在C printf格式化输出中需要帮助

Question

I am displaying a partition table, and the table is displayed somewhat like:

Number     Device name       Partition type      Size in MB
------------------------------------------------------------
1           /dev/sda1         NTFS                    300
2           /dev/sda2         *Win95 FAT32             99
3           /dev/sda3         Unknown                 128
4           /dev/sda4         NTFS                  19472
120         /dev/sda120       NTFS                   3000

*=Active partition

Now for displaying the above, we are using formatted output printf and the format string is

"%-6d=partition number    %-25.25s=device name  %c=active partition    %-30.30s=part type          %7Ld=size"

Now i want to display the same partition table, but with some slight modification, such that the gaps in partition slots would be displayed by a range, like:

5-119     /dev/sda5.../dev/sda119    Empty          0

I am using the formatted string as:

%d-%-6d=partition range   %s%d...%s%d=(/dev/sda5.../dev/sda119)   %c  %-30.30s  %7Ld

but it does not help me.

What should be the correct format string? I am using a gcc compiler.

Answer 1

I think you need to use snprintf() to prepare the two composite strings, and then a simpler printf() to do the actual printing. Since you've not shown your actual code, we have to guess at everything, which is a nuisance...

int   min = 5;
int   max = 119;
char *dev = "/dev/sda";

char  num_range[32];
char  dev_range[60];

snprintf(num_range, sizeof(num_range), "%d-%d", min, max);
snprintf(dev_range, sizeof(dev_range), "%s%d...%s%d", dev, min, dev, max);

printf("%-10s   %-50.50s   %c%-30.30s  %7d", num_range, dev_range, ' ', "Empty", 0);

You specified %-25.25s for a single device, so it isn't clear whether you should double that for the range, or you should use some other value (or even the same value); you'll need to tweak that part of the format string to suit yourself. This technique is also how I get a colon at the end of a name — format the name and the colon into a string, and then format that string into the final print operation.