我想在C ++中使用等效（在C中）getline：
std::string s;
getline(std::cin,s); // reads an arbitrarily long line and
// inserts its contents on s
有没有办法在C中做这样的事情？我正在寻找看起来像这样的东西：
char* s;
getline(stdin,s); // allocates the space necessary to fit the read
// line and make s point to it
编辑：我决定在Linux上使用POSIX getline函数，因为我在Linux上运行{运行man getline如果你不知道我在做什么谈到），但Michael Burr提供了getline的实现，该实现适用于默认情况下getline不可用的其他操作系统。即使他的实施不是最有效的，我也可以按照自己的意愿完成工作，所以我将其标记为我的问题的答案。
答案 0 :(得分：6)
如果您无法访问POSIX getline()实施，这里是我所处的公共域实施。
我添加了一个getline_simple()函数，它只返回动态分配的缓冲区中的下一行。如果您对详细的错误处理不感兴趣，可以使用该函数逐行读取文件：
#define _CRT_SECURE_NO_WARNINGS 1
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <limits.h>
#include <sys/types.h>
#if !__GNUC__
#if _WIN64
typedef long long ssize_t;
#else
typedef long ssize_t;
#endif
#endif
#if !defined(SSIZE_MAX)
#define SSIZE_MAX ((ssize_t)(SIZE_MAX/2))
#endif
#if !defined(EOVERFLOW)
#define EOVERFLOW (ERANGE) /* is there something better to use? */
#endif
/*
nx_getdelim()
a version of the POSIX getdelim() function that return error codes directly
instead of messing with the global `errno` value.
*/
ssize_t nx_getdelim(char **lineptr, size_t *n, int delim, FILE *stream);
/*
getdelim_calc_new_alloc()
Helper function for getdelim() to figure out an appropriate new
allocation size that's not too small or too big.
These numbers seem to work pretty well for most text files.
returns the input value if it decides that new allocation block
would be just too big (the caller should handle this as
an error).
*/
static
size_t nx_getdelim_get_realloc_size( size_t current_size)
{
enum {
k_min_realloc_inc = 32,
k_max_realloc_inc = 1024,
};
if (SSIZE_MAX < current_size) return current_size;
if (current_size <= k_min_realloc_inc) return current_size + k_min_realloc_inc;
if (current_size >= k_max_realloc_inc) return current_size + k_max_realloc_inc;
return current_size * 2;
}
/*
getdelim_append()
a helper function for getdelim() that adds a new character to
the outbuffer, reallocating as necessary to ensure the character
and a following null terminator can fit
*/
static
int nx_getdelim_append( char** lineptr, size_t* bufsize, size_t count, char ch)
{
char* tmp = NULL;
size_t tmp_size = 0;
// assert the contracts for this functions inputs
assert( lineptr != NULL);
assert( bufsize != NULL);
if (count >= (((size_t) SSIZE_MAX) + 1)) {
// writing more than SSIZE_MAX to the buffer isn't supported
return -1;
}
tmp = *lineptr;
tmp_size = tmp ? *bufsize : 0;
// need room for the character plus the null terminator
if ((count + 2) > tmp_size) {
tmp_size = nx_getdelim_get_realloc_size( tmp_size);
tmp = (char*) realloc( tmp, tmp_size);
if (!tmp) {
return -1;
}
}
*lineptr = tmp;
*bufsize = tmp_size;
// remember, the reallocation size calculation might not have
// changed the block size, so we have to check again
if (tmp && ((count+2) <= tmp_size)) {
tmp[count++] = ch;
tmp[count] = 0;
return 1;
}
return -1;
}
/*
nx_getdelim()
A getdelim() function modeled on the Linux/POSIX/GNU
function of the same name.
Read data into a dynamically resizable buffer until
EOF or until a delimiter character is found. The returned
data will be null terminated (unless there's an error allocating
memory that prevents it).
params:
lineptr - a pointer to a char* allocated by malloc()
(actually any pointer that can legitimately be
passed to free()). *lineptr will be updated
by getdelim() if the memory block needs to be
reallocated to accommodate the input data.
*lineptr can be NULL (though lineptr itself cannot),
in which case the function will allocate any necessary
buffer.
n - a pointer to a size_t object that contains the size of
the buffer pointed to by *lineptr (if non-NULL).
The size of whatever buff the resulting data is
returned in will be passed back in *n
delim - the delimiter character. The function will stop
reading one this character is read form the stream.
It will be included in the returned data, and a
null terminator character will follow it.
stream - A FILE* stream object to read data from.
Returns:
The number of characters placed in the returned buffer, including
the delimiter character, but not including the terminating null.
If no characters are read and EOF is set (or attempting to read
from the stream on the first attempt caused the eof indication
to be set), a null terminator will be written to the buffer and
0 will be returned.
If an error occurs while reading the stream, a 0 will be returned.
A null terminator will not necessarily be at the end of the data
written.
On the following error conditions, the negative value of the error
code will be returned:
ENOMEM: out of memory
EOVERFLOW: SSIZE_MAX character written to te buffer before
reaching the delimiter
(on Windows, EOVERFLOW is mapped to ERANGE)
The buffer will not necessarily be null terminated in these cases.
Notes:
The returned data might include embedded nulls (if they exist
in the data stream) - in that case, the return value of the
function is the only way to reliably determine how much data
was placed in the buffer.
If the function returns 0 use feof() and/or ferror() to determine
which case caused the return.
If EOF is returned after having written one or more characters
to the buffer, a normal count will be returned (but there will
be no delimiter character in the buffer).
If 0 is returned and ferror() returns a non-zero value,
the data buffer may not be null terminated.
In other cases where a negative value is returned, the data
buffer is not necessarily null terminated and there
is no reliable means to determining what data in the buffer is
valid.
The pointer returned in *lineptr and the buffer size
returned in *n will be valid on error returns unless
NULL pointers are passed in for one or more of these
parameters (in which case the return value will be -EINVAL).
*/
ssize_t nx_getdelim(char **lineptr, size_t *n, int delim, FILE *stream)
{
int retval = 0;
ssize_t result = 0;
char* line = NULL;
size_t size = 0;
size_t count = 0;
int err = 0;
int ch = 0;
if (!lineptr || !n) {
return -EINVAL;
}
line = *lineptr;
size = *n;
for (;;) {
ch = fgetc( stream);
if (ch == EOF) {
break;
}
result = nx_getdelim_append( &line, &size, count, ch);
// check for error adding to the buffer (ie., out of memory)
if (result < 0) {
err = -ENOMEM;
break;
}
++count;
// check if we're done because we've found the delimiter
if ((unsigned char)ch == (unsigned char)delim) {
break;
}
// check if we're passing the maximum supported buffer size
if (count > SSIZE_MAX) {
err = -EOVERFLOW;
break;
}
}
// update the caller's data
*lineptr = line;
*n = size;
// check for various error returns
if (err != 0) {
return err;
}
if (ferror(stream)) {
return 0;
}
if (feof(stream) && (count == 0)) {
if (nx_getdelim_append( &line, &size, count, 0) < 0) {
return -ENOMEM;
}
}
return count;
}
ssize_t nx_getline(char **lineptr, size_t *n, FILE *stream)
{
return nx_getdelim( lineptr, n, '\n', stream);
}
/*
versions of getline() and getdelim() that attempt to follow
POSIX semantics (ie. they set errno on error returns and
return -1 when the stream error indicator or end-of-file
indicator is set (ie., ferror() or feof() would return
non-zero).
*/
ssize_t getdelim(char **lineptr, size_t *n, char delim, FILE *stream)
{
ssize_t retval = nx_getdelim( lineptr, n, delim, stream);
if (retval < 0) {
errno = -retval;
retval = -1;
}
if (retval == 0) {
retval = -1;
}
return retval;
}
ssize_t getline(char **lineptr, size_t *n, FILE *stream)
{
return getdelim( lineptr, n, '\n', stream);
}
/*
A simple function to return the next line of text in a dynamically
allocated buffer
On error a NULL pointer is returned. When the caller no longer needs the
returned data, the pointer returned should be passed to `free()`.
*/
char* getline_simple( FILE* stream)
{
char* p = NULL;
size_t size = 0;
ssize_t result = getline( &p, &size, stream);
if (result < 0) {
free(p);
p = NULL;
}
return p;
}
免责声明 - 此代码适用于我的目的，但我不保证会出现这种情况。如果打算使用此代码，请使用尽职调查。
答案 1 :(得分：2)
您需要在循环中逐步执行此操作，使用fgets()读取数据块，并使用realloc()扩大内存缓冲区（如果您没有捕获到行的末尾）。
但是可能有一些库函数可以帮助你。
#define BLOCKSIZE 1024
char *readAllocLine(FILE *fp) {
char *line = NULL;
size_t maxlength = 0;
assert(fp != NULL);
for(;;) { // Read the line in BLOCKSIZE -blocks.
char *crlf, *block;
maxlength += BLOCKSIZE;
// This exploits realloc behaviour upon hitting NULL
if (NULL == (line = realloc(line, maxlength+1))) {
break; // realloc error returns NULL.
}
block = line + maxlength - BLOCKSIZE;
// BLOCKSIZE+1 to accommodate final zero
if (NULL == fgets(block, BLOCKSIZE+1, fp)) {
// TODO: rewind fp in case of error.
if (block == line) {
// Error.
free(line); line = NULL;
}
break;
}
// This was the last block iff we find a CRLF inside.
if (NULL != (crlf = strchr(block, '\n'))) {
*crlf = 0x0;
if (crlf != block) {
if ('\r' == *(--crlf))
*crlf = 0x0;
}
break;
} /* if */
} /* for */
return line;
}
使用此主
进行测试int main(int argc, char **argv) {
FILE *fp;
if (argc !=2 ) {
fprintf(stderr, "Syntax: %s testfile.txt\n", argv[0]);
return -1;
}
fp = fopen(argv[1], "r");
while(!feof(fp)) {
char *s = readAllocLine(fp);
if (NULL != s) {
printf("\"%s\"\n", s);
free(s);
} else {
printf("--- end of file ---\n");
break;
}
}
fclose(fp);
return 0;
}
和这个脚本：
for i in $( seq 1020 1028 ); do
# Didn't want to think over
yes x | tr -d "\n" | dd of=test bs=1 count=$i 2>/dev/null
./readline test | wc -c | tr "\n" ","
echo "" >> test
./readline test | wc -c | tr "\n" ","
echo "" >> test
./readline test | wc -c
done