标准文件IO

标准文件IO和系统调用open/read/write的关系:

标准文件IO会去调用open/read/write系统调用,内部是对系统调用的封装

系统调用open/read/write有什么问题:

程序中使用open/read/write系统调用,程序会频繁的在用户态和内核态去切换;

为了减少系统调用的次数,标准文件IO增加了缓冲区,来提高效率

针对缓冲区Buffer的几个函数:

• void setbuf(FILE *stream, char *buf);
• void setbuffer(FILE *stream, char *buf, size_t size);
• void setlinebuf(FILE *stream);
• int setvbuf(FILE *stream, char *buf, int mode, size_t size);
• int fflush(FILE *stream);

参数mode:

• IONBF: unbuffered 不带缓冲
• IOLBF: line buffered 行缓冲
• IOFBF: fully buffered 全缓冲

#include <stdio.h>
#include <stdio_ext.h>

int main(void) {
    FILE* f;
    size_t bufsize;

    f = fopen("test.txt", "wb");
    if (f == NULL) {
        perror("fopen failed\n");
        return -1;
    }

    bufsize = __fbufsize(f);
    printf("The buffer size is %zd\n", bufsize);

    putc('\n', f);
    bufsize = __fbufsize(f);
    printf("The buffer size is %zd\n", bufsize);

    fclose(f);
    return 0;
}

操作相关的函数:

• FILE *fopen(const char *path, const char *mode);
• int fclose(std::FILE* stream);
• size_t fread(void *buffer, size_t size, size_t count, FILE *stream);
• size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);

"光标"位置相关函数:

• int fseek(FILE *stream, long offset, int whence);
• long ftell(FILE *stream);
• void rewind(FILE *stream);

"工具"函数:

• int fgetc(FILE *stream);
• char *fgets(char *s, int size, FILE *stream);
• int getc(FILE *stream);

格式化IO:

• int fprintf(FILE *stream, const char *format, ... );
• int fscanf(FILE *stream, const char *format, ... );

other:

• void clearerr(FILE *stream);
• int feof(FILE *stream);
• int ferror(FILE *stream);
• int fileno(FILE *stream);

临时文件:

• FILE *tmpfile(void);