//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

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

//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

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。

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!

提供一个可移植的通用getdelim函数，通过msvc, clang, gcc测试。

/*
 * An implementation conform IEEE Std 1003.1-2017:
 * https://pubs.opengroup.org/onlinepubs/9699919799/functions/getdelim.html
 *
 * <nio.h>:
 * https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nio.h
 * <nio.c>:
 * https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nio.c
 *
*/

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


/*
 * LINE_MAX dependents on OS' implementations so check it first.
 * https://github.com/junjiemars/c/blob/c425bd0e49df35a2649327664d3f6cd610791996/src/posix/nlim_auto_check
 */
#define NM_LINE_MAX  4096       /* Linux */

#if (MSVC)
typedef SSIZE_T  ssize_t;
#  if !defined(SSIZE_MAX)
#    define SSIZE_MAX  ((ssize_t)((size_t)((ssize_t)-1) >> 1))
#  endif
#endif


ssize_t getdelim(char **restrict lineptr, size_t *restrict n, int delimiter,
                 FILE *restrict stream);

#if defined(getline)
#  undef getline
#endif
#define getline(lp, n, f)  getdelim((lp), (n), 0x0a, (f))


ssize_t
getdelim(char **restrict lineptr, size_t *restrict n, int delimiter,
         FILE *restrict stream)
{
    int       c;
    char     *p, *p1;
    ssize_t   len;

  if (NULL == lineptr || NULL == n || NULL == stream
      || (UCHAR_MAX < delimiter || delimiter < 0))
    {
      errno = EINVAL;
      return EOF;
    }

  if (feof(stream) || ferror(stream))
    {
      return EOF;
    }

    if (0 == *lineptr)
    {
      if (0 == *n)
        {
          *n = NM_LINE_MAX;
        }

      *lineptr = malloc(*n);
      if (0 == *lineptr)
        {
          return EOF;
        }
    }

  p = *lineptr;
  len = 0;

    while (EOF != (c = fgetc(stream)))
    {
      if (SSIZE_MAX == (ssize_t) len)
        {
          errno = EOVERFLOW;
          return EOF;
        }

      if ((size_t) len == (*n - 1))
        {
          *n <<= 1;
          p1 = realloc(*lineptr, *n);
          if (0 == p1)
            {
              return EOF;
            }
          *lineptr = p1;
          p = p1 + len;
        }
      *p++ = (char) c;
      len++;

      if (c == delimiter)
        {
          break;
        }
    }

  if (ferror(stream))
    {
      return EOF;
    }

  *p = 0;
    return len;
}



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

  fp = fopen("/some-file", "r");
  if (fp == NULL)
    exit(1);
  while ((read = getline(&line, &len, fp)) != -1) {
    printf("Retrieved line of length %zu :\n", read);
    printf("%s", line);
  }
  if (ferror(fp)) {
    /* handle error */
  }
  free(line);
  fclose(fp);

  return 0;

}