concat const char * with char *


 
Thread Tools Search this Thread
Top Forums Programming concat const char * with char *
# 1  
Old 02-16-2009
concat const char * with char *

hello everybody!
i have aproblem! i dont know how to concatenate const char* with char
const char *buffer;
char *b;
sprintf(b,"result.txt");
strcat(buffer,b);

thanx in advance
# 2  
Old 02-16-2009
You cannot strcpy( const char *buffer, char *src)

This is because you told the compiler with "const" that you promised not to alter buffer.
You will have to create a temorary char * variable then sprintf buffer + "results.txt" into the temporary char variable.

post your code and we can help.
# 3  
Old 02-17-2009
Hi,

const char *buffer does not mean that buffer is a constant. It only says buffer points to a constant char. To declare buffer as a constant, you will have give,

char * const buffer;

Here in your code, you cannot give

char *b
sprintf(b,"result.txt");

Either you have to allocate memory to b and then use sprintf or strcpy or you can use,

b="result.txt";

To append something to buffer, you dont want to concatinate the new string with buffer, you may append the string to the array buffer points to. You cannot give strcat(buffer,b) as strcat does not allow const char * as its first argument.

You may try this

char b[1024]={"result.txt"}; //You may try dynamic allocation also
const char *buffer=b;

cout<<"\n Value of buffer 1 : "<<buffer<<endl; //It will be result.txt

strcat(b,",result1.txt");

cout<<"\n Value of buffer 2 : "<<buffer<<endl; //It will be result.txt,result1.txt

.
# 4  
Old 02-17-2009
I get this with a C99-compliant compiler:
Code:
csaprd:/home/jmcnama> cc t.c
cc: "t.c", line 6: error 1549: Modifiable lvalue required for assignment operator.
cc: "t.c", line 7: warning 611: Type conversion loses "const" qualifier.
cc: "t.c", line 7: warning 563: Argument #1 is not the correct type.

on this code:
Code:
#include <string.h>
#include <stdlib.h>

const char *mystrcpy(const char *dest,const char *source, const size_t len)
{
    *dest=0x0;
	memcpy(dest, source, len);
	return dest;
}

int main()
{
    char dest[10]={0x0};    
    const char *p=mystrcpy(dest, "test", 4);
    printf("%s\n", dest);
    return 0;
}

const char *foo means it cannot be modified (the "modifiable lvalue" complaint), for example. ujeshm may use a non-standard compiler or a really old one that allows it.

You can work around it by copying a const char into a regular string then modifying the new string.

Note: A warning means the code did not compile correctly, and if it runs you are then coding by coincidence and will sooner or later cause serious problems that are difficult to resolve.

If you are using gcc, compile
Code:
gcc -Wall mycode.c

to see full errors and warnings.
# 5  
Old 02-18-2009
"const char *foo means it cannot be modified" -> This is not completly correct. 'const char *foo' means foo is a pointer which points to a constant char (pointer to a constant variable, not a constant pointer variable). That means, the address in foo can be changed(foo is not a constant), but we cannot modify the value contained in the address held by foo(the value is considered as constant).

For example

Code:
const char *foo;

foo="Test1";

cout<<"\n foo 1 : "<<foo<<endl;  // Result will be 'Test1'

foo="Foo is changed now."       

cout<<"\n foo 2 : "<<foo<<endl;  // Result will be 'Foo is changed now'

In your code,

you got error in line 6 because, you are trying to change the value contained in the address held by 'dest'. That is if you remove the '*' it wont generate any errors. But the meaning will be different.

you got error in 7 because the dest is a const char * variable. memcpy does not allow const char * as its first argument (memcpy(void *,const void *,size_t))

The work around you are suggesting is similar to what I have given in my previous post. Only difference is you are asking to assign the address of the const char * variable to a char * variable and change the content of the char * variable. I declared the char * variable first and assigned its address to the const char * variable. After that I am also doing the same thing.

I think my previous post created some miss understandings in you. Sorry if it is so.

Last edited by ujeshm; 02-18-2009 at 02:15 PM..
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. Programming

Invalid conversion from char* to char

Pointers are seeming to get the best of me and I get that error in my program. Here is the code #include <stdio.h> #include <stdlib.h> #include <string.h> #define REPORTHEADING1 " Employee Pay Hours Gross Tax Net\n" #define REPORTHEADING2 " Name ... (1 Reply)
Discussion started by: Plum
1 Replies

2. Programming

Double to const char conversion

Dear all, I am using C and ROOT for programming. And I need to incorporate following in my code. char *fps=NULL; int dec=0,sign=0; float mean = h1->GetMean(1); //0.001298 fps= fcvt(mean,6 , &dec, &sign); I need to provide this mean as const char to some other function to get... (8 Replies)
Discussion started by: emily
8 Replies

3. Programming

Using const char*

I am writing some code in C++ to print a message using fprintf Here is an example void pr_desc( FILE* stream, int shift, const char* desc) { const char* format="%*s\e; fprintf(stream,format,shift,"",desc); } I call it using const char* desc; ... (4 Replies)
Discussion started by: kristinu
4 Replies

4. Programming

error: invalid conversion from ‘const char*’ to ‘char*’

Compiling xpp (The X Printing Panel) on SL6 (RHEL6 essentially): xpp.cxx: In constructor ‘printFiles::printFiles(int, char**, int&)’: xpp.cxx:200: error: invalid conversion from ‘const char*’ to ‘char*’ The same error with all c++ constructors - gcc 4.4.4. If anyone can throw any light on... (8 Replies)
Discussion started by: GSO
8 Replies

5. Shell Programming and Scripting

how first char in odd line and second char in even line

Hi I m having ifconfig -a o/p like sbanlab1:ksh# ifconfig -a | egrep "flags|inet" | awk -F' ' '{print $1,$2}' lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> inet 127.0.0.1 lo0:1: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> inet 127.0.0.1 bge0:... (1 Reply)
Discussion started by: tarunn.dubeyy
1 Replies

6. Programming

Adding a single char to a char pointer.

Hello, I'm trying to write a method which will return the extension of a file given the file's name, e.g. test.txt should return txt. I'm using C so am limited to char pointers and arrays. Here is the code as I have it: char* getext(char *file) { char *extension; int i, j;... (5 Replies)
Discussion started by: pallak7
5 Replies

7. Shell Programming and Scripting

How to replace any char with newline char.

Hi, How to replace any character in a file with a newline character using sed .. Ex: To replace ',' with newline Input: abcd,efgh,ijkl,mnop Output: abcd efgh ijkl mnop Thnx in advance. Regards, Sasidhar (5 Replies)
Discussion started by: mightysam
5 Replies

8. UNIX for Dummies Questions & Answers

int open(const char *pathname, int flags, mode_t mode) doubt...

hello everybody! I want to create a file with permissions for read, write, and execute to everybody using C, so I write this code: #include <stdio.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> int main(){ int fileDescriptor; fileDescriptor =... (2 Replies)
Discussion started by: csnmgeek
2 Replies

9. Programming

char *p and char p[].

Can anyone please explain me the difference between char *p and char p ? Thanks in Advance, Arun. (4 Replies)
Discussion started by: arunviswanath
4 Replies

10. Programming

diff b/w char const in C/C++

hi, what is the difference b/w char in C and C++.and give me the examples. Thanks... sarwan (3 Replies)
Discussion started by: sarwan
3 Replies
Login or Register to Ask a Question

Featured Tech Videos