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在头文件<stdio.h>中定义



int ungetc（int ch，FILE * stream）;



如果ch不等于EOF，则将字符ch（重新解释为unsigned char）推入与流相关联的输入缓冲区，stream以便后续读取操作stream将检索该字符。与流关联的外部设备不会被修改。

流重新定位操作fseek，fsetpos并rewind丢弃效果ungetc。

如果ungetc在没有插入读取或重新定位的情况下多次调用，则可能会失败（换句话说，保证大小为1的推回缓冲区，但是任何较大的缓冲区都是实现定义的）。如果ungetc执行了多个成功操作，则读取操作将按照相反的顺序检索后退字符ungetc。

如果ch等于EOF，则操作失败并且流不受影响。

成功调用以ungetc清除文件状态标志的结尾feof。

成功调用ungetc二进制流将流位置指示符递减1（如果流位置指示符为零，则行为不确定）。

ungetc在文本流上的成功调用以未指定的方式修改流位置指示符，但保证在用读取操作检索到所有后推字符后，流位置指示符等于其之前的值ungetc。

参数

CH

-

字符被压入输入流缓冲区

流

-

文件流将字符放回

返回值

成功则返回ch。

失败时EOF返回并且给定的流保持不变。

注意

推回缓冲区的大小实际上从4k（Linux，MacOS）到4（Solaris）或保证的最小1（HPUX，AIX）不等。

如果推回的字符等于外部字符序列中存在于该位置的字符（实现可简单地递减读取文件位置指示符并避免维持推回缓冲区），则推回缓冲区的表观大小可较大。

例

演示了ungetc的最初目的：实现scanf。

#include <ctype.h>#include <stdio.h> void demo_scanf(const char* fmt, FILE* s) {    if(*fmt == '%') {
        int c;        switch(*++fmt) {            case 'u': while(isspace(c=getc(s))) {} // skip leading white space
                      unsigned int num = 0;                      while(isdigit(c)) {
                          num = num*10 + c-'0';
                          c = getc(s);                      }                      printf("%%u scanned %u\n", num);                      ungetc(c, s); // reprocess the non-digit            case 'c': c = getc(s);                      printf("%%c scanned '%c'\n", c);        }    } }
 int main(void){
    FILE* f = fopen("input.txt", "w+");    fputs("123x", f);    rewind(f); 
    demo_scanf("%u%c", f);    fclose(f);}

输出：

%u scanned 123%c scanned 'x'

参考

  • C11标准（ISO / IEC 9899：2011）：

    • 7.21.7.10 ungetc函数（p：334）

  • C99标准（ISO / IEC 9899：1999）：

    • 7.19.7.11 ungetc函数（p：300）

  • C89 / C90标准（ISO / IEC 9899：1990）：

    • 4.9.7.11 ungetc函数

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