将行读入动态分配的char指针数组时出现段错误

Question

In C, I am trying to implement a function that uses getline() to read all the lines from a file. It is implemented similarly to getline() , specifically the fact that it is using realloc() to resize a char** if there is not enough memory allocated to store the next pointer to a line. Unfortunately I am getting seg faults during the string dupilcation process.

After a little poking around, I discovered that the segfault happens during the second iteration while attempting to store the second line in the char pointer array.

ssize_t fgetlines(char*** linesptr, size_t* n, FILE* fp)
{
    char* line = NULL;
    size_t sz_line = 0;
    size_t cur_len = 0;
    size_t needed;

    if (linesptr == NULL || n == NULL) {
        errno = EINVAL;
        return -1;
    }

    if (*linesptr == NULL) {
        if (*n == 0)
            *n = sizeof(**linesptr) * 30; /* assume 30 lines */
        *linesptr = malloc(*n);
        if (*linesptr == NULL) {
            *n = 0;
            return -1;
        }
    }

    while (getline(&line, &sz_line, fp) > 0) {
        needed = (cur_len + 1) * sizeof(**linesptr);
        while (needed > *n) {
            char** new_linesptr;
            *n *= 2;
            new_linesptr = realloc(*linesptr, *n);
            if (new_linesptr == NULL) {
                *n /= 2;
                free(line);
                return -1;
            }
            *linesptr = new_linesptr;
        }
        *linesptr[cur_len] = strdup(line);
        printf("%s", *linesptr[cur_len]);
        if (*linesptr[cur_len] == NULL) {
            free(line);
            free(*linesptr);
            return -1;
        }
        ++cur_len;
    }

    free(line);
    return cur_len;
}

And I call the function like so:

    char **settings = NULL;
    size_t sz_settings = sizeof(*settings) * 6;
    int count = fgetlines(&settings, &sz_settings, f_cfg);

Due to the function not being able to successfully complete I do not get any output. But after printing back the string after strdup() I managed to get one line of f_cfg , "Hello World" before a seg fault.

Answer 1

As chux pointed out, the intended precedence here was incorrect. References to *linesptr[cur_len] must be changed to (*linesptr[cur_len]) . Also the code hole *n == 0 and *n *= 2 has been fixed.

Answer 2

Should change

*linesptr[cur_len]　 =>  (*linesptr)[cur_len]

The modified function is as follows:

ssize_t fgetlines(char *** linesptr, size_t *n, FILE *fp)
{
    char *line = NULL;
    size_t sz_line = 0;
    size_t cur_len = 0;
    size_t needed;

    if (linesptr == NULL || n == NULL) {
        errno = EINVAL;
        return -1;
    }

    if (*linesptr == NULL) {
        if (*n == 0)
            *n = sizeof(**linesptr) * 30; /* assume 30 lines */
        *linesptr = malloc(*n);
        if (*linesptr == NULL) {
            *n = 0;
            return -1;
        }
    }

    while (getline(&line, &sz_line, fp) > 0) {
        needed = (cur_len + 1) * sizeof(**linesptr);
        while (needed > *n) {
            char **new_linesptr;
            *n *= 2;
            new_linesptr = realloc(*linesptr, *n);
            if (new_linesptr == NULL) {
                *n /= 2;
                free(line);
                return -1;　// Possible memory leak
            }
            *linesptr = new_linesptr;
        }
        (*linesptr)[cur_len] = strdup(line);
        printf("%s", (*linesptr)[cur_len]);
        if ((*linesptr)[cur_len] == NULL) {
            free(line);
            free(*linesptr);
            return -1;  // Possible memory leak
        }
        ++cur_len;
    }

    free(line);
    return cur_len;
}

In addition, when your memory allocation fails, the memory of "strdup" is not free, which will lead to memory leak.