I'm posting this as a vent for my questions (I will have a multitude). I decided it would be rather annoying to keep asking the same person one question at a time (said person is very busy), so I'll be posting questions as I come across them in my project. If you feel like helping, please do, and I would greatly appreciate it!
Note: this means I'll be updating this post frequently. Help is greatly, greatly appreciated as always.
EDIT so you guys want me to just keep posting different questions if I come across them? Of course I always do research before asking you guys, you talented group of men and women only get the most persistent of errors.
My first question:
I keep getting the error:
lvalue required as left operand of assignment
THE PURPOSE of this code is to copy the first n character up to ':'. For instance, if currentline is: "LABEL: .long 100" then GetLabelName would return "LABEL".
NOTE strncpy isn't working for this. It returns the remaining characters after ignoring the first n characters instead of just returning the first n characters...
Here's the code that's causing the error:
char *GetLabelName(char *currentline){
char *labelname[200];
while((((*labelname)++)=(*currentline)++)!=':');
return labelname;
}
Something is fishy about this code I guess, but I can't figure out what. Any ideas?
What I think you're trying to do is extract/copy all of the characters in a string up until a certain point (':' or NUL) and return that buffer. If that's the case, you're going to need to dynamically allocate memory for the new string (you can't return a local buffer allocated on the stack), and you should also take advantage of functions in <string.h> like strchr and memcpy.
Here's an alternative working example:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *extract_string(char *str, char delim)
{
size_t len;
char *new_str;
char *delim_pos = strchr(str, delim);
/* new string is the length from the start of the old string to the
* delimiter, or if it doesn't exist, a copy of the whole string */
if (delim_pos == NULL)
return strdup(str);
len = delim_pos - str;
new_str = malloc(len + 1);
memcpy(new_str, str, len);
new_str[len] = '\0'; /* NUL terminate the new string */
return new_str;
}
int main(void)
{
char *extracted1 = extract_string("some:string", ':');
char *extracted2 = extract_string("no delimiter", ':');
puts(extracted1);
puts(extracted2);
/* free the dynamically allocated buffers */
free(extracted1);
free(extracted2);
return 0;
}
Output:
some
no delimiter
If you don't want to make a copy when the delimiter isn't found, you could alternatively return NULL.
Alternatively, if you don't mind mangling your initial string, you could use strtok to extract tokens.
The problem is (*labelname)++. You are incrementing the value that's pointed to by labelname and simultaneously assigning to it the value that's pointed to by currentline. If you want to increment the pointers, use *labelname++ and *currentline++
Related
I'm trying to make a simple code that converts one type of data into another. I use strtok to extract one part of the data and then I run a long serial of if conditions to find the right output. However, when the correct output is found and written in the variable currentNote, it also seems to overwrite the variable comma for a reason I can't figure out. Here is the problematic part of the code :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char sequence[] = "c1[M],c1[M],d3[L],c3[M],b2[M],(#A-2)[IKN],(#A-1)[L]";
char copy[] = "";
char *comma;
char currentNote[4] = "";
strcpy(copy, sequence);
comma = strtok(copy, ",");
if(strstr(comma, "c1") != 0) //c1
{
printf("%s\n\n", comma); //Here ...
strcpy(currentNote, "C5 ");
printf("%s\n\n", comma);
}
return 0;
}
And here's the outcome :
c1[M]
cC5
No need to say that strcpy(currentNote, "C5 "); is causing it. I don't know why though, I thought it would return c1[M] like I would like it to do. After some more experimentation it turns out that the secondprintf("%s\n\n", comma); will always return the first character of sequence followed by C5. I hope someone can find out, it would be very great.
You are calling
strcpy(copy, sequence) while copy is a 1 element length array. You need to define it by giving it sufficient size. Your code as is, is simply a UB. Redefine it like
char copy[100];
Or allocate sufficient memory to it dynamically using malloc.
So I'm new to C and the whole string manipulation thing, but I can't seem to get strtok() to work. It seems everywhere everyone has the same template for strtok being:
char* tok = strtok(source,delim);
do
{
{code}
tok=strtok(NULL,delim);
}while(tok!=NULL);
So I try to do this with the delimiter being the space key, and it seems that strtok() no only reads NULL after the first run (the first entry into the while/do-while) no matter how big the string, but it also seems to wreck the source, turning the source string into the same thing as tok.
Here is a snippet of my code:
char* str;
scanf("%ms",&str);
char* copy = malloc(sizeof(str));
strcpy(copy,str);
char* tok = strtok(copy," ");
if(strcasecmp(tok,"insert"))
{
printf(str);
printf(copy);
printf(tok);
}
Then, here is some output for the input "insert a b c d e f g"
aaabbbcccdddeeefffggg
"Insert" seems to disappear completely, which I think is the fault of strcasecmp(). Also, I would like to note that I realize strcasecmp() seems to all-lower-case my source string, and I do not mind. Anyhoo, input "insert insert insert" yields absolutely nothing in output. It's as if those functions just eat up the word "insert" no matter how many times it is present. I may* end up just using some of the C functions that read the string char by char but I would like to avoid this if possible. Thanks a million guys, i appreciate the help.
With the second snippet of code you have five problems: The first is that your format for the scanf function is non-standard, what's the 'm' supposed to do? (See e.g. here for a good reference of the standard function.)
The second problem is that you use the address-of operator on a pointer, which means that you pass a pointer to a pointer to a char (e.g. char**) to the scanf function. As you know, the scanf function want its arguments as pointers, but since strings (either in pointer to character form, or array form) already are pointer you don't have to use the address-of operator for string arguments.
The third problem, once you fix the previous problem, is that the pointer str is uninitialized. You have to remember that uninitialized local variables are truly uninitialized, and their values are indeterminate. In reality, it means that their values will be seemingly random. So str will point to some "random" memory.
The fourth problem is with the malloc call, where you use the sizeof operator on a pointer. This will return the size of the pointer and not what it points to.
The fifth problem, is that when you do strtok on the pointer copy the contents of the memory pointed to by copy is uninitialized. You allocate memory for it (typically 4 or 8 bytes depending on you're on a 32 or 64 bit platform, see the fourth problem) but you never initialize it.
So, five problems in only four lines of code. That's pretty good! ;)
It looks like you're trying to print space delimited tokens following the word "insert" 3 times. Does this do what you want?
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
char str[BUFSIZ] = {0};
char *copy;
char *tok;
int i;
// safely read a string and chop off any trailing newline
if(fgets(str, sizeof(str), stdin)) {
int n = strlen(str);
if(n && str[n-1] == '\n')
str[n-1] = '\0';
}
// copy the string so we can trash it with strtok
copy = strdup(str);
// look for the first space-delimited token
tok = strtok(copy, " ");
// check that we found a token and that it is equal to "insert"
if(tok && strcasecmp(tok, "insert") == 0) {
// iterate over all remaining space-delimited tokens
while((tok = strtok(NULL, " "))) {
// print the token 3 times
for(i = 0; i < 3; i++) {
fputs(tok, stdout);
}
}
putchar('\n');
}
free(copy);
return 0;
}
I am trying to copy certain parts of a string into other, new strings, but when i try to do it and print the results it gives me weird output.. I really hope someone can help. I have a feeling that it is something about missing pointers.. Here is my source;
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void getData(char code[], char ware[], char prod[], char qual[])
{
printf("Bar code: %s\n", code);
/* Copy warehouse name from barcode */
strncpy(ware, &code[0], 3);
ware[4] = "\0";
strncpy(prod, &code[3], 4);
prod[5] = "\0";
strncpy(qual, &code[7], 3);
qual[4] = "\0";
}
int main(){
/* allocate and initialize strings */
char barcode[] = "ATL1203S14";
char warehouse[4];
char product[5];
char qualifier[4];
getData(&barcode, &warehouse, &product, &qualifier);
/* print it */
printf("Warehouse: %s\nID: %s\nQualifier: %s", warehouse, product, qualifier);
return 0;
}
EDIT:
The wierd output is:
Bar code: ATL1203S14
Warehouse: ATL
ID: ♫203(♫>
Qualifier: S14u♫203(♫>
I think you meant '\0' instead of "\0" and 3 instead of 4:
ware[4] = "\0";
Try:
ware[3] = 0;
Also the & in getData(&barcode, &warehouse...) are useless. Just use getData(barcode, warehouse...);.
You're writing past the end of the chars in your getData() function. You've defined char product[5], which allocates 5 bytes of memory. That gives you array indexes 0,1,2,3,4. In getData, you write the product's null terminator to index 5, which is past the end of product, and will overwrite the next var's first character.
The same applies for barecode, warehouse, and qualifier.
Arrays in C and C++ are zero-based. The last index is one less than the length. You're setting a value in the memory after the array, for each of the arrays ware, prod and qual.
For example, instead of
char warehouse[4];
ware[4] = "\0";
you'd want:
char warehouse[4];
ware[3] = "\0";
getData(&barcode, &warehouse, &product, &qualifier);
This is not the way you should call getData. getData takes pointers, arrays are automatically converted to pointers, so theres no need to use the address-of operator &.
You should use
getData(barcode, warehouse, product, qualifier);
The sizes of the strings inside main() don't include a place for the sentinel.
You need to have:
char warehouse[5];
char product[6];
char qualifier[5];
Also, You are assigning a pointer to the string "\0" into a character, where you should be assigning the character '\0' itself.
I think I'd do things a bit differently. In particular, strncpy is almost never really useful (I'm reasonably certain it was invented for file names in the original Unix FS, and while it fits their specific requirements quite nicely, those requirements are sufficiently unusual that it's rarely good for much of anything else).
Instead, I'd use sscanf: sscanf(code, "%4c%5c%4c", ware, prod, qual);
Your question does not make it clear whether this is really correct. As others have pointed out, you're writing past the ends of the space you've allocated. Above, I've assumed you specified the number of characters you want to copy, so you'd have to expand each of the allocations by one character to make room for the terminator. Alternative, if you've already left room for the terminator and want one fewer character copied, you'd have to reduce each of the lengths above by one so the format string would be "%3c%4c%3c".
At a recent job interview, I was asked to implement my own string copy function. I managed to write code that I believe works to an extent. However, when I returned home to try the problem again, I realized that it was a lot more complex than I had thought. Here is the code I came up with:
#include <stdio.h>
#include <stdlib.h>
char * mycpy(char * d, char * s);
int main() {
int i;
char buffer[1];
mycpy(buffer, "hello world\n");
printf("%s", buffer);
return 0;
}
char * mycpy (char * destination, char * source) {
if (!destination || !source) return NULL;
char * tmp = destination;
while (*destination != NULL || *source != NULL) {
*destination = *source;
destination++;
source++;
}
return tmp;
}
I looked at some other examples online and found that since all strings in C are null-terminated, I should have read up to the null character and then appended a null character to the destination string before exiting.
However one thing I'm curious about is how memory is being handled. I noticed if I used the strcpy() library function, I could copy a string of 10 characters into a char array of size 1. How is this possible? Is the strcpy() function somehow allocating more memory to the destination?
Good interview question has several layers, to which to candidate can demonstrate different levels of understanding.
On the syntactic 'C language' layer, the following code is from the classic Kernighan and Ritchie book ('The C programming language'):
while( *dest++ = *src++ )
;
In an interview, you could indeed point out the function isn't safe, most notably the buffer on *dest isn't large enough. Also, there may be overlap, i.e. if dest points to the middle of the src buffer, you'll have endless loop (which will eventually creates memory access fault).
As the other answers have said, you're overwriting the buffer, so for the sake of your test change it to:
char buffer[ 12 ];
For the job interview they were perhaps hoping for:
char *mycpy( char *s, char *t )
{
while ( *s++ = *t++ )
{
;
}
return s;
}
No, it's that strcpy() isn't safe and is overwriting the memory after it, I think. You're supposed to use strncpy() instead.
No, you're writing past the buffer and overwriting (in this case) the rest of your stack past buffer. This is very dangerous behavior.
In general, you should always create methods that supply limits. In most C libraries, these methods are denoted by an n in the method name.
C does not do any run time bounds checking like other languages(C#,Java etc). That is why you can write things past the end of the array. However, you won't be able to access that string in some cases because you might be encroaching upon memory that doesn't belong to you giving you a segementation fault. K&R would be a good book to learn such concepts.
The strcpy() function forgoes memory management entirely, therefore all allocation needs to be done before the function is called, and freed afterward when necessary. If your source string has more characters than the destination buffer, strcpy() will just keep writing past the end of the buffer into unallocated space, or into space that's allocated for something else.
This can be very bad.
strncpy() works similarly to strcpy(), except that it allows you to pass an additional variable describing the size of the buffer, so the function will stop copying when it reaches this limit. This is safer, but still relies on the calling program to allocate and describe the buffer properly -- it can still go past the end of the buffer if you provide the wrong length, leading to the same problems.
char * mycpy (char * destination, char * source) {
if (!destination || !source) return NULL;
char * tmp = destination;
while (*destination != NULL || *source != NULL) {
*destination = *source;
destination++;
source++;
}
return tmp;
}
In the above copy implementation, your tmp and destination are having the same data. Its better your dont retrun any data, and instead let the destination be your out parameter. Can you rewrite the same.
The version below works for me. I'm not sure if it is bad design though:
while(source[i] != '\0' && (i<= (MAXLINE-1)))
{
dest[i]=source[i];
++i;
}
In general it's always a good idea to have const modifier where it's possible, for example for the source parameter.
This is my first time posting here, hopefully I will not make a fool of myself.
I am trying to use a function to allocate memory to a pointer, copy text to the buffer, and then change a character. I keep getting a segfault and have tried looking up the answer, my syntax is probably wrong, I could use some enlightenment.
/* My objective is to pass a buffer to my Copy function, allocate room, and copy text to it. Then I want to modify the text and print it.*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int Copy(char **Buffer, char *Text);
int main()
{
char *Text = malloc(sizeof(char) * 100);
char *Buffer;
strncpy(Text, "1234567890\n", 100);
Copy(&Buffer, Text);
}
int Copy(char **Buffer, char *Text)
{
int count;
count = strlen(Text)+1;
*Buffer = malloc(sizeof(char) * count);
strncpy(*Buffer, Text, 5);
*Buffer[2] = 'A'; /* This results in a segfault. "*Buffer[1] = 'A';" results in no differece in the output. */
printf("%s\n", *Buffer);
}
Your problem is simply one of precedence. The [] operator has higher precendence that unary-*, so the line is parsed as if it was:
*(Buffer[2]) = 'A';
...which is not what you want. You actually want the * to happen first, so you need to use parantheses:
(*Buffer)[2] = 'A';
Additionally, your strncpy() call is wrong. strncpy() does not nul-terminate the result if the number of characters copied is equal to the length; and since your memory comes straight from malloc(), there may not be a nul-terminator there already. strncpy() is actually the wrong tool in 99.99% of the cases that you will encounter - search on this site for numerous other answers explaining why.
A call to strncat() can be used instead:
(*Buffer)[0] = '\0'; /* Truncate to an empty string */
strncat(*Buffer, Text, 5);
*Buffer[2] is getting interpreted as *(Buffer[2]). What you want is (*Buffer)[2].
The problem is that *Buffer[2] means *(Buffer[2]) - you're trying to dereference the wrong pointer. You want to use (*Buffer)[2].