下用行读取文件中的行 [英] C read file line by line

问题描述

我写了这个功能从文件中读取一个行：

I wrote this function to read a line from a file:

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;
}

函数正确读取该文件，并用printf我看到constLine串并得到正确读取以及

The function reads the file correctly, and using printf I see that the constLine string did get read correctly as well.

不过，如果我使用功能例如像这样的：

However, if I use the function e.g. like this:

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

的printf输出的乱码。为什么呢？

printf outputs gibberish. Why?

推荐答案

如果你的任务不是去创造行由行阅读功能，但只是为了读取文件中的行由行，你可以使用一个典型code段涉及函数getline（）功能（请参见手册页的这里）：

If your task is not to invent the line-by-line reading function, but just to read the file line-by-line, you may use a typical code snippet involving the getline() function (see the manual page here):

#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);
}

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