign = '-';
            value = -value;
          } else if (flags & PLUS_SIGN) {
            sign = '+';
          } else if (flags & SPACE) {
            sign = ' ';
          }
          base = 10;
          digits = "0123456789";
          break;
        case 'o':
          if (flags & LLONG) {
            value = va_arg(ap, unsigned long long);
          } else if (flags & LONG) {
            value = va_arg(ap, unsigned long);
          } else if (flags & SHORT) {
            value = (unsigned short)va_arg(ap, unsigned int);
          } else if (flags & CHAR_SHORT) {
            value = (unsigned char)va_arg(ap, unsigned int);
          } else {
            value = va_arg(ap, unsigned int);
          }
          base = 8;
          digits = "01234567";
          if (flags & ALTERNATE_FORM && value != 0) {
            prefix_str = "0";
            prefix_char = '0';
            prefix = 1;
          }
          break;
        case 'u':
          if (flags & LLONG) {
            value = va_arg(ap, unsigned long long);
          } else if (flags & LONG) {
            value = va_arg(ap, unsigned long);
          } else if (flags & SHORT) {
            value = (unsigned short)va_arg(ap, unsigned int);
          } else if (flags & CHAR_SHORT) {
            value = (unsigned char)va_arg(ap, unsigned int);
          } else {
            value = va_arg(ap, unsigned int);
          }
          base = 10;
          digits = "0123456789";
          break;
        case 'x':
        case 'X':
          if (flags & LLONG) {
            value = va_arg(ap, unsigned long long);
          } else if (flags & LONG) {
            value = va_arg(ap, unsigned long);
          } else if (flags & SHORT) {
            value = (unsigned short)va_arg(ap, unsigned int);
          } else if (flags & CHAR_SHORT) {
            value = (unsigned char)va_arg(ap, unsigned int);
          } else {
            value = va_arg(ap, unsigned int);
          }
          base = 16;
          digits = (type == 'X') ? "0123456789ABCDEF" : "0123456789abcdef";
          if (flags & ALTERNATE_FORM && value != 0) {
            prefix_str = (type == 'X') ? "0X" : "0x";
            prefix_char = (type == 'X') ? 'X' : 'x';
            prefix = 2;
          }
          break;
        case 'p':
          value = (uintptr_t)va_arg(ap, void *);
          base = 16;
          digits = "0123456789abcdef";
          prefix_str = "0x";
          prefix_char = 'x';
          prefix = 2;
          break;
        case 'c':
          n = va_arg(ap, int);
          buf[0] = n;
          s = buf;
          len = 1;
          break;
        case 's':
          s = va_arg(ap, char *);
          if (s == NULL) {
            s = "(null)";
          }
          len = strlen(s);
          if (flags & PRECISION && len > prec) {
            len = prec;
          }
          break;
        case '%':
          buf[0] = '%';
          s = buf;
          len = 1;
          break;
        default:
          buf[0] = '%';
          buf[1] = type;
          s = buf;
          len = 2;
          break;
      }

      /* 计算输出长度 */
      if (type == 's') {
        count += len;
      } else {
        if (value == 0 && (flags & PRECISION) && prec == 0) {
          len = 0;
        } else {
          len = calc_digits(value, base);
        }
        if (sign || prefix) {
          len++;
        }
        count += (len < width) ? width : len;
      }

      /* 输出前导空格或零 */
      if (!(flags & LEFT_ADJUSTMENT)) {
        if (flags & ZERO_PAD) {
          pad_char = '0';
        }
        while (count < width) {
          fputc(pad_char, stream);
          count++;
        }
      }

      /* 输出符号或前缀 */
      if (sign) {
        fputc(sign, stream);
        count++;
      } else if (prefix) {
        fputs(prefix_str, stream);
        count += prefix;
      }

      /* 输出数字或字符串 */
      if (type == 's') {
        fwrite(s, len, 1, stream);
      } else {
        output_number(value, base, digits, prefix_char, stream, len);
      }

      /* 输出后缀空格 */
      if (flags & LEFT_ADJUSTMENT) {
        while (count < width) {
          fputc(' ', stream);
          count++;
        }
      }
    } else {
      /* 处理普通字符 */
      fputc(*p, stream);
      count++;
    }
  }

  /* 返回输出字符数 */
  if (writelen) {
    *writelen = count;
  }
  ret = ferror(stream) ? EOF : count;
  return ret;
}

该函数首先解析格式字符串，然后根据格式字符串逐个处理输出。具体而言，它会根据格式字符串中的格式控制指示符，处理标志位、字段宽度、精度、长度修饰符和转换类型等信息，并据此输出相应的内容。输出时会考虑前导空格、符号或前缀、数字或字符串以及后缀空格等因素。最后，该函数会返回输出字符数，并将其写入指针 `writelen` 所指向的变量中。