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C语言fwrite函数不能写入文件的原因记录

时间：2024-01-26 10:18:51

最近在写一个C语言的小程序，中间需要对文件进行读写操作，使用到了fwrite函数，示例代码如下：

#include<stdio.h>#include<stdlib.h>int main(void){const char* fileName = "./test.bin";FILE* tarFile;int i,j;short tarSource[10]={0,0,0,0,0,0,0,0,0,0};tarFile = fopen(fileName,"wb+");//打开文件for(i=0;i<10;i++){fread(tarSource,2,10,tarFile);//读取二进制文件for(j=0;j<10;j++){tarSource[j]+=1;}fseek(tarFile,0,SEEK_SET);//移动流到文件头fwrite(tarSource,2,10,tarFile);//二进制写入文件}fclose(tarFile);//关闭文件}

期望最后的文件内容如下：

但是以上代码运行出来的内容如下：

百思不得其解之后终于在度娘上找到问题的原因[1]，原来fopen函数在使用"r+"、"w+"或"a+"对文件进行读写时，在fwrite和fread之间必须插入fflush、fsetpos、fseek或rewind函数，否则将无法写入。详细说明如下：

"r"
Opens for reading. If the file does not exist or cannot be found, the fopen call fails.
"w"
Opens an empty file for writing. If the given file exists, its contents are destroyed.
"a"
Opens for writing at the end of the file (appending) without removing the EOF marker before writing new data to the file; creates the file first if it doesn’t exist.
"r+"
Opens for both reading and writing. (The file must exist.)
"w+"
Opens an empty file for both reading and writing. If the given file exists, its contents are destroyed.
"a+"
Opens for reading and appending; the appending operation includes the removal of the EOF marker before new data is written to the file and the EOF marker is restored after writing is complete; creates the file first if it doesn’t exist.
When a file is opened with the “a” or “a+” access type, all write operations occur at the end of the file. The file pointer can be repositioned using fseek or rewind, but is always moved back to the end of the file before any write operation is carried out. Thus, existing data cannot be overwritten.
The “a” mode does not remove the EOF marker before appending to the file. After appending has occurred, the MS-DOS TYPE command only shows data up to the original EOF marker and not any data appended to the file. The “a+” mode does remove the EOF marker before appending to the file. After appending, the MS-DOS TYPE command shows all data in the file. The “a+” mode is required for appending to a stream file that is terminated with the CTRL+Z EOF marker.
When the “r+”, “w+”, or “a+” access type is specified, both reading and writing are allowed (the file is said to be open for “update”).However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation.The current position can be specified for the fsetpos or fseek operation, if desired.
In addition to the above values, the following characters can be included in mode to specify the translation mode for newline characters:
t
Open in text (translated) mode. In this mode, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading/writing with “a+”, fopen checks for a CTRL+Z at the end of the file and removes it, if possible. This is done because using fseek and ftell to move within a file that ends with a CTRL+Z, may cause fseek to behave improperly near the end of the file.
Also, in text mode, carriage return–linefeed combinations are translated into single linefeeds on input, and linefeed characters are translated to carriage return–linefeed combinations on output. When a Unicode stream-I/O function operates in text mode (the default), the source or destination stream is assumed to be a sequence of multibyte characters. Therefore, the Unicode stream-input functions convert multibyte characters to wide characters (as if by a call to the mbtowc function). For the same reason, the Unicode stream-output functions convert wide characters to multibyte characters (as if by a call to the wctomb function).
b
Open in binary (untranslated) mode; translations involving carriage-return and linefeed characters are suppressed.
If t or b is not given in mode, the default translation mode is defined by the global variable _fmode. If t or b is prefixed to the argument, the function fails and returns NULL.
For more information about using text and binary modes in Unicode and multibyte stream-I/O, see Text and Binary Mode File I/O and Unicode Stream I/O in Text and Binary Modes.
c
Enable the commit flag for the associated filename so that the contents of the file buffer are written directly to disk if either fflush or _flushall is called.
n
Reset the commit flag for the associated filename to “no-commit.” This is the default. It also overrides the global commit flag if you link your program with COMMODE.OBJ. The global commit flag default is “no-commit” unless you explicitly link your program with COMMODE.OBJ.