C语言 如何使用 fgets() 从 stdin 读取？
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How to read from stdin with fgets()?
提问by robdavies35
I've written the following code to read a line from a terminal window, the problem is the code gets stuck in an infinite loop. The line/sentence is of undefined length, therefore I plan to read it in parts into the buffer, then concatenate it to another string which can be extended via reallocaccordingly. Please can somebody spot my mistake or suggest a better way of achieving this?
我编写了以下代码来从终端窗口读取一行，问题是代码陷入无限循环。行/句子的长度未定义，因此我计划将其部分读入缓冲区，然后将其连接到另一个可以相应地扩展的字符串realloc。请有人发现我的错误或提出更好的方法来实现这一目标吗？
#include <stdio.h>
#include <string.h>
#define BUFFERSIZE 10
int main (int argc, char *argv[])
{
char buffer[BUFFERSIZE];
printf("Enter a message: \n");
while(fgets(buffer, BUFFERSIZE , stdin) != NULL)
{
printf("%s\n", buffer);
}
return 0;
}
采纳答案by user411313
here a concatenation solution:
这是一个串联解决方案：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFFERSIZE 10
int main() {
char *text = calloc(1,1), buffer[BUFFERSIZE];
printf("Enter a message: \n");
while( fgets(buffer, BUFFERSIZE , stdin) ) /* break with ^D or ^Z */
{
text = realloc( text, strlen(text)+1+strlen(buffer) );
if( !text ) ... /* error handling */
strcat( text, buffer ); /* note a '\n' is appended here everytime */
printf("%s\n", buffer);
}
printf("\ntext:\n%s",text);
return 0;
}
回答by slezica
You have a wrong idea of what fgets returns. Take a look at this: http://www.cplusplus.com/reference/clibrary/cstdio/fgets/
您对 fgets 返回的内容有错误的认识。看看这个：http: //www.cplusplus.com/reference/clibrary/cstdio/fgets/
It returns null when it finds an EOF character. Try running the program above and pressing CTRL+D (or whatever combination is your EOF character), and the loop will exit succesfully.
当它找到一个 EOF 字符时它返回 null。尝试运行上面的程序并按 CTRL+D（或任何组合是您的 EOF 字符），循环将成功退出。
How do you want to detect the end of the input? Newline? Dot (you said sentence xD)?
你想如何检测输入的结尾？新队？点（你说的句子xD）？
回答by Steve Emmerson
Assuming that you only want to read a single line, then use LINE_MAX, which is defined in <limits.h>:
假设您只想读取一行，然后使用LINE_MAX，其定义在<limits.h>：
#include <stdio.h>
#include <limits.h>
...
char line[LINE_MAX];
...
if (fgets(line, LINE_MAX, stdin) != NULL) {
...
}
...
回答by sapitando
Exits the loop if the line is empty(Improving code).
如果该行为空，则退出循环（改进代码）。
#include <stdio.h>
#include <string.h>
// The value BUFFERSIZE can be changed to customer's taste . Changes the
// size of the base array (string buffer )
#define BUFFERSIZE 10
int main(void)
{
char buffer[BUFFERSIZE];
char cChar;
printf("Enter a message: \n");
while(*(fgets(buffer, BUFFERSIZE, stdin)) != '\n')
{
// For concatenation
// fgets reads and adds '\n' in the string , replace '\n' by '#include <stdio.h>
#include <stdbool.h>
#include <string.h>
#include <assert.h>
#define BUFFERSIZE 16
int main(void)
{
char buffer[BUFFERSIZE];
printf("Enter a message: \n");
while(true)
{
assert(fgets(buffer, BUFFERSIZE, stdin) != NULL);
// Verifies that the previous character to the last character in the
// buffer array is '\n' (The last character is '/* Autor : Tiago Portela
Email : [email protected]
Sobre : Compilado com TDM-GCC 5.10 64-bit e LCC-Win32 64-bit;
Obs : Apenas tentando aprender algoritimos, sozinho, por hobby. */
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <assert.h>
#define BUFFERSIZE 8
typedef struct _Node {
char *lpBuffer;
struct _Node *LpProxNode;
} Node_t, *LpNode_t;
int main(void)
{
char acBuffer[BUFFERSIZE] = {0};
LpNode_t lpNode = (LpNode_t)malloc(sizeof(Node_t));
assert(lpNode!=NULL);
LpNode_t lpHeadNode = lpNode;
char* lpBuffer = (char*)calloc(1,sizeof(char));
assert(lpBuffer!=NULL);
char cChar;
printf("Enter a message: \n");
// Exit when Enter is pressed
/* while(true)
{
assert(fgets(acBuffer, BUFFERSIZE, stdin)!=NULL);
lpNode->lpBuffer = (char*)malloc((strlen(acBuffer) + 1) * sizeof(char));
assert(lpNode->lpBuffer!=NULL);
strcpy(lpNode->lpBuffer, acBuffer);
if(lpNode->lpBuffer[strlen(acBuffer) - 1] == '\n')
{
lpNode->lpBuffer[strlen(acBuffer) - 1] = '#include <stdio.h>
#include <string.h>
#define BUFFERSIZE 10
#define BIG_BUFFERSIZE 1024
int main (int argc, char *argv[])
{
char buffer[BUFFERSIZE];
char *big_buffer = new char[BIG_BUFFERSIZE];
big_buffer[0] = '##代码##';
printf("Enter a message: \n");
while(fgets(buffer, BUFFERSIZE , stdin) != NULL)
{
if ((strlen(big_buffer) + strlen(buffer)) < BIG_BUFFERSIZE)
{
strcat(big_buffer, buffer);
}
}
return 0;
}
';
lpNode->LpProxNode = NULL;
break;
}
lpNode->LpProxNode = (LpNode_t)malloc(sizeof(Node_t));
lpNode = lpNode->LpProxNode;
assert(lpNode!=NULL);
}*/
// Exits the loop if the line is empty(Improving code).
while(true)
{
assert(fgets(acBuffer, BUFFERSIZE, stdin)!=NULL);
lpNode->lpBuffer = (char*)malloc((strlen(acBuffer) + 1) * sizeof(char));
assert(lpNode->lpBuffer!=NULL);
strcpy(lpNode->lpBuffer, acBuffer);
if(acBuffer[strlen(acBuffer) - 1] == '\n')
lpNode->lpBuffer[strlen(acBuffer) - 1] = '##代码##';
if(strlen(acBuffer) == (BUFFERSIZE - 1) && (acBuffer[strlen(acBuffer) - 1] != '\n'))
{
cChar = fgetc(stdin);
if(cChar != '\n')
ungetc(cChar, stdin);
}
if(acBuffer[0] == '\n')
{
lpNode->LpProxNode = NULL;
break;
}
lpNode->LpProxNode = (LpNode_t)malloc(sizeof(Node_t));
lpNode = lpNode->LpProxNode;
assert(lpNode!=NULL);
}
printf("\nPseudo String :\n");
lpNode = lpHeadNode;
while(lpNode != NULL)
{
printf("%s", lpNode->lpBuffer);
lpNode = lpNode->LpProxNode;
}
printf("\n\nMemory blocks:\n");
lpNode = lpHeadNode;
while(lpNode != NULL)
{
printf("Block \"%7s\" size = %lu\n", lpNode->lpBuffer, (long unsigned)(strlen(lpNode->lpBuffer) + 1));
lpNode = lpNode->LpProxNode;
}
printf("\nConcatenated string:\n");
lpNode = lpHeadNode;
while(lpNode != NULL)
{
lpBuffer = (char*)realloc(lpBuffer, (strlen(lpBuffer) + strlen(lpNode->lpBuffer)) + 1);
strcat(lpBuffer, lpNode->lpBuffer);
lpNode = lpNode->LpProxNode;
}
printf("%s", lpBuffer);
printf("\n\n");
// Deallocate memory
lpNode = lpHeadNode;
while(lpNode != NULL)
{
lpHeadNode = lpNode->LpProxNode;
free(lpNode->lpBuffer);
free(lpNode);
lpNode = lpHeadNode;
}
lpBuffer = (char*)realloc(lpBuffer, 0);
lpBuffer = NULL;
if((lpNode == NULL) && (lpBuffer == NULL))
{
printf("Deallocate memory = %s", (char*)lpNode);
}
printf("\n\n");
return 0;
}
') if the
// character is '\n' leaves loop.
if(buffer[strlen(buffer) - 1] == '\n')
{
// fgets reads and adds '\n' in the string, replace '\n' by '##代码##' to
// remove the line break .
buffer[strlen(buffer) - 1] = '##代码##';
printf("%s", buffer);
break;
}
printf("%s", buffer);
}
return 0;
}
' to
// remove the line break .
/* if(buffer[strlen(buffer) - 1] == '\n')
buffer[strlen(buffer) - 1] = '##代码##'; */
printf("%s", buffer);
// Corrects the error mentioned by Alain BECKER.
// Checks if the string buffer is full to check and prevent the
// next character read by fgets is '\n' .
if(strlen(buffer) == (BUFFERSIZE - 1) && (buffer[strlen(buffer) - 1] != '\n'))
{
// Prevents end of the line '\n' to be read in the first
// character (Loop Exit) in the next loop. Reads
// the next char in stdin buffer , if '\n' is read and removed, if
// different is returned to stdin
cChar = fgetc(stdin);
if(cChar != '\n')
ungetc(cChar, stdin);
// To print correctly if '\n' is removed.
else
printf("\n");
}
}
return 0;
}
Exit when Enter is pressed.
按下 Enter 时退出。
##代码##Concatenation and dinamic allocation(linked list) to a single string.
连接和动态分配（链表）到单个字符串。
##代码##回答by jholl
If you want to concatenate the input, then replace printf("%s\n", buffer);with strcat(big_buffer, buffer);. Also create and initialize the big buffer at the beginning: char *big_buffer = new char[BIG_BUFFERSIZE];big_buffer[0] = '\0';. You should also prevent a buffer overrun by verifying the current buffer length plus the new buffer length does not exceed the limit: if ((strlen(big_buffer) + strlen(buffer)) < BIG_BUFFERSIZE). The modified program would look like this:
如果要连接输入，请替换printf("%s\n", buffer);为strcat(big_buffer, buffer);. 还要在开始时创建和初始化大缓冲区：char *big_buffer = new char[BIG_BUFFERSIZE];big_buffer[0] = '\0';。你也应该防止缓冲区溢出通过验证当前缓冲区长度加上新的缓冲区长度不超过极限：if ((strlen(big_buffer) + strlen(buffer)) < BIG_BUFFERSIZE)。修改后的程序如下所示：
