I have a char buf[x], int s and void* data.
I want to write a string of size s into data from buf.
How can I accomplish it?
Thanks in advance.
5 Answers
Assuming that
- by “string” you mean a null-terminated string as is normally meant in C;
- you haven't yet allocated memory in
data; - you already know that s <= x
First you need to allocate memory in data. Don't forget the room for the 0 byte at the end of the string.
data = malloc(s+1);
if (data == NULL) {
... /*out-of-memory handler*/
}
Assuming malloc succeeds, you can now copy the bytes.
EDIT:
The best function for the job, as pointed out by caf, is strncat. (It's fully portable, being part of C89.) It appends to the destination string, so arrange for the destination to be an empty string beforehand:
*(char*)data = 0;
strncat(data, buf, s);
Other inferior possibilities, kept here to serve as examples of related functions:
If you have
strlcpy(which is not standard C but is common on modern Unix systems; there are public domain implementations floating around):strlcpy(data, buf, s+1);If you know that there are at least
scharacters in the source string, you can usememcpy:memcpy(data, buf, s);((char*)data)[s+1] = 0;
Otherwise you can compute the length of the source string first:
size_t bytes_to_copy = strlen(buf); if (bytes_to_copy > s) bytes_to_copy = s; memcpy(data, buf, bytes_to_copy); ((char*)data)[s+1] = 0;Or you can use
strncpy, though it's inefficient if the actual length of the source string is much smaller thans:strncpy(data, buf, s); ((char*)data)[s+1] = 0;
answered Jul 18, 2010 at 16:18
Gilles 'SO- stop being evil'
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2 Comments
caf
Your
memcpy() implementations have bugs - they both place the nul terminator incorrectly, and they try to dereference a void *, which you can't do. The first should use ((char *)data)[s] = 0, and the second should use ((char *)data)[bytes_to_copy] = 0;. For the last one, instead of strncpy() you can use strncat(): ((char *)data)[0] = 0; strncat(data, buf, s);Gilles 'SO- stop being evil'
@caf: you're right on both counts. Unfortunately my answer has been accepted, so I've edited it rather than delete it because I don't want to leave a wrong answer too visible. But if you write an strncat answer, I'll upvote it.
If data is not allocated:
char buf[] = "mybuffer";
void *data = malloc(strlen(buf)+1);
strcpy((char*)data,buf);
Actually if data is really to be defined you can also do
char buf[] = "mybuffer";
void *data= (void*)strdup(buf);
3 Comments
sje397
You should
malloc(strlen(buf)+1)Gilles 'SO- stop being evil'
Yes, and here
buf is null-terminated even though it isn't a string literal. But strlen(buf) doesn't include the terminator byte in its count.
R.. GitHub STOP HELPING ICE
You could use
sizeof buf instead of strlen(buf)+1 here since buf is an array.memcpy(data, buf, s);
This assumes that you have enough space in data (and in buf).
Depending on what you are doing (you don't say, but you do say that you are copying strings), you may want to add a null at the end of your newly copied string if you did not copy a null already in buff, and you are going to use data in a function that expects strings.
Comments
data = malloc(s);
strcpy((char*)data,buf);
free(data);
Comments
int n = MIN((x - 1), s);
char *bp = buf;
char *dp = (char *)data;
while (n--) {
*bp++ = *dp++;
}
*dp = '\0';
Comments
bufa null-delimited string or is its size defined bys? Are you trying to copy just a part of the string? What isx? Does the string copied todataneed to be null-delimited? What have you tried so far?