fgets 之用法

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xingyuan1hao 2022-12-08 19:10:37

今天在看ObjectiveC,看到了一个fgets的例子，才发现对于fgets的理解不够透彻。
 fgets 的使用方法：char *fgets(char *string, int n, FILE *stream) 
 从文件stream中读取n-1个字符/一行（若一行不满n-1个），string接收字符串
  如果n <= 0，返回NULL
  如果n == 1，返回" "，也就是一个空串
  如果成功，返回值等于string， 也就是获得字符串的首地址
  如果出错，或者读到FILE的结尾，返回NULL.
 下面看例子:
 
 #include <stdio.h>
 #include <string.h> int main( )
 {
  FILE* wordFile = fopen("words.txt","r");
  char word[100];
  
  while(fgets(word, 100, wordFile))
  {
   word[strlen(word)-1] = '\0';
   printf("%s is %d characters long\n", word, strlen(word));
  }
  fclose(wordFile);
  return (0);
 } words.txt的内容如下(共4行，第4行，为空)：
 apple trees
 many books on the desk
 have a cup of water 运行后，输出结果如下：
 apple trees is 11 characters long
 many books on the desk is 22 characters long
 have a cup of water is 19 charcters long
 从上面的例子看出，若一行不满100个字符时，fgets可以从文件中读取一行。这点，以前，没有注意到。
 
 fgets 函数的使用
 fgets 既可以读文件，又可以读标准输入，而且可以防止溢出。但是它只能输入字符串(且能读到回车符\n)，故而用scanf语句的较多。scanf语句可以输入各种格式的数据，其功能较为强大。
 fgets 的使用方法：char *fgets(char *string, int n, FILE *stream) 
 从文件stream中读取n-1个字符/一行（若一行不满n-1个），string接收字符串
  如果n <= 0，返回NULL
  如果n == 1，返回" "，也就是一个空串
  如果成功，返回值等于string， 也就是获得字符串的首地址
  如果出错，或者读到FILE的结尾，返回NULL
 //通过while循环一行行取，读到文件末尾就是NULL了 ----读取整个文件
 #include <stdio.h> void main( void ) 
 { 
 FILE *stream; 
 char line[100]; if( (stream = fopen( "file.txt", "r" )) != NULL ) 
 { 
 while( fgets( line, 100, stream ) != NULL) 
 printf( "%s", line); 
 fclose( stream ); 
 } 
 } 以下是fgets这个函数的实现：
 /*** 
 *fgets.c - get string from a file 
 * 
 * Copyright (c) Microsoft Corporation. All rights reserved. 
 * 
 *Purpose: 
 * defines fgets() - read a string from a file 
 * 
 *******************************************************************************/ #include <cruntime.h> 
 #include <stdio.h> 
 #include <dbgint.h> 
 #include <file2.h> 
 #include <internal.h> 
 #include <mtdll.h> 
 #include <tchar.h> /*** 
 *char *fgets(string, count, stream) - input string from a stream 
 * 
 *Purpose: 
 * get a string, up to count-1 chars or '\n', whichever comes first, 
 * append '\0' and put the whole thing into string. the '\n' IS included 
 * in the string. if count<=1 no input is requested. if EOF is found 
 * immediately, return NULL. if EOF found after chars read, let EOF 
 * finish the string as '\n' would. 
 * 
 *Entry: 
 * char *string - pointer to place to store string 
 * int count - max characters to place at string (include \0) 
 * FILE *stream - stream to read from 
 * 
 *Exit: 
 * returns string with text read from file in it. 
 * if count <= 0 return NULL 
 * if count == 1 put null string in string 
 * returns NULL if error or end-of-file found immediately 
 * 
 *Exceptions: 
 * 
 *******************************************************************************/ _TSCHAR * __cdecl _fgetts ( 
 _TSCHAR *string, 
 int count, 
 FILE *str 
 ) 
 { 
 REG1 FILE *stream; 
 REG2 _TSCHAR *pointer = string; 
 _TSCHAR *retval = string; 
 int ch; _VALIDATE_RETURN(( string != NULL ) || ( count == 0 ), EINVAL, NULL); 
 _VALIDATE_RETURN(( count >= 0 ), EINVAL, NULL); 
 _VALIDATE_RETURN(( str != NULL ), EINVAL, NULL); if (count == 0) 
 { 
 return NULL; 
 } /* The C Standard states the input buffer should remain 
 unchanged if EOF is encountered immediately. Hence we 
 do not blank out the input buffer here */ /* Init stream pointer */ 
 stream = str; _lock_str(stream); 
 __try { #ifndef _UNICODE 
 _VALIDATE_STREAM_ANSI_SETRET(stream, EINVAL, retval, NULL); 
 #endif /* _UNICODE */ 
 if(retval!=NULL) 
 { 
 while (--count) 
 { 
 if ((ch = _fgettc_nolock(stream)) == _TEOF) 
 { 
 if (pointer == string) { 
 retval=NULL; 
 goto done; 
 } break; 
 } if ((*pointer++ = (_TSCHAR)ch) == _T('\n')) 
 break; 
 } 
 *pointer = _T('\0'); 
 } 
 /* Common return */ 
 done: ; } 
 __finally { 
 _unlock_str(stream); 
 } return(retval); 
 } -----------------------------------------------------------------------------------------------------------------
 另： fgetc 和 getchar 是一个一个字符的读取，