Writing and Reading unit32_t values using htons and ntohs

This is a discussion on Writing and Reading unit32_t values using htons and ntohs within the C Programming forums, part of the General Programming Boards category; Hey there, I'm having an issue with writing uint32_t values to file and then reading them back. Code: //My code ...

  1. #1
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    Writing and Reading unit32_t values using htons and ntohs

    Hey there,

    I'm having an issue with writing uint32_t values to file and then reading them back.

    Code:
    //My code to write the value is as follows,
    
    //eklen = 128
    
    FILE* pFilOutput = open_memstream(pChrOutputBuffer, &pSizTotalSize);
    uint32_t eklen_n = htonl(eklen);
    if (fwrite(&eklen_n, sizeof(uint32_t), 1, pFilOutput) != 1) {
       //Failed to write data
    }
    
    //My code to read the value back again is as follows,
    
    FILE* pFilInput = fmemopen(pChrData, strlen(pChrData), "r");
    uint32_t eklen_n;
    fread(&eklen_n, sizeof(eklen_n), 1, pFilInput);
    int eklen = ntohl(eklen_n);
    Wrote = -2147483648
    Read 1344601458

    ---

    I'm guessing I'm making a school boy error here but I just can't see it. I thought maybe I needed to specify "b" in the fmemopen mode parameter but that didn't help and on further reading I assume it's binary by default anyway.

    Any ideas what I'm doing wrong? Many thanks in advance for any help!

    Nick.

  2. #2
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    Are you checking your fread() call for success?

    This doesn't look like a problem with htonl/ntohl, since the values aren't the byte-reversed values of one another.

  3. #3
    spurious conceit MK27's Avatar
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    Yeah, the problem is not your use of htonl, ntohl, fread, or fwrite. Have you tried this with a straight sequence of bytes to check if fmemopen and open_memstream are doing what you think they are?
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  4. #4
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    Quote Originally Posted by JohnGraham View Post
    Are you checking your fread() call for success?

    This doesn't look like a problem with htonl/ntohl, since the values aren't the byte-reversed values of one another.
    Hey John, yup, I've added checks for that and it's returning without error.

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