[英]C - Unexpected Segmentation Fault on strtok(…)
I am using strtok(...) of the library and it appears to be working fine until the end condition, where it results in a segmentation fault and program crash. 我正在使用库的strtok(...),它似乎工作正常,直到结束条件,它导致分段错误和程序崩溃。 The API claims that strtok(...) will output a NULL when there are no more tokens to be found, which meant, I thought, that you had to catch this NULL in order to terminate any loops that you were running using strtok(...).
API声称strtok(...)将在没有找到更多令牌时输出NULL,这意味着,我想,你必须捕获这个NULL才能终止你使用strtok运行的任何循环( ...)。 What do I need to do to catch this NULL to prevent my program from crashing?
我需要做什么才能捕获此NULL以防止程序崩溃? I imagined the NULL was allowed for use as a terminating condition.
我想象允许使用NULL作为终止条件。
I have prepared a SSCCE for you to observe this behavior. 我准备了一个SSCCE供你观察这种行为。 I need strtok(...) to work for a much larger piece of software I am writing, and I am getting the exact same segmentation behavior.
我需要strtok(...)来处理我正在编写的更大的软件,并且我得到完全相同的分段行为。 The output at the command line is shown below this code vignette (yes I know you use <...> to enclose libraries, but I was having difficulty getting this post to display the code libraries).
命令行的输出显示在此代码插图下方(是的,我知道您使用<...>来封装库,但是我很难获得此帖子来显示代码库)。 I am using gcc version 4.5.3, on a Windows 8 OS, and below shows two different flavors of how I imagine one could try to catch the NULL in a loop.
我在Windows 8操作系统上使用gcc 4.5.3版本,下面显示了我想象的两种不同风格,我可以尝试在循环中捕获NULL。
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
main(){
char* from = "12.34.56.78";
char * ch = ".";
char * token = strtok(from, ch);
printf("%s\n",token);
while(token != NULL){
token = strtok(NULL, ch);
printf("%s\n", token);
}
printf("Broke out of loop!");
while(strcmp(token, 0) != 0){
printf("%s\n",token);
token = strtok(NULL, ch);
}
}
############ OUTPUT: ############ $ ./test 12 34 56 78 Segmentation fault (core dumped)
strtok
modifies its first argument. strtok
修改了它的第一个参数。 You are passing it a string from read-only memory, and the segfault occurs when strtok
tries to change it. 您从只读内存传递一个字符串,当
strtok
尝试更改它时发生segfault。 Try changing from: 尝试改变:
char* from = "12.34.56.78";
to 至
char from[] = "12.34.56.78";
you are first checking if token
is not equal to NULL(when it is, it breaks out of the while
loop). 你首先检查
token
是否不等于NULL(当它是,它突破了while
循环)。 Then you are comparing token
, which is a NULL
with a constant NUMBER? 然后你比较
token
,这是一个NULL
与常数NUMBER? here: strcmp(token, 0)
when strcmp
expects 2 strings, you provide a number. 这里:
strcmp(token, 0)
当strcmp
需要2个字符串时,你提供一个数字。 strcmp
will try to fetch a string at 0th address(or NULL) giving you a segmentation fault. strcmp
将尝试在第0个地址(或NULL)获取字符串,从而为您提供分段错误。
while(strcmp(token, 0) != 0){
token = strtok(NULL, ch);
printf("%s\n",token);
}
Also this piece of code should be something like the following: 此代码也应如下所示:
change 更改
char * token = strtok(from, ch);
printf("%s\n",token);
while(token != NULL){
token = strtok(NULL, ch);
printf("%s\n", token);
}
to 至
char * token = strtok(from, ch);
printf("%s\n",token);
while(token != NULL){
printf("%s\n", token);
token = strtok(NULL, ch);
}
This is a problem: 这是个问题:
while(token != NULL){
token = strtok(NULL, ch);
printf("%s\n", token);
}
You're checking for NULL, but then calling strtok
again and not checking after that but before printing. 你正在检查NULL,但是再次调用
strtok
而不是在那之后但是在打印之前检查。
There are other problems with the code, but I suspect this is why it crashes where it does now. 代码还有其他问题,但我怀疑这就是它崩溃的原因。
The problem is that even though you terminate the loop when strtok()
returns NULL
, you try to print the NULL
first: 问题是即使你在
strtok()
返回NULL
时终止循环,你首先尝试打印NULL
:
while(token != NULL){
token = strtok(NULL, ch);
printf("%s\n", token); // not good when token is NULL
}
It turns out there are several opportunities in addition to this one for segfaults in this example, as pointed out by other answers. 事实证明,除了这个例子之外,在这个例子中还有几个机会用于segfaults,正如其他答案所指出的那样。
Here's one way to handle your example tokenization: 这是处理示例标记化的一种方法:
char from[] = "12.34.56.78";
char * ch = ".";
char * token = strtok(from, ch);
while (token != NULL){
printf("%s\n", token);
token = strtok(NULL, ch);
}
If purpose of code is only to print element separated by '.', Only change in char declaration and before printing token check for its value NULL or not ! 如果代码的目的只是打印以'。'分隔的元素,则只更改char声明并在打印令牌之前检查其值是否为NULL!
main(){
char from[] = "12.34.56.78.100.101";
char * ch = ".";
char * token = strtok(from, ch);
//printf("%s\n",token);
while(token != NULL){
printf("%s\n", token);
token = strtok(NULL, ch);
}
}
OUTPUT
./test1 12 12 34 56 78 100 101
You have both memory access errors and logic errors. 您有内存访问错误和逻辑错误。 I will only address the memory access errors that are causing your program to crash.
我只会解决导致程序崩溃的内存访问错误。
strtok
modifies it's first argument. strtok
修改了它的第一个参数。 Since you are passing in a string literal, it is unable to modify the string (string literals are not modifiable.) 由于传入字符串文字,因此无法修改字符串(字符串文字不可修改。)
Here's a possible fix to define from
as a modifiable string array: 这是一个可能的修复,
from
一个可修改的字符串数组定义:
char from[] = "12.34.56.78";
Because strtok
modifies the string passed into it, you cannot process that string again in your second while loop. 因为
strtok
修改了传入其中的字符串,所以在第二个while循环中不能再次处理该字符串。 You are essentially passing in a NULL into the strcmp
function there. 您实际上是将NULL传入
strcmp
函数。 A possible fix would be to copy the from
array into another buffer each time you wish to use strtok
. 可能的解决方法是每次要使用
strtok
时将from
数组复制到另一个缓冲区。
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