从我的 cmets 开始，您的思路是正确的，只是您没有按照正确的顺序将各个部分放在一起。在main() 中，您正在循环调用您的函数，分配一行，然后输出一行文本并一遍又一遍地执行此操作。 （并且没有释放你分配的任何内存——每行读取都会造成内存泄漏）
如果您正在分配存储来保存行，您通常希望通过文件分配和存储所有行并返回指向您的行集合的指针在main()（或任何调用函数）中打印。
您可以通过添加一个额外的间接级别并让您的函数返回char ** 来做到这一点。这是一个简单的两步过程，您可以：
-
分配一块包含指针的内存（每行一个指针）。因为你不会事先知道多少行，你只需分配一些指针，然后当你用完时realloc()更多的指针；
-
对于您读取的每一行，您分配length + 1 个字符的存储空间并将当前行复制到该内存块，并将该行的地址分配给您的下一个可用指针。
（您可以跟踪分配的指针和行数，或者在最后一个指针分配行之后提供一个设置为NULL 的附加指针作为哨兵m> - 由您决定，简单地用计数器跟踪可能在概念上更容易）
读完最后一行后，您只需将指针返回到分配给调用者使用的指针集合。 （您也可以将char ** 的地址作为参数传递给您的函数，从而导致类型为char ***，但成为三星级程序员 并不总是一种恭维）。但是，这样做并没有错，在某些情况下，这是必需的，但如果您有其他选择，那通常是首选路线。
那么这在实践中如何运作？
只需将您的函数返回类型更改为char ** 并将一个附加指针传递给一个计数变量，这样您就可以使用从函数返回之前读取的行数更新该地址的值。例如，你可以这样做：
char **readfile (FILE *fp, size_t *n);
这会将您的文件指针指向打开的文件流，然后从中读取每一行，为该行分配存储空间并将该分配的地址分配给您的一个指针。在该函数中，您将使用一个足够大的字符数组来保存您使用fgets() 读取的每一行。从末尾修剪'\n' 并获取长度，然后分配length + 1 字节来保存该行。将新分配的块的地址分配给一个指针，然后从数组复制到新分配的块。
（strdup() 既可以分配也可以复制，但它不是标准库的一部分，它是 POSIX ——尽管如果您提供适当的选项，大多数编译器都支持它作为扩展）
在readfile() 函数下方将其全部放在一起，从单个指针开始，并在用完时重新分配两倍于当前数字的内存（这在所需的分配数量和指针数量之间提供了合理的权衡。（在只需 20 次调用 realloc()，您将有 1M 指针）。您可以选择任何您喜欢的重新分配和增长方案，但您要避免为每一行调用 realloc() -- realloc() 仍然是一个相对昂贵的调用。
#define MAXC 1024 /* if you need a constant, #define one (or more) */
#define NPTR 1 /* initial no. of pointers to allocate */
/* readfile reads all lines from fp, updating the value at the address
* provided by 'n'. On success returns pointer to allocated block of pointers
* with each of *n pointers holding the address of an allocated block of
* memory containing a line from the file. On allocation failure, the number
* of lines successfully read prior to failure is returned. Caller is
* responsible for freeing all memory when done with it.
*/
char **readfile (FILE *fp, size_t *n)
{
char buffer[MAXC], **lines; /* buffer to hold each line, pointer */
size_t allocated = NPTR, used = 0; /* allocated and used pointers */
lines = malloc (allocated * sizeof *lines); /* allocate initial pointer(s) */
if (lines == NULL) { /* validate EVERY allocation */
perror ("malloc-lines");
return NULL;
}
while (fgets (buffer, MAXC, fp)) { /* read each line from file */
size_t len; /* variable to hold line-length */
if (used == allocated) { /* is pointer reallocation needed */
/* always realloc to a temporary pointer to avoid memory leak if
* realloc fails returning NULL.
*/
void *tmp = realloc (lines, 2 * allocated * sizeof *lines);
if (!tmp) { /* validate EVERY reallocation */
perror ("realloc-lines");
break; /* lines before failure still good */
}
lines = tmp; /* assign reallocted block to lines */
allocated *= 2; /* update no. of allocated pointers */
}
buffer[(len = strcspn (buffer, "\n"))] = 0; /* trim \n, save length */
lines[used] = malloc (len + 1); /* allocate storage for line */
if (!lines[used]) { /* validate EVERY allocation */
perror ("malloc-lines[used]");
break;
}
memcpy (lines[used], buffer, len + 1); /* copy buffer to lines[used] */
used++; /* increment used no. of pointers */
}
*n = used; /* update value at address provided by n */
/* can do final realloc() here to resize exactly to used no. of pointers */
return lines; /* return pointer to allocated block of pointers */
}
在main() 中，您只需传递您的文件指针和size_t 变量的地址，并在迭代指针之前检查返回值，以使用您需要的任何行（它们简单地打印在下面），例如
int main (int argc, char **argv) {
char **lines; /* pointer to allocated block of pointers and lines */
size_t n; /* number of lines read */
/* use filename provided as 1st argument (stdin by default) */
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
perror ("file open failed");
return 1;
}
lines = readfile (fp, &n);
if (fp != stdin) /* close file if not stdin */
fclose (fp);
if (!lines) { /* validate readfile() return */
fputs ("error: no lines read from file.\n", stderr);
return 1;
}
for (size_t i = 0; i < n; i++) { /* loop outputting all lines read */
puts (lines[i]);
free (lines[i]); /* don't forget to free lines */
}
free (lines); /* and free pointers */
return 0;
}
(注意：完成后不要忘记释放分配的内存。当您调用在其他函数中分配的函数时，这变得至关重要。在main() 中，内存将退出时自动释放，但要养成良好的习惯。）
使用/输出示例
$ ./bin/readfile_allocate dat/captnjack.txt
This is a tale
Of Captain Jack Sparrow
A Pirate So Brave
On the Seven Seas.
程序将读取任何文件，无论它有 4 行还是 400,000 行，直到系统内存的物理限制（如果您的行长度超过 1023 个字符，请调整 MAXC）。
内存使用/错误检查
在您编写的任何动态分配内存的代码中，对于分配的任何内存块，您都有 2 个职责：(1)始终保留指向起始地址的指针内存块，因此，(2) 当不再需要它时可以释放。
您必须使用内存错误检查程序来确保您不会尝试访问内存或写入超出/超出分配块的边界，尝试读取或基于未初始化的值进行条件跳转，最后, 以确认您已释放所有已分配的内存。
对于 Linux，valgrind 是正常的选择。每个平台都有类似的内存检查器。它们都易于使用，只需通过它运行您的程序即可。
$ valgrind ./bin/readfile_allocate dat/captnjack.txt
==4801== Memcheck, a memory error detector
==4801== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==4801== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==4801== Command: ./bin/readfile_allocate dat/captnjack.txt
==4801==
This is a tale
Of Captain Jack Sparrow
A Pirate So Brave
On the Seven Seas.
==4801==
==4801== HEAP SUMMARY:
==4801== in use at exit: 0 bytes in 0 blocks
==4801== total heap usage: 10 allocs, 10 frees, 5,804 bytes allocated
==4801==
==4801== All heap blocks were freed -- no leaks are possible
==4801==
==4801== For counts of detected and suppressed errors, rerun with: -v
==4801== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
始终确认您已释放已分配的所有内存并且没有内存错误。
示例中使用的完整代码仅包含标题，但出于完整性考虑，将其包含在下面：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXC 1024 /* if you need a constant, #define one (or more) */
#define NPTR 1 /* initial no. of pointers to allocate */
/* readfile reads all lines from fp, updating the value at the address
* provided by 'n'. On success returns pointer to allocated block of pointers
* with each of *n pointers holding the address of an allocated block of
* memory containing a line from the file. On allocation failure, the number
* of lines successfully read prior to failure is returned. Caller is
* responsible for freeing all memory when done with it.
*/
char **readfile (FILE *fp, size_t *n)
{
char buffer[MAXC], **lines; /* buffer to hold each line, pointer */
size_t allocated = NPTR, used = 0; /* allocated and used pointers */
lines = malloc (allocated * sizeof *lines); /* allocate initial pointer(s) */
if (lines == NULL) { /* validate EVERY allocation */
perror ("malloc-lines");
return NULL;
}
while (fgets (buffer, MAXC, fp)) { /* read each line from file */
size_t len; /* variable to hold line-length */
if (used == allocated) { /* is pointer reallocation needed */
/* always realloc to a temporary pointer to avoid memory leak if
* realloc fails returning NULL.
*/
void *tmp = realloc (lines, 2 * allocated * sizeof *lines);
if (!tmp) { /* validate EVERY reallocation */
perror ("realloc-lines");
break; /* lines before failure still good */
}
lines = tmp; /* assign reallocted block to lines */
allocated *= 2; /* update no. of allocated pointers */
}
buffer[(len = strcspn (buffer, "\n"))] = 0; /* trim \n, save length */
lines[used] = malloc (len + 1); /* allocate storage for line */
if (!lines[used]) { /* validate EVERY allocation */
perror ("malloc-lines[used]");
break;
}
memcpy (lines[used], buffer, len + 1); /* copy buffer to lines[used] */
used++; /* increment used no. of pointers */
}
*n = used; /* update value at address provided by n */
/* can do final realloc() here to resize exactly to used no. of pointers */
return lines; /* return pointer to allocated block of pointers */
}
int main (int argc, char **argv) {
char **lines; /* pointer to allocated block of pointers and lines */
size_t n; /* number of lines read */
/* use filename provided as 1st argument (stdin by default) */
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
perror ("file open failed");
return 1;
}
lines = readfile (fp, &n);
if (fp != stdin) /* close file if not stdin */
fclose (fp);
if (!lines) { /* validate readfile() return */
fputs ("error: no lines read from file.\n", stderr);
return 1;
}
for (size_t i = 0; i < n; i++) { /* loop outputting all lines read */
puts (lines[i]);
free (lines[i]); /* don't forget to free lines */
}
free (lines); /* and free pointers */
return 0;
}
如果您还有其他问题，请告诉我。