不兼容的指针类型，将“ char（*）[128]”传递给类型为“ char **”的参数

Question

I can't figure out why does the following code generates this error:

incompatible pointer types passing 'char (*)[128]' to parameter of type 'char **'

int main(int argc, char *argv) 
{
    char line[128];
    size_t n;

    FILE *fp = fopen(argv[1], "r");
    if (NULL == fp)
    {
        log_error("%d. %s", errno, strerror(errno));
        exit(EXIT_FAILURE);
    }

    while(-1 != getline(&line, &n, fp))
    {
        // do something
    }

    return 0;
}

The error is generated by the following line -1 != getline(&line, &n, fp) Here is the prototype for getline,

ssize_t getline(char **lineptr, size_t *n, FILE *stream);

What am I doing wrong?

Answer 1

getline will allocate a buffer for you (that you should free when you are finished with it). Instead of passing it a pointer to a statically allocated buffer, just pass it a pointer to a char * . If the pointer is NULL, it'll allocate a new buffer and point you to it. Changing char line[128]; to char *line = NULL; should do the trick; just remember to free it when you're done with it.

From the man page:

If *lineptr is NULL, then getline() will allocate a buffer for storing the line, which should be freed by the user program. (In this case, the value in *n is ignored.)

Alternatively, before calling getline(), *lineptr can contain a pointer to a malloc(3)-allocated buffer *n bytes in size. If the buffer is not large enough to hold the line, getline() resizes it with realloc(3), updating *lineptr and *n as necessary.

In either case, on a successful call, *lineptr and *n will be updated to reflect the buffer address and allocated size respectively.

How it might look in your example main function:

int main(int argc, char **argv) 
{
    char *line = NULL;
    size_t n;

    FILE *fp = fopen(argv[1], "r");
    if (NULL == fp)
    {
        log_error("%d. %s", errno, strerror(errno));
        exit(EXIT_FAILURE);
    }

    while(-1 != getline(&line, &n, fp))
    {
        // do something
    }
    free(line);
    return 0;
}