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Editorial Team
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Asked: 2026-05-15T15:45:07+00:00

Which is an error-free way to get a http POST in CGI in C?

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Which is an error-free way to get a http POST in CGI in C?

I’m currently using this. Is it ok? What if “CONTENT_LENGTH” is incorrect and is larger than stdin is; how will read behave then?


char *STDIN = NULL;
char *pointer = getenv("CONTENT_LENGTH");
if(pointer != NULL)
{
    char charlength[5] = "";
    strncat(charlength, pointer, 4); // limit length to a reasonable number
    unsigned long int length = strtoul(charlength, NULL, 0);
    STDIN = malloc(length + 2);
    ssize_t readbytes = read(0, STDIN, length);
    STDIN[readbytes] = '&'; // for use later on in my program
    memset(STDIN + readbytes + 1, (int)'\0', (length + 2) - (size_t)(readbytes + 1));
}

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1 Answer

  1. Editorial Team

    read(2) will return no more bytes than you asked for. This might leave more data in the stdin file descriptor for reading (in case the client sends a CONTENT_LENGTH of 0 but hands you their /dev/urandom) but that’s okay. Your process is free to go away without reading it all.

    read(2) may return fewer bytes than you ask for. This could be because not all the data has arrived yet and the kernel is tired of blocking, or it could be that the content is smaller than the CONTENT_LENGTH. I’m glad you’re limiting the length to something ‘reasonable’, as it’d be pretty easy to pass in a CONTENT_LENGTH that is the maximum size_t value, or that value minus one, or that value minus two, and play games with your malloc() allocating 0, 1, or 2 bytes, and let you happily scribble all over your memory.

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