c,pointers,linked-list , I can't find the pointer error that's causing an intermittent crash. Can you?


I can't find the pointer error that's causing an intermittent crash. Can you?

Question:

Tag: c,pointers,linked-list

This works most of the time, but I get an occasional crash. There's a pointer problem somewhere but I can't see it yet.

The code takes words out of a string, and builds a linked list of them. The words have to include adjacent punctuation, but no whitespace. For example, the string:

He said, 'blah blah!' and then died.

Would become the strings

He

said,

'blah

blah!'

and

then

died.

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

unsigned longestword(char *str);

typedef struct _list{
    char *p;
    struct _list *next;
}list;


int main(void){
    char str[]="   'Well!' thought Alice to herself, 'after such a fall as this, I shall think nothing of tumbling down stairs!'";
    unsigned k, i=0, l, j, wordscout;
    list *curr, *root=NULL;

    k =longestword(str);
    puts(str);
    printf("\nlongest word in string is %u letters long.", k);

    do{
        //  skip over any leading whitespace.
        for(; str[i] && isspace(str[i]); i++);
        if(!str[i]) break;

        //  count length of word that begins with str[i].
        for(wordscout=i, l=0; str[wordscout] && !isspace(str[wordscout]); wordscout++, l++);

        //  if this is first word, malloc space to root.
        if(root==NULL){
            if((root = malloc(sizeof(list))) == NULL){
                printf("\nmalloc() failed.");
                exit(1);
            }
            curr = root;
        }

        //  if not first word, malloc space to curr->next.
        else{
            if((curr->next = malloc(sizeof(list))) == NULL){
                printf("\nmalloc() failed.");
                exit(1);
            }
            curr = curr->next;
        }

        //  malloc space in current struct for string.
        if((curr->p = malloc(1+l*sizeof(char))) == NULL){
            printf("\nmalloc() failed.");
            exit(1);
        }

        //  read first word into new space.
        j=0;
        while(!isspace(str[i])) curr->p[j++] = str[i++];
        curr->p[j] = '\0';

        //  check if word is there.
        printf("\n<<%s>>", curr->p);
    }while(str[wordscout]);
}


//  takes a null-terminated string, returns length of longest word in the string. word includes adjacent punctuation, but not whitespace.
unsigned longestword(char *str){

    //  check that word is null-terminated before carrying on.
    unsigned j,k,i,l;
    l = strlen(str);
    for(i=j=k=0; i<=l; i++){
        if(isalpha(str[i]) || ispunct(str[i])) j++;
        else if(j>k){ k=j; j=0; }
        else j=0;
    }
    return k;
}

The longestword() function can be ignored. It works and it's for something else later on.

My output is the following, which is what I want. But every now and again, it crashes after displaying this:

   'Well!' thought Alice to herself, 'after such a fall as this, I shall think n
othing of tumbling down stairs!'

longest word in string is 8 letters long.
<<'Well!'>>
<<thought>>
<<Alice>>
<<to>>
<<herself,>>
<<'after>>
<<such>>
<<a>>
<<fall>>
<<as>>
<<this,>>
<<I>>
<<shall>>
<<think>>
<<nothing>>
<<of>>
<<tumbling>>
<<down>>
<<stairs!'>>
Process returned 0 (0x0)   execution time : 0.062 s
Press any key to continue.

Answer:

while(!isspace(str[i])) will never terminate if the string doesn't end in whitespace.

The segfault might depend on how long it takes before whitespace turns up in the junk after the end of the string.

Instead you could do while( i < wordscout ). Note that l is redundant as you can and should use wordscout - i.


Related:


Does strlen() always correctly report the number of char's in a pointer initialized string?


c,strlen
As long as I use the char and not some wchar_t type to declare a string will strlen() correctly report the number of chars in the string or are there some very specific cases I need to be aware of? Here is an example: char *something = "Report all my...

Parameters to use in a referenced function c++


c++,pointers,reference
I am very confused as to what kind of variables I would put into my function here: names. I am doing a practice problem in a C++ book, because I am learning C++ and am on References and pointers right now, and cannot find a solution. Just for background information,...

Reverse ^ operator for decryption


c,algorithm,security,math,encryption
I'm trying to reverse the following code in order to provide a function which takes the buffer and decrypts it. void crypt_buffer(unsigned char *buffer, size_t size, char *key) { size_t i; int j; j = 0; for(i = 0; i < size; i++) { if(j >= KEY_SIZE) j = 0;...

Array breaking in Pebble C


c,arrays,pebble-watch,cloudpebble
I'm trying to create a simple dice-rolling application in Pebble using C on CloudPebble. I have an array of different die sizes you can scroll through using Up/Down, and you roll (currently just generate a random number, it'll get fancier later) using the middle button. There's also a label at...

Can assignment from a const_iterator dereference cause undefined behaviour?


c++,pointers,pass-by-reference,undefined-behavior,const-iterator
This code is a simplified test for something I am trying to do for real elsewhere. I have a function which takes a "ref-to-ptr" argument and modifies it to return a pointer from a list of pointers. #include <iostream> #include <list> using namespace std; typedef int* intp; typedef std::list<intp> intplist;...

How does this code print odd and even?


c,if-statement,macros,logic
#define MACRO(num, str) {\ printf("%d", num);\ printf(" is");\ printf(" %s number", str);\ printf("\n");\ } int main(void) { int num; printf("Enter a number: "); scanf("%d", &num); if (num & 1) { MACRO(num, "Odd"); } else { MACRO(num, "Even"); } return 0; } Please explain the above code (if/else condition and how...

memory content not erased after deleting my pointer (on a simple example) [duplicate]


c++,pointers,memory-management
This question already has an answer here: Can a local variable's memory be accessed outside its scope? 16 answers Here is a simple of example of a question I have: I create a pointer of integer (value 5), I print the pointer (hence the address) of the memory case...

What all local variables goto Data/BSS segment?


c++,c,nm
The man page of nm here: MAN NM says that The symbol type. At least the following types are used; others are, as well, depending on the object file format. If lowercase, the symbol is usually local; if uppercase, the symbol is global (external) And underneath it has "b" and...

getchar() not working in c


c,while-loop,char,scanf,getchar
getchar() is not working in the below program, can anyone help me to solve this out. I tried scanf() function in place of getchar() then also it is not working. I am not able to figure out the root cause of the issue, can anyone please help me. #include<stdio.h> int...

Text justification C language


c,text,alignment
I have to solve a problem that involves left justification string length and leading zeros. I have the following table : BEGIN CLOSE CONCATENATE DELETE END INITIALIZE PRINT WRITE This is produced by a simple program. My problem is to find out how to convert it like that : It...

Galois LFSR - how to specify the output bit number


c,prng,shift-register
I am trying to understand how change the galois LFSR code to be able to specify the output bit number as a parameter for the function mentioned below. I mean I need to return not the last bit of LFSR as output bit, but any bit of the LFSR (...

Segmentation fault with generating an RSA and saving in ASN.1/DER?


c,openssl,cryptography,rsa
#include <string.h> #include <openssl/aes.h> #include <openssl/rand.h> #include <openssl/bio.h> #include <openssl/rsa.h> #include <openssl/evp.h> #include <openssl/pem.h> #define RSA_LEN 2048 #define RSA_FACTOR 65537 int genRSA2048(unsigned char **pub,unsigned int *pub_l,unsigned char **priv,unsigned int *priv_l){ RSA *pRSA = NULL; pRSA = RSA_generate_key(RSA_LEN,RSA_FACTOR,NULL,NULL); if (pRSA){ pub_l = malloc(sizeof(pub_l)); *pub_l = i2d_RSAPublicKey(pRSA,pub); priv_l = malloc(sizeof(priv_l));...

double* android ndk crash


c++,pointers,android-ndk
I'm facing a peculiar problem with the android ndk. I'm trying to read a previously stored set of bytes in an unsigned char* readBytes (previous allocated and read from file). The data needs to be interpreted as double. unsigned char* svDataOffset=(unsigned char*)(readBytes+byteCounter); unsigned char* chars=(unsigned char*)(svDataOffset+2); //offset of 2 is...

Efficient comparison of small integer vectors


c,integer,compare,bit-manipulation,string-comparison
I have small vectors. Each of them is made of 10 integers which are between 0 and 15. This means that every element in a vector can be written using 4 bits. Hence I can concatenate my vector elements and store the whole vector in a single long type (in...

Program to reverse a string in C without declaring a char[]


c,string,pointers,char
I need to reverse a given string and display it without using the value At[index] notation , I tried the below program using pointers,but it does not print anything for the reverse string, Please help! int main() { char* name=malloc(256); printf("\nEnter string\n"); scanf("%s",name); printf("\nYou entered%s",name); int i,count; count=0; //find the...

pointer to pointer dynamic array in C++


c++,arrays,pointers
I've been having bad luck with dynamic pointers when I want to close it. why the application wrote to memory after end of heap buffer? how can I close my array? int main() { . . int **W; W = new int* [n]; for (int i=1; i <= n; i++)...

How to control C Macro Precedence


c,macros
#define VAL1CHK 20 #define NUM 1 #define JOIN(A,B,C) A##B##C int x = JOIN(VAL,NUM,CHK); With above code my expectation was int x = 20; But i get compilation error as macro expands to int x = VALNUMCHK; // Which is undefined How to make it so that NUM is replaced first...

Is there Predefined-Macros define about byte order in armcc


c,armcc,predefined-macro
Is there Predefined-Macros define about byte order in armcc. I am a novice on the armcc.and sorry for my English. In gcc these are macros: __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__ __ORDER_BIG_ENDIAN__ __ORDER_PDP_ENDIAN__ ... Now I have to use armcc, Is there same like these with armcc? Thank a lot. by the way,the armcc...

Function pointer to singleton class instance function


c++,oop,pointers,singleton
What I'm trying to do is create a function pointer to a single class instance function. I want to do this so I can do something like this: C->member_method(); instead of: Config::inst()->member_method(); but I'm not sure how to go about it. Here is my singleton class: class Config { private:...

How convert unsigned int to unsigned char array


c++,c
I just need to extract those bytes using bitwise & operator. 0xFF is a hexadecimal mask to extract one byte. For 2 bytes, this code is working correctly: #include <stdio.h> int main() { unsigned int i = 0x7ee; unsigned char c[2]; c[0] = i & 0xFF; c[1] = (i>>8) &...

C binary tree sort - extending it


c,binary-tree,binary-search-tree
I need some help in C Help me to extend the binary tree sort on C. I need to return a sorted array in sort function. here it is: #include <stdio.h> #include <stdlib.h> struct btreenode { struct btreenode *leftchild ; int data ; struct btreenode *rightchild ; } ; void...

C++ / C #define macro calculation


c++,c,macros
Suppose I have #define DETUNE1 sqrt(7)-sqrt(5) #define DETUNE2 sqrt(11)-sqrt(7) And I call these multiple times in my program. Are DETUNE1 and DETUNE2 calculated every time it is called? Thanks. Please don't downvote this, I really want to know and a search didn't turn up anything definite. ...

What does `strcpy(x+1, SEQX)` do?


c,strcpy
I'm wondering what this syntax of strcpy() does in line 65 and 66: 24 #define SEQX "TTCATA" 25 #define SEQY "TGCTCGTA" 61 M = strlen(SEQX); 62 N = strlen(SEQY); 63 x = malloc(sizeof(char) * (M+2)); /* +2: leading blank, and trailing \0 */ 64 y = malloc(sizeof(char) * (N+2)); 65...

Disadvantages of calling realloc in a loop


c,memory-management,out-of-memory,realloc
I'm trying to implement some math algorithms in C on Windows 7, and I need to repeatedly increase size of my array. Sometimes it fails because realloc can't allocate memory. But if I allocate a lot of memory at once in the beginning it works fine. Is it a problem...

Passing iterator's element to a function: wrong type of pointer


c++,pointers,stl,iterator
I'm attempting to solve Project Euler's problem #3 using C++ to gain an understanding of how to use C++ iterators. According to the examples I've seen online, I can use the dereferened iterator as a parameter for cout, and it will print the elements successfully. By that same logic, I...

How to increment the value of an unsigned char * (C)


c++,c,openssl,byte,sha1
I have a value stored as an unsigned char * (in C). This holds the SHA1 hash of a string. My goal is to cover the SHA1 key space. Since I'm using <openssl/evp.h> to generate the hashes, I end up with an unsigned char* holding the SHA1 value. Now I...

Passing int using char pointer in C


c,exec,ipc
I'm trying to figure out how to pass an int using a char pointer. It fails once the int value is too large for the char. This is what I'm trying to figure out: char *args[5]; int i = 20; /*some other code/assignments*/ args[2] = (char *)&i; execv(path, args); How...

Is it safe to read and write on an array of 32 bit data byte by byte?


c,memory,memory-alignment
So I have a void * data of 32 bit unsigned integers which represents the pixels. Is it okay for me to access one of the pixels with a char * and modify the values directly? Or is it better to store my new pixel in a temporary uint32_t variable...

OpenGL glTexImage2D memory issue


c,opengl
I'm loading a cubemap to create a skybox, everything is fine and the skybox renders properly with a correct texture application. However, I decided to check my program safety with valgrind, Valgrind gives this error: http://pastebin.com/seqmXjyx The line 53 in sky.c is: glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X + i, 0, GL_RGB, texture.width, texture.height, 0,...

execl() works on one of my code, but doesn't work on another


c,execl
I already used execl() in code, and it worked well. But this time, I really have no idea why it doesn't work. So here's the code that do not work #include <unistd.h> #include <stdio.h> int main() { int i = 896; printf("please\n"); execl("home/ubuntu/server/LC/admin/admin", (char*)i, NULL); printf("i have no idea why\n");...

C programming - Confusion regarding curly braces


c,scope
The following code is for replacing multiple consecutive spaces into 1 space. Although I manage to do it, I am confused in the use of curly braces. This one is actually running fine: #include <stdio.h> #include <stdlib.h> int main() { int ch, lastch; lastch = 'a'; while((ch = getchar())!= EOF)...

Set precision dynamically using sprintf


c,printf,format-string
Using sprintf and the general syntax "%A.B" I can do this: double a = 0.0000005l; char myNumber[50]; sprintf(myNumber,"%.2lf",a); Can I set A and B dynamically in the format string?...

free causing different results from malloc


c,string,malloc,free
Below is a C program i have written to print different combination of characters in a string. This is not an efficient way as this algorithm creats a lot of extra strings. However my question is NOT about how to solve this problem more efficiently. The program works(inefficiently though) and...

CallXXXMethod undefined using JNI in C


java,c,jni
So I've tried to use the JNI interface to call Java methods from C. Calling static methods is no problem, but I get stuck when I want to call a method on an object. The code is as follows: #include <stdio.h> #include <string.h> #include <jni.h> int main() { JavaVMOption options[1];...

Counting bytes received by posix read()


c,function,serial-port,posix
I get confused with one line of code: temp_uart_count = read(VCOM, temp_uart_data, 4096); I found more about read function at http://linux.die.net/man/3/read, but if everything is okay it returns 0, so how we can get num of bytes received from that? temp_uart_count is used to count how much bytes we received...

Call template function for the value of a pointer out of a template function, in C++


c++,templates,pointers,c++11
I am trying to call a template function for the pointer that was given as template parameter in my calling function. My code is: template <> struct serialize_helper<std::string> { // not important code... } }; template <class T> inline void serializer(const T& obj, StreamType::iterator& res) { if(std::is_pointer<T>::value) { //THIS doesn'...

CGO converting Xlib XEvent struct to byte array?


c,go,xlib,cgo
I am creating a simple window manager (code based of the c code in tinywm) in Golang. To use Xlib, I am using cgo, so my header is: // #cgo LDFLAGS: -lX11 // #include <X11/Xlib.h> And I have a variable declaration, like: event := C.XEvent{} And then, I use this...

Is post-increment operator guaranteed to run instantly?


c,c89,post-increment,ansi-c
Let's say I have the following code: int i = 0; func(i++, i++); The increment is happening right after returning the value? Is it guaranteed that the first argument will be 0, and the second argument will be 1?...

C language, vector of struct, miss something?


c,vector,struct
This is a part of my program that I want to create a vector of struct typedef struct { char nome[501]; int qtd; int linha; int coluna; } tPeca; tPeca* criarPecas(FILE *pFile, int tam) { int i; tPeca *pecaJogo = (tPeca*)malloc(tam*sizeof(tPeca)); if (pecaJogo == NULL) return NULL; for (i =...

Segmentation Fault if I don't say int i=0


c,arrays,segmentation-fault,initialization,int
void removeVowels(char* array){ int i,j,v; i=0; char vowel[]={'a','e','i','o','u'}; while(array[i]!='\0') { for(v=0;v<5;v++) { if (array[i]==vowel[v]) { j=i; while(array[j]!='\0') { array[j]=array[j+1]; j++; } i--; break; } } i++; } } in function removeVowels() if I don't include i=0; and just say int i; why does it give segmentation fault? Isn't it automatically...

How to read string until two consecutive spaces?


c,format,sscanf,c-strings
A well known function of the scanf() functions is that you can pass a format to scan input according to this format. For my case, I cannot seem to find a solution searching this and this documentation. I have a string (sInput) as the following: #something VAR1 this is a...

How can I align stack to the end of SRAM?


c,embedded,stm32,gnu-arm,coocox
I have a STM32F103VCT6 microcontroller with 48kb of SRAM, and recently i've got a memory collision: I have some static variable (lets call it A) located in heap with size of 0x7000 and I wrote some simple function to get info about stack and heap: void check(int depth) { char...

How does ((a++,b)) work? [duplicate]


c,function,recursion,comma
This question already has an answer here: What does the comma operator `,` do in C? 8 answers In the below block of code, I am trying to understand how the line return reverse((i++, i)) is working. #include <stdio.h> void reverse(int i); int main() { reverse(1); } void reverse(int...

VS2012 Identifer not found when part of static lib


c,visual-studio-2012,linker,static-libraries
Using VS2012 C/C++: I created and linked a static lib called "libtools" to my project. Calls to functions in the libtools lib worked as expected. I created and linked a second static lib called "shunt" to my project. But when I incorporate a call to a function in shunt, I...

Loop through database table and compare user input


mysql,c
I am trying to loop through the rows in a MySql table and compare the data in a certain column to some user input using C. Currently my code looks like this: MYSQL *cxn = mysql_init(NULL); MYSQL_RES *result; unsigned int num_fields; unsigned int num_rows; char *query_string; MYSQL_ROW *row; if (mysql_real_connect(cxn,...

scanf get multiple values at once


c,char,segmentation-fault,user-input,scanf
I need to get in one single shot different inputs from one single line. In particular I need to get a single char and then, depending on which char value I just read, it can be a string and an int or a string, an int and another string and...

Infinite loop with fread


c,arrays,loops,malloc,fread
I'm trying to allocate an array 64 bytes in size and then loop over the array indexes to put a read a byte each from the inputfile. but when I don't malloc() the array indexes, the loop stays in index0 (so each time it loops it replaces the content in...

Does realloc() invalidate all pointers?


c,pointers,dynamic-memory-allocation,behavior,realloc
Note, this question is not asking if realloc() invalidates pointers within the original block, but if it invalidates all the other pointers. I'm new to C, and am a bit confused about the nature of realloc(), specifically if it moves any other memory. For example: void* ptr1 = malloc(2); void*...