Removing first word in a string in C

Question

Hello I am trying to remove the first word in a string. For example: buffer = "Hello my name is Code" After implementing removeToken() , buffer should now be "my name is Code" .

The code I have currently is giving me a segmentation fault.

I have the following:

char *buffer = NULL;
char *buffercopy = NULL;

void startBuffer(char *inputLine) {
    int length = strlen(inputLine);
    buffer = (char *)malloc(length);
    buffercopy = (char *)malloc(length);
    strcpy(buffer, inputLine);
    strcpy(buffercopy, inputLine);
}

char *removeToken() {
    /* removes the first token from the buffer, buffer is reduced in size */
    char *token;
    char delimiters[2] = " ,";
    int origLen = strlen(buffer);
    token = strtok(buffer, delimiters);
    printf("%s \n", token);
    int p = strlen(token);
    int i = 0;
    while (buffer[i] != '\0') {
        buffer[i] = buffer[i + p];
        i++;
    }
    return buffer;
}

Answer 1

There are multiple problems in your code:

you do not allocate enough memory to copy the strings, you must allocate one extra byte for the null terminator:

void startBuffer(char *inputLine) {
    int length = strlen(inputLine);
    buffer = (char *)malloc(length + 1);
    buffercopy = (char *)malloc(length + 1);
    strcpy(buffer, inputLine);
    strcpy(buffercopy, inputLine);
}

Note that the above code can be simplified to this:

void startBuffer(const char *inputLine) {
    buffer = strdup(inputline);
    buffercopy = strdup(inputline);
}

In removeToken , you allocate the separator string with:

char delimiters[2] = " ,";

This is incorrect as you need a third byte for the '\\0' null terminator. You can write this instead:

char delimiters[] = " ,";

You only count the length of the initial string, not the separators, the first of which has been changed to a '\\0' .

The copy loop therefore stops immediately.

You also forget to copy the final '\\0' in the copy loop.

Here is a corrected version:

char *removeToken(char *buffer) {
    const char *delimiters = " ,";
    size_t i, skip;

    skip = strcspn(buffer, delimiters); /* skip the word */
    skip += strspn(buffer + skip, delimiters); /* skip the delimiters */

    for (i = 0; buffer[skip + i] != '\0'; i++) {
        buffer[i] = buffer[skip + i];
    }
    buffer[i] = '\0';

    return buffer;
}