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###Binary mode###

You should use binary mode to open your file instead of text mode. So change:

FILE* inptr = fopen(infile, "r");

to

FILE* inptr = fopen(infile, "rb");

Otherwise, your CR/LF characters may get converted when you don't want them to be.

###Buffer overrun###

Your filename buffer is too short:

char filename[7];
char * name()
{
 if (filenum < 10)
 sprintf(filename, "00%d.jpg", filenum);
 else
 sprintf(filename, "0%d.jpg", filenum);
 // ...

Here, your filenames will be something like "000.jpg" which require 7 characters plus a terminating null character, or 8 characters in total. Also, if you generate 100 or more files, you will need even more characters.

###Pad string with leading zeros###

There is a better way to generate filenames padded with leading zeros. Instead of:

if (filenum < 10)
 sprintf(filename, "00%d.jpg", filenum);
else
 sprintf(filename, "0%d.jpg", filenum);

you can do this:

sprintf(filename, "%03d.jpg", filenum);

Binary mode

You should use binary mode to open your file instead of text mode. So change:

FILE* inptr = fopen(infile, "r");

to

FILE* inptr = fopen(infile, "rb");

Otherwise, your CR/LF characters may get converted when you don't want them to be.

Buffer overrun

Your filename buffer is too short:

char filename[7];
char * name()
{
 if (filenum < 10)
 sprintf(filename, "00%d.jpg", filenum);
 else
 sprintf(filename, "0%d.jpg", filenum);
 // ...

Here, your filenames will be something like "000.jpg" which require 7 characters plus a terminating null character, or 8 characters in total. Also, if you generate 100 or more files, you will need even more characters.

Pad string with leading zeros

There is a better way to generate filenames padded with leading zeros. Instead of:

if (filenum < 10)
 sprintf(filename, "00%d.jpg", filenum);
else
 sprintf(filename, "0%d.jpg", filenum);

you can do this:

sprintf(filename, "%03d.jpg", filenum);