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Editorial Team
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Asked: 2026-05-29T03:56:13+00:00

In C or C++, the following can be used to return a file size:

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In C or C++, the following can be used to return a file size:

const unsigned long long at_beg = (unsigned long long) ftell(filePtr);
fseek(filePtr, 0, SEEK_END);
const unsigned long long at_end = (unsigned long long) ftell(filePtr);
const unsigned long long length_in_bytes = at_end - at_beg;
fprintf(stdout, "file size: %llu\n", length_in_bytes);

Are there development environments, compilers, or OSes which can return the wrong file size from this code, based on padding or other information that is situation-specific? Were there changes in the C or C++ specification around 1999, which would have lead to this code no longer working in certain cases?

For this question, please assume I am adding large file support by compiling with the flags -D_FILE_OFFSET_BITS=64 -D_LARGEFILE64_SOURCE=1. Thanks.

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1 Answer

  1. Editorial Team

    It won’t work on unseekable files like /proc/cpuinfo or /dev/stdin or /dev/tty, or pipe files gotten with popen

    And it won’t work if that file is written by another process at the same time.

    Using the Posix stat function is probably more efficient and more reliable. Of course, this function might not be available on non Posix systems.

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