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

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。

2013-03-25 23:25:40

其他回答

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

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

2013-03-25 23:25:40

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

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);
}

2010-08-17 11:03:30

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;

}

2019-08-25 12:03:47

C逐行读取文件

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