file access
open a file
std::fopen| std::FILE* fopen( const char* filename, const char* mode ); |
Opens filenamethe file indicated by and returns the stream associated with the file. Use modeto determine the file access mode.
parameter
| filename | - | The filename to associate the file stream to |
| mode | - | A null-terminated string that determines the file access mode |
|
return value
On success, returns a pointer to the object controlling the open file stream, with the end-of-file and error bits cleared. Streams are fully buffered unless filename refers to an interactive device.
On error, returns a null pointer. POSIX requires errno to be set in this case.
Notice
filenameThe format of is implementation-defined and does not necessarily refer to a file (eg it could be a console or another device accessible through the file system API). On supported platforms, filenamemay contain absolute or relative filesystem paths.
Portable directory and file naming, see the C++ filesystem library or boost.filesystem .
call example
#include <cstdio>
#include <cstdlib>
int main()
{
FILE* fp = std::fopen("test.txt", "r");
if (!fp)
{
std::perror("File opening failed");
return EXIT_FAILURE;
}
int c; // 注意：是 int 而非 char ，要求处理 EOF
while ((c = std::fgetc(fp)) != EOF) // 标准 C I/O 文件读取循环
{
std::putchar(c);
}
if (std::ferror(fp))
{
std::puts("I/O error when reading");
}
else if (std::feof(fp))
{
std::puts("End of file reached successfully");
}
std::fclose(fp);
return 0;
}output
open an existing stream with a different name
std::freopen| std::FILE* freopen( const char* filename, const char* mode, std::FILE* stream ); |
First, an attempt is made to close streamthe file associated with , ignoring any errors. Then, if filenamenon- null, an attempt is made to modeopen filenamethe file specified by by as if by fopen, and streamassociate the file with the file stream pointed to by . If filenameis a null pointer, the function attempts to reopen streamthe file already associated with (it is implementation-defined whether mode changes are allowed in this case).
parameter
| filename | - | The file to associate the file stream to |
| mode | - | A null-terminated string that determines the new file opening mode |
|
| stream | - | the file stream to modify |
return value
on success stream, NULL on failure.
Notice
freopenis the only way to change the narrow/wide aspect of a stream once it has been established by an I/O operation or by std::fwide.
call example
#include <cstdio>
int main()
{
std::printf("stdout is printed to console\n");
if (std::freopen("redir.txt", "w", stdout))
{
std::printf("stdout is redirected to a file\n"); // 这被写入 redir.txt
std::fclose(stdout);
}
return 0;
}output
close file
std::fclose| int fclose( std::FILE* stream ); |
Close the given file stream. Flush any unwritten buffered data to the OS. Any unread buffered data is discarded.
Regardless of the success of the operation, the stream is no longer associated with the file, and buffers allocated by std::setbuf or std::setvbuf are also deassociated, if present, and deallocated if automatic allocation was used.
The behavior is undefined if the value of fcloseis used after the return of stream.
parameter
| stream | - | the file stream to close |
return value
0 on success, EOF otherwise.
call example
#include <cstdio>
#include <cstdlib>
int main()
{
FILE* fp = std::fopen("test.txt", "r");
if (!fp)
{
std::perror("File opening failed");
return EXIT_FAILURE;
}
int c; // 注意：是 int 而非 char ，要求处理 EOF
while ((c = std::fgetc(fp)) != EOF) // 标准 C I/O 文件读取循环
{
std::putchar(c);
}
if (std::ferror(fp))
{
std::puts("I/O error when reading");
}
else if (std::feof(fp))
{
std::puts("End of file reached successfully");
}
std::fclose(fp);
return 0;
}