多次打开和关闭文本文件并从延续中阅读

Question

我有一个文本文件，然后将其打开并阅读其中一行，然后关闭文本文件。我在循环下调用我的功能，但是每次此函数都读取文本文件的第一行

Answer 1

您 can 使用 fseek 在关闭和重新打开后将自己重新定位在文件中，但是这样做是非常不寻常的。实际上，如此不寻常，以至于我认为这是完全错误的。这是一些示例代码，演示了如何执行此操作以及更典型的循环。这里的每个循环都会读取文件的前2行，假设每行都足够小。处理长行超出了这个问题的范围。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

FILE * xfopen(const char *path, const char *mode);
void xfseek(FILE *stream, long offset, int whence, const char *);
long xftell(FILE *stream, const char *);
void xfclose(FILE *stream, const char *);


int
main(int argc, char *argv[])
{
    const char *path = argc > 1 ? argv[1] : "input";

    /* Read the first two lines of the file, closing the file on each
     * iteration. This is ** not ** the usual way to do this, and is
     * included here for demonstration
     * purposes only.  DO NOT DO THIS.
     * It is very unusual to close and re-open the file on each iteration.
     */
    long position = 0;
    for( int line = 1; line < 3; line++ ){
        FILE *ifp = xfopen(path, "r");
        char buf[1024];
        xfseek(ifp, position, SEEK_SET, path);
        fgets(buf, sizeof buf, ifp);  /* (1) */
        printf("line %d: %s", line, buf);
        position = xftell(ifp, path);
        xfclose(ifp, path);  /* !! */
    }

    /* The more usual way to read each line of a file is to simply
     * read it with repeated calls to the appropriate read method
     * (fgets, fread, fgetc, etc.)  Each subsequent read starts
     * where the previous read finished.
     */
    FILE *ifp = xfopen(path, "r");
    for( int line = 1; line < 3; line++ ){
        char buf[1024];
        fgets(buf, sizeof buf, ifp);  /* (1) */
        printf("line %d: %s", line, buf);
    }
    xfclose(ifp, path);
    return 0;
}



FILE *
xfopen(const char *path, const char *mode)
{
    FILE *fp = path[0] != '-' || path[1] != '\0' ? fopen(path, mode) :
        *mode == 'r' ? stdin : stdout;
    if( fp == NULL ){
        perror(path);
        exit(EXIT_FAILURE);
    }
    return fp;
}


void
xfseek(FILE *stream, long offset, int whence, const char *name)
{
    if( fseek(stream, offset, whence) == -1){
        perror(name);
        exit(EXIT_FAILURE);
    }
}

long
xftell(FILE *stream, const char *name)
{
    long ret = ftell(stream);
    if( ret == -1 ){
        perror(name);
        exit(EXIT_FAILURE);
    }
    return ret;
}

void
xfclose(FILE *stream, const char *name)
{
    if( fclose(stream) ){
        perror(name);
        exit(EXIT_FAILURE);
    }
}

注意：（1）它是读者的练习。将其作为一种练习也许有点像是有点淘汰，但是处理这些问题的烦恼强烈指向使用标准成语的原因。如果要打印文件的前两行，请使用的某些变体（int count = 0;（c = fgetc（fp））！= null＆amp;＆amp; count＆lt; 2;）{如果（c =='\ n'）count += 1; putchar（c）; } 。将读取功能作为循环的条件（几乎）总是最好的选择。

Answer 2

这些评论已经就您正在尝试的其他替代方案提出了建议。但是，无论是正确的方法，您所陈述的问题似乎很清楚，这很清楚要使用 fseek（） et。在打开和关闭文件时查看连续的行。

要打开和关闭文件，每次访问并显示连续的行，您必须首先知道要查看的每个位置都位于该文件中。确实，正如您已经标记的那样， fseek（fseek（） =“ https://www.tutorialspoint.com/c_standard_library/c_function_ftell.htm” rel =“ nofollow noreferrer”> ftell（））可以使用。以下伪代码步骤说明了一种可能性：

//store file pointer locations of each line in file:
FILE *fp = fopen(fn, "r");
if(fp)
{
    for(int i = 0; i < l_cnt; i++)
    {
        pos[i] = ftell(fp);
        fgets(line, sizeof line, fp);
    }
}
fclose(fp);

然后...

//alternately open and close file to view successive lines at stored positions
for(int i = 0; i < line_cnt; i++)
{
    FILE *fp = fopen(fn, "r");
    if(fp)
    {
        fseek(fp, pos[i], 0);
        fgets(line, sizeof line, fp);
        printf("line %d: %s\n", i, line);
        fclose(fp);
    }       
} 

更完整