C逐行读取文件

我写这个函数是从文件中读取一行:

const char *readLine(FILE *file) {

    if (file == NULL) {
        printf("Error: file pointer is null.");
        exit(1);
    }

    int maximumLineLength = 128;
    char *lineBuffer = (char *)malloc(sizeof(char) * maximumLineLength);

    if (lineBuffer == NULL) {
        printf("Error allocating memory for line buffer.");
        exit(1);
    }

    char ch = getc(file);
    int count = 0;

    while ((ch != '\n') && (ch != EOF)) {
        if (count == maximumLineLength) {
            maximumLineLength += 128;
            lineBuffer = realloc(lineBuffer, maximumLineLength);
            if (lineBuffer == NULL) {
                printf("Error reallocating space for line buffer.");
                exit(1);
            }
        }
        lineBuffer[count] = ch;
        count++;

        ch = getc(file);
    }

    lineBuffer[count] = '\0';
    char line[count + 1];
    strncpy(line, lineBuffer, (count + 1));
    free(lineBuffer);
    const char *constLine = line;
    return constLine;
}

该函数正确地读取文件，使用printf我看到constLine字符串也被正确读取。

然而，如果我像这样使用函数:

while (!feof(myFile)) {
    const char *line = readLine(myFile);
    printf("%s\n", line);
}

Printf输出胡言乱语。为什么?

当前回答

这是我的几个小时…逐行读取整个文件。

char * readline(FILE *fp, char *buffer)
{
    int ch;
    int i = 0;
    size_t buff_len = 0;

    buffer = malloc(buff_len + 1);
    if (!buffer) return NULL;  // Out of memory

    while ((ch = fgetc(fp)) != '\n' && ch != EOF)
    {
        buff_len++;
        void *tmp = realloc(buffer, buff_len + 1);
        if (tmp == NULL)
        {
            free(buffer);
            return NULL; // Out of memory
        }
        buffer = tmp;

        buffer[i] = (char) ch;
        i++;
    }
    buffer[i] = '\0';

    // Detect end
    if (ch == EOF && (i == 0 || ferror(fp)))
    {
        free(buffer);
        return NULL;
    }
    return buffer;
}

void lineByline(FILE * file){
char *s;
while ((s = readline(file, 0)) != NULL)
{
    puts(s);
    free(s);
    printf("\n");
}
}

int main()
{
    char *fileName = "input-1.txt";
    FILE* file = fopen(fileName, "r");
    lineByline(file);
    return 0;
}

2017-11-28 21:16:42

其他回答

您应该使用ANSI函数来读取一行，例如。fgets。调用后，你需要在调用上下文中使用free()，例如:

...
const char *entirecontent=readLine(myFile);
puts(entirecontent);
free(entirecontent);
...

const char *readLine(FILE *file)
{
  char *lineBuffer=calloc(1,1), line[128];

  if ( !file || !lineBuffer )
  {
    fprintf(stderr,"an ErrorNo 1: ...");
    exit(1);
  }

  for(; fgets(line,sizeof line,file) ; strcat(lineBuffer,line) )
  {
    if( strchr(line,'\n') ) *strchr(line,'\n')=0;
    lineBuffer=realloc(lineBuffer,strlen(lineBuffer)+strlen(line)+1);
    if( !lineBuffer )
    {
      fprintf(stderr,"an ErrorNo 2: ...");
      exit(2);
    }
  }
  return lineBuffer;
}

2010-08-17 14:00:43

readLine()返回指向局部变量的指针，这会导致未定义的行为。

你可以:

在调用函数中创建变量，并将其地址传递给readLine() 使用malloc()为行分配内存——在这种情况下，行将是持久的使用全局变量，尽管这通常是一个不好的做法

2010-08-17 10:10:36

完整的fgets()解决方案:

#include <stdio.h>
#include <string.h>

#define MAX_LEN 256

int main(void)
{
    FILE* fp;
    fp = fopen("file.txt", "r");
    if (fp == NULL) {
      perror("Failed: ");
      return 1;
    }

    char buffer[MAX_LEN];
    while (fgets(buffer, MAX_LEN, fp))
    {
        // Remove trailing newline
        buffer[strcspn(buffer, "\n")] = 0;
        printf("%s\n", buffer);
    }

    fclose(fp);
    return 0;
}

输出:

First line of file
Second line of file
Third (and also last) line of file

记住，如果你想从标准输入(而不是像这种情况下的文件)中读取，那么你所要做的就是将stdin作为fgets()方法的第三个参数传递，就像这样:

while(fgets(buffer, MAX_LEN, stdin))

附录

从fgets()输入中删除尾随换行符

如何在c中检测文件是否打开

2020-10-17 19:13:46

FILE* filePointer;
int bufferLength = 255;
char buffer[bufferLength]; /* not ISO 90 compatible */

filePointer = fopen("file.txt", "r");

while(fgets(buffer, bufferLength, filePointer)) {
    printf("%s\n", buffer);
}

fclose(filePointer);

2016-08-30 21:47:47

实现从文件中读取和获取内容的方法(input1.txt)

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

void testGetFile() {
    // open file
    FILE *fp = fopen("input1.txt", "r");
    size_t len = 255;
    // need malloc memory for line, if not, segmentation fault error will occurred.
    char *line = malloc(sizeof(char) * len);
    // check if file exist (and you can open it) or not
    if (fp == NULL) {
        printf("can open file input1.txt!");
        return;
    }
    while(fgets(line, len, fp) != NULL) {
        printf("%s\n", line);
    }
    free(line);
}

希望这对你有所帮助。编码快乐!

2017-11-16 15:12:07

C逐行读取文件

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