const char *readLine(FILE *file, char* line) {

    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);
    return line;

}


char linebuffer[256];
while (!feof(myFile)) {
    const char *line = readLine(myFile, linebuffer);
    printf("%s\n", line);
}

注意，'line'变量是在调用函数中声明的，然后传递，所以你的readLine函数填充预定义的缓冲区并返回它。这是大多数C库的工作方式。

我知道还有其他办法

defining the char line[] as static (static char line[MAX_LINE_LENGTH] -> it will hold it's value AFTER returning from the function). -> bad, the function is not reentrant, and race condition can occur -> if you call it twice from two threads, it will overwrite it's results malloc()ing the char line[], and freeing it in calling functions -> too many expensive mallocs, and, delegating the responsibility to free the buffer to another function (the most elegant solution is to call malloc and free on any buffers in same function)

顺便说一下，从char*到const char*的显式强制转换是多余的。

在btw2中，不需要malloc() lineBuffer，只需要定义它为char lineBuffer[128]，所以你不需要释放它

btw3不使用“动态大小堆栈数组”(将数组定义为char arrayName[some_nonconstant_variable])，如果你不确切地知道你在做什么，它只在C99中工作。

我的工具从头开始:

FILE *pFile = fopen(your_file_path, "r");
int nbytes = 1024;
char *line = (char *) malloc(nbytes);
char *buf = (char *) malloc(nbytes);

size_t bytes_read;
int linesize = 0;
while (fgets(buf, nbytes, pFile) != NULL) {
    bytes_read = strlen(buf);
    // if line length larger than size of line buffer
    if (linesize + bytes_read > nbytes) {
        char *tmp = line;
        nbytes += nbytes / 2;
        line = (char *) malloc(nbytes);
        memcpy(line, tmp, linesize);
        free(tmp);
    }
    memcpy(line + linesize, buf, bytes_read);
    linesize += bytes_read;

    if (feof(pFile) || buf[bytes_read-1] == '\n') {
        handle_line(line);
        linesize = 0;
        memset(line, '\0', nbytes);
    }
}

free(buf);
free(line);

你犯了一个错误，返回了一个指向自动变量的指针。 变量行被分配到堆栈中，并且只在函数存在的时间内存在。 你不允许返回指向它的指针，因为一旦它返回，内存就会被分配到其他地方。

const char* func x(){
    char line[100];
    return (const char*) line; //illegal
}

为了避免这种情况，你可以返回一个指向堆上内存的指针。lineBuffer 当用户使用完free()时，调用它应该是用户的责任。 或者，您可以要求用户将写入行内容的内存地址作为参数传递给您。

如果你的任务不是发明逐行读取函数，而只是逐行读取文件，你可以使用一个典型的代码片段，包括getline()函数(参见手册页):

#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);
}

//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

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;
}