多个fscanf

Question

I have written the following program that is intended to read a string from a file into variable "title":

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int m, b;
    char *title;
    FILE *fp;

    fp = fopen("input2.txt", "r");
    if (fp == NULL)
    {
         printf ("Error: file cannot be found\n");
         return 1;
    }

    fscanf(fp, "<%d>\n<%d>", &m, &b);
    printf("%d\n%d", m, b);
    fscanf(fp, "<%s>", title);

    fclose(fp);
    return 0;
}

The above program crashes at the second call to fscanf . Why does this happen?

Answer 1

Your main problem is that you've not allocated space for the string to be read into. You can do this in multiple ways:

char title[256];

or:

char *title = malloc(256);
if (title == NULL)
{
    fprintf(stderr, "Out of memory\n");
    exit(1);
}

either of which should then be used with:

if (fscanf(fp, " <%255[^>]>", title) != 1)
{
    fprintf(stderr, "Oops: format error\n");
    exit(1);
}

or, if you have a system with an implementation of fscanf() that's compliant with POSIX 2008, you can use the m modifier to %s (or with %c , or, in this case, a scanset %[...] — more on that below):

char *title = 0;

if (fscanf(fp, " <%m[^>]>", &title) != 1)  // Note the crucial &
{
    fprintf(stderr, "Oops: format error\n");
    exit(1);
}

This way, if the fscanf() succeeds in its entirety, the function will allocate the memory for the title. If it fails, the memory will have been released (or never assigned).

Note that I changed %s to %m[^>] . This is necessary because the original conversions will never match the > . If there is a > in the input, it will be incorporated into the result string because that reads up to white space, and > is not white space. Further, you won't be able to tell whether the trailing context was ever matched — that's the > in the original format, and it's still a problem (or not) in the revised code I'm suggesting.

I also added a space at the start of the string to match optional white space. Without that, the < at the start of the string must be on the same line as the > after the second number, assuming that the > is present at all. You should also check the return from the first fscanf() :

if (fscanf(fp, "<%d>\n<%d>", &m, &b) != 2)
{
    fprintf(stderr, "Oops: format error\n");
    exit(1);
}

Note that the embedded newline simply looks for white space between the > and the < — that's zero or more blanks, tabs or newlines. Also note that you'll never know whether the second > was matched or not.

You could use exit(EXIT_FAILURE); in place of exit(1); — or, since this code is in main() , you could use either return 1; or return(EXIT_FAILURE); where the parentheses are optional in either case but their presence evokes unwarranted ire in some people.

You could also improve the error messages. And you should consider using fgets() or POSIX's getline() followed by sscanf() because it makes it easier (by far) to do good error reporting, plus you can rescan the data easily if the first attempt at converting it fails.

Answer 2

This:

    char *title;

is just a pointer to a char. If fscanf writes more than one character to it, you will corrupt whatever happens to be in memory after

You need to do one of two things:

    char title[50]; // Holds up to 49 characters, plus termination

Or:

    #include <stdlib.h>
    // ...
    char *title = malloc(50 * sizeof(char)); // Same capacity as above
    if (title == NULL) {
        // handle out of mem error
    }
    // ...
    free (title);

The first option is obviously much simpler, but requires you to know your array size at compile time.

If you are new to programming, and haven't encountered pointers and dynamic memory allocation yet, stick with the first option for now.