anything wrong with this trim() method in C

Question

this method is for trimming a string in C by deleting spaces from the beginning and end. BTW iam not that good with C and actually facing some issues with dealding with strings

char* trim(char s[])
{
    int i, j;
    int size = strlen(s);

    //index i will point to first char, while j will point to the last char
    for(i = 0; s[i] == ' '; i++);
    for(j = size - 1; s[j] == ' '; j--);

    if(i > 0)
        s[i - 1] = '\0';

    if(j < size - 1)
        s[j + 1] = '\0';

    s = &s[i];

    return s;
}

Answer 1

This loop

for(j = size - 1; s[j] == ' '; j--);

will access an out-of-bounds index when:

• the input string consists entirely of spaces (e.g., " "), in which case there is nothing stopping j from reaching -1 and beyond, or

• the input string is the empty string (""), where j starts at -1.

You need to guard against this in some way

for (j = size - 1; j >= 0 && s[j] == ' '; j--)

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The other thing to consider is that you are both: modifying the contents of the original string buffer, and potentially returning an address that does not match the beginning of the buffer.

This is somewhat awkward, as the user must take care to keep the original value of s, as its memory contains the trimmed string (and might be the base pointer for allocated memory), but can only safely access the trimmed string from the returned pointer.

Some things to consider:

• moving the trimmed string to the start of the original buffer.
• returning a new, dynamically allocated string.

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Some people have warned that you cannot pass a string literal to this function, which is true, but passing a string literal to a non-const argument is a terrible idea, in general.