Here is a weird problem:
token = strtok(NULL, s);
printf(" %s\n", token); // these two lines can read the token and print
However!
token = strtok(NULL, s);
printf("%s\n", token); // these two lines give me a segmentation fault
Idk whats happened, because I just add a space before %s\n, and I can see the value of token.
my code:
int main() {
FILE *bi;
struct _record buffer;
const char s[2] = ",";
char str[1000];
const char *token;
bi = fopen(DATABASENAME, "wb+");
/*get strings from input, and devides it into seperate struct*/
while(fgets(str, sizeof(str), stdin)!= NULL) {
printf("%s\n", str); // can print string line by line
token = strtok(str, s);
strcpy(buffer.id, token);
printf("%s\n", buffer.id); //can print the value in the struct
while(token != NULL){
token = strtok(NULL, s);
printf("%s\n", token); // problem starts here
/*strcpy(buffer.lname, token);
printf("%s\n", buffer.lname); // cant do anything with token */
}}
fclose(bi);
return 1;}
Here is the example of string I read from stdin and after parsed(I just tried to strtok the first two elements to see if it works):
<15322101,MOZNETT,JOSE,n/a,n/a,2/23/1943,MALE,824-75-8088,42 SMITH AVENUE,n/a,11706,n/a,n/a,BAYSHORE,NY,518-215-5848,n/a,n/a,n/a
<
< 15322101
< MOZNETT
In the first version your compiler transforms printf() into a
puts() and puts does not allow null pointers, because internally
invokes the strlen() to determine the lenght of the string.
In the case of the second version you add a space in front of format
specifier. This makes it impossible for the compiler to call puts
without appending this two string together. So it invokes the actual
printf() function, which can handle NULL pointers. And your code
works.
Your problem reduces to the following question What is the behavior of printing NULL with printf's %s specifier?
.
In short NULL as an argument to a printf("%s") is undefined. So you need to check for NULL as suggested by #kninnug
You need to change you printf as follows:
token = strtok(NULL, s);
if (token != NULL) printf("%s\n", token);
Or else
printf ("%s\n", token == NULL ? "" : token);
Related
Assume we need to copy user's input into another string by concatenating the tokens of input, e.g., "hello world" -> "helloworld".
#include <stdio.h>
#include <string.h>
int main(void) {
char buffer[50];
printf("\nEnter a string: ");
while (fgets(buffer, sizeof(buffer), stdin) != 0) {
size_t size = strlen(buffer);
if (size > 0 && buffer[size - 1] == '\n') {
char input[1]; // set it too small
buffer[size - 1] = '\0';
char *tok = strtok(buffer, " "); // works fine
do {
strcat(input, tok); // append to "input" that has not enough space
printf("\nfound token: %s", tok);
tok = strtok(NULL, " "); // produces garbage
} while (tok);
break;
}
}
Running the code above:
Enter a string: hello world
found token: hello
found token: w
found token: r
*** stack smashing detected ***: <unknown> terminated
I struggle to understand how is strtok related to strcat failing to append tok. They are not sharing variables except for tok which is (according to the docs) copied by strcat, so whatever strcat is doing shouldn't affect the strtok behavior and the program should crash on the second strcat call at least, right? But we see that strcat is getting called 3 times before stack smashing gets detected. Can you please explain why?
For starters this array
char input[1];
is not initialized and does not contain a string.
So this call of strcat
strcat(input, tok);
invokes undefined behavior also because the array input is not large enough to store the copied string. It can overwrite memory beyond the array.
There are multiple problems in the code:
char input[1]; is too small to do anything. You cannot concatenate the tokens from the line into this minuscule array. You must define it with a sufficient length, namely the same length as buffer for simplicity.
input must be initialized as an empty string for strcat(input, tok); to have defined behavior. As coded, the first call to strcat corrupts other variables causing the observed behavior, but be aware anything else could happen as a result of this undefined behavior.
char *tok = strtok(buffer, " "); works fine but may return a null pointer if buffer contains only whitespace if anything. The do loop will then invoke undefined behavior on strcat(input, tok). Use a for or while loop instead.
there is a missing } in the code, it is unclear whether you mean to break from the while loop after the first iteration or only upon getting the end of the line.
Here is a modified version:
#include <stdio.h>
#include <string.h>
int main(void) {
char buffer[50];
char input[sizeof buffer] = "";
printf("Enter a string: ");
if (fgets(buffer, sizeof(buffer), stdin)) {
char *tok = strtok(buffer, " \n");
while (tok) {
strcat(input, tok);
printf("found token: %s\n", tok);
tok = strtok(NULL, " \n");
}
printf("token string: %s\n", input);
}
return 0;
}
I have this code in my program:
char* tok = NULL;
char move[100];
if (fgets(move, 100, stdin) != NULL)
{
/* then split into tokens using strtok */
tok = strtok(move, " ");
while (tok != NULL)
{
printf("Element: %s\n", tok);
tok = strtok(NULL, " ");
}
}
I have tried adding printf statements before and after fgets, and the one before gets printed, but the one after does not.
I cannot see why this fgets call is causing a segmentation failure.
If someone has any idea, I would much appreciate it.
Thanks
Corey
The strtok runtime function works like this
the first time you call strtok you provide a string that you want to tokenize
char s[] = "this is a string";
in the above string space seems to be a good delimiter between words so lets use that:
char* p = strtok(s, " ");
what happens now is that 's' is searched until the space character is found, the first token is returned ('this') and p points to that token (string)
in order to get next token and to continue with the same string NULL is passed as first argument since strtok maintains a static pointer to your previous passed string:
p = strtok(NULL," ");
p now points to 'is'
and so on until no more spaces can be found, then the last string is returned as the last token 'string'.
more conveniently you could write it like this instead to print out all tokens:
for (char *p = strtok(s," "); p != NULL; p = strtok(NULL, " "))
{
puts(p);
}
EDITED HERE:
If you want to store the returned values from strtok you need to copy the token to another buffer e.g. strdup(p); since the original string (pointed to by the static pointer inside strtok) is modified between iterations in order to return the token.
I am trying to read a txt file, and I can get the line which I want, but I can not print every words in this line one by one;
for example: the line looks like:
hello world 1 2 3
and I need print them one by one which looks like:
hello
world
1
2
3
I got the segmentation fault core dumped error
char temp[256]；
while(fgets(temp, 256, fp) != NULL) {
...
int tempLength = strlen(temp);
char *tempCopy = (char*) calloc(tempLength + 1, sizeof(char));
strncpy(temCopy, temp, tempLength); // segmentation fault core dumped here；
// works fine with temp as "name country"
name = strtok_r(tempCopy, delimiter, &context);
country = strtok_r(Null, delimiter, &context);
printf("%s\n", name);
printf("%s\n", country);
}
Can anyone help me fix the code?
Thanks!
Impleted with strtok()
char *p;
char temp[256];
while(fgets(temp,256,fp) != NULL){
p = strtok (temp," ");
while (p != NULL)
{
printf ("%s\n",p);
p = strtok (NULL, " ");
}
}
If you see man strtok You will found
BUGS
Be cautious when using these functions. If you do use them, note that:
* These functions modify their first argument.
* These functions cannot be used on constant strings.
* The identity of the delimiting character is lost.
* The strtok() function uses a static buffer while parsing, so it's not thread safe. Use strtok_r() if this matters to you.
Try to make changes with strtok_r()
While read a line from a file you can invoke the following function:
if( fgets (str, 60, fp)!=NULL ) {
puts(str);
token = strtok(str," ");
while(token != NULL)
{
printf("%s\n",token);
token = strtok(NULL," ");
}
}
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. 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(...). What do I need to do to catch this NULL to prevent my program from crashing? I imagined the NULL was allowed for use as a terminating condition.
I have prepared a SSCCE for you to observe this behavior. I need strtok(...) to work for a much larger piece of software I am writing, and I am getting the exact same segmentation behavior. 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). 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.
#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. You are passing it a string from read-only memory, and the segfault occurs when strtok tries to change it. Try changing from:
char* from = "12.34.56.78";
to
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). Then you are comparing token, which is a NULL with a constant NUMBER? here: strcmp(token, 0) when strcmp expects 2 strings, you provide a number. strcmp will try to fetch a string at 0th address(or NULL) giving you a segmentation fault.
while(strcmp(token, 0) != 0){
token = strtok(NULL, ch);
printf("%s\n",token);
}
Also this piece of code should be something like the following:
change
char * token = strtok(from, ch);
printf("%s\n",token);
while(token != NULL){
token = strtok(NULL, ch);
printf("%s\n", token);
}
to
char * token = strtok(from, ch);
printf("%s\n",token);
while(token != NULL){
printf("%s\n", token);
token = strtok(NULL, ch);
}
This is a problem:
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.
There are other problems with the code, but I suspect this is why it crashes where it does now.
The problem is that even though you terminate the loop when strtok() returns NULL, you try to print the NULL first:
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.
Here's one way to handle your example tokenization:
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 !
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. I will only address the memory access errors that are causing your program to crash.
strtok modifies it's first argument. Since you are passing in a string literal, it is unable to modify the string (string literals are not modifiable.)
Here's a possible fix to define from as a modifiable string array:
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. You are essentially passing in a NULL into the strcmp function there. A possible fix would be to copy the from array into another buffer each time you wish to use strtok.
I am trying to work with strtok and strcat but the second printf never shows up. Here is the code:
int i = 0;
char *token[128];
token[i] = strtok(tmp, "/");
printf("%s\n", token[i]);
i++;
while ((token[i] = strtok(NULL, "/")) != NULL) {
strcat(token[0], token[i]);
printf("%s", token[i]);
i++;
}
If my input is 1/2/3/4/5/6 for tmp then the console output would be 13456. The 2 is always missing. Does anyone know how to fix this?
The two is always missing because on the first iteration of your loop you overwrite it with the call to strcat.
After entry to the loop your buffer contains: "1\02\03/4/5/6" internal strtok pointer is pointing to "3". tokens[1] points to "2".
You then call strcat: "12\0\03/4/5/6" so your token[i] pointer is pointing to "\0". The first print prints nothing.
Subsequent calls are OK because the null characters do not overwrite the input data.
To fix it you should build up your output string into a second buffer, not the one you are parsing.
A working(?) version:
#include <stdio.h>
#include <string.h>
int main(void)
{
int i = 0;
char *token[128];
char tmp[128];
char removed[128] = {0};
strcpy(tmp, "1/2/3/4/5/6");
token[i] = strtok(tmp, "/");
strcat(removed, token[i]);
printf("%s\n", token[i]);
i++;
while ((token[i] = strtok(NULL, "/")) != NULL) {
strcat(removed, token[i]);
printf("%s", token[i]);
i++;
}
return (0);
}
strtok modifies the input string in place and returns pointers to that string. You then take one of those pointers (token[0]) and pass it to another operation (strcat) that writes to that pointer. The writes are clobbering each other.
If you want to concatenate all the tokens, you should allocate a separate char* to strcpy to.