将子字符串复制到C中的字符串开头

Question

I am trying to remove the whitespace at the start of a string, I have the index of the first non whitespace character, so I tried to do this:

int firstNonWhitespace = ...;
char *line = ...;
char *realStart = line + firstNonWhiteSpace;
strcpy(line, realStart);

but got Abort Trap 6 when at runtime.

However it works if I copy the realStart string to a temporary string, and then copy the temporary string to line:

int firstNonWhitespace = ...;
char *line = ...;
char *realStart = line + firstNonWhiteSpace;
char *tstring = malloc(strlen(realStart) + 1);
strcpy(tstring, realStart);
strncpy(line, tstring, strlen(line));
free(tstring);

Answer 1

There are two problems with your code.

1. The source and destination in the call to strcpy() do overlap, which results in Undefined Behaviour.

2. It might well be the case that realStart points to some non-writeable area of memory.

Answer 2

The faster way is

line += firstNonWhiteSpace;

but that might have consequences for your memory management, in case that part of memory was dynamically allocated. Only do this if you know what you are doing.

Answer 3

int main()
{
    char a[] = "        hey";
    int i = 0;
    char *p = a;
    while(a[i++] == ' ');

    strcpy(p, p + i - 1);
    printf("%s\n", a);
}

Answer 4

Your problem is likely that you are not allowed to modify string literals, ie the code

int main() {
    int firstNonWhitespace = 3;
    char *line = "   foo";
    char *realStart = line + firstNonWhiteSpace;
    strcpy(line, realStart);
}

may or may not work depending on whether your platform protects against modifying the string literal " foo" . Copying the string first is required by the language standard.

Also, since strcpy() is not guaranteed to work correctly on overlapping strings (you might get lucky, though), use memmove() to do the moving.