C编程fwrite跳转到文件末尾

Question

I'm writing a C module and I'm running into an interesting problem I never seen before.

// Many other operations before this point
fseek(samples_file, 0, SEEK_SET);
printf("ftell A1 %llu\n", ftell(samples_file));
count = fwrite(channel_buffer+chan_type.size*set_index, 1, chan_type.size, samples_file);
printf("count %llu\n", count);
printf("ftell A2 %llu\n", ftell(samples_file));
// Many more operations to come after this point

When I run the module, I get a printout like the following:

ftell A1 0
count 8
ftell A2 6018

I have set the file pointer to the very start of the file. When I write some data, it should write the data out at the position I seek'd to and then increment the file position with the number of bytes written (in this case, 8). However, when I do an ftell, it seems that the position suddenly jumped to 6018 (which happens to be the original size of the file plus 8).

Why does this occur and how do I prevent this behavior?

Answer 1

It sounds like the file has been opened in append mode. Check that there's no "a" in the second argument to fopen() .