C逐行读取文件

Question

我写了这个函数来从文件中读取一行:

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;
}

该函数正确读取文件,并使用printf我看到constLine字符串也正确读取.

但是,如果我使用这样的功能:

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

printf输出乱码.为什么？

Answer 1

如果您的任务不是发明逐行读取功能,而只是逐行读取文件,您可以使用涉及该getline()功能的典型代码片段(请参见此处的手册页):

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

int main(void)
{
    FILE * fp;
    char * line = NULL;
    size_t len = 0;
    ssize_t read;

    fp = fopen("/etc/motd", "r");
    if (fp == NULL)
        exit(EXIT_FAILURE);

    while ((read = getline(&line, &len, fp)) != -1) {
        printf("Retrieved line of length %zu:\n", read);
        printf("%s", line);
    }

    fclose(fp);
    if (line)
        free(line);
    exit(EXIT_SUCCESS);
}

Answer 2

在你的readLine函数中,你返回一个指向line数组的指针(严格地说,指向它的第一个字符的指针,但这里的差异是无关紧要的).由于它是一个自动变量(即它在"堆栈中"),因此在函数返回时回收内存.你看到胡言乱语,因为printf它把自己的东西放在堆栈上.

您需要从函数返回动态分配的缓冲区.你已经拥有一个,它是lineBuffer; 你所要做的就是将它截断到所需的长度.

    lineBuffer[count] = '\0';
    realloc(lineBuffer, count + 1);
    return lineBuffer;
}

ADDED(对注释中的后续问题的响应):readLine返回指向组成该行的字符的指针.此指针是您处理行内容所需的.free当你使用完这些字符所占用的内存时,它也是你必须传递的内容.以下是您可以使用该readLine功能的方法:

char *line = readLine(file);
printf("LOG: read a line: %s\n", line);
if (strchr(line, 'a')) { puts("The line contains an a"); }
/* etc. */
free(line);
/* After this point, the memory allocated for the line has been reclaimed.
   You can't use the value of `line` again (though you can assign a new value
   to the `line` variable if you want). */

Answer 3

FILE* fp;
char buffer[255];

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

while(fgets(buffer, 255, (FILE*) fp)) {
    printf("%s\n", buffer);
}

fclose(fp);

Answer 4

//open and get the file handle
FILE* fh;
fopen_s(&fh, filename, "r");

//check if file exists
if (fh == NULL){
    printf("file does not exists %s", filename);
    return 0;
}


//read line by line
const size_t line_size = 300;
char* line = malloc(line_size);
while (fgets(line, line_size, fh) != NULL)  {
    printf(line);
}
free(line);    // dont forget to free heap memory

Answer 5

readLine() 返回指向局部变量的指针,这会导致未定义的行为.

您可以:

1. 在调用函数中创建变量并将其地址传递给 readLine()
2. 分配内存以供line使用malloc()- 在这种情况下line将是持久的
3. 使用全局变量,虽然这通常是一种不好的做法

Answer 6

一个完整的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];
    // -1 to allow room for NULL terminator for really long string
    while (fgets(buffer, MAX_LEN - 1, 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

请记住，如果您想从标准输入（而不是本例中的文件）中读取，那么您所要做的就是将其stdin作为fgets()方法的第三个参数传递，如下所示：

while(fgets(buffer, MAX_LEN - 1, stdin))

_附录

从 fgets() 输入中删除尾随换行符

c - 如何检测文件是否在c中打开

Answer 7

用于fgets()从文件句柄中读取一行.