Note: This is the first time I've written anything whatsoever in straight C. In other words:I have no idea what I'm doing.

I recently had a task that involved temporarily relaying responses from one server, through a web-facing server, and then on to the client. Along the way, the relay server had to dynamically add an HTTP header of its own to the relayed response. Came up with a quick and dirty scripted solution, but it worked for the duration it had to.

Still, I thought it might be a good exercise for getting to know a little C, just for fun, since something like this would benefit from being written closer to the metal.

So below is a program (I call itheadshape for lack of a better name) where you pipe in (say, from a curl command) an HTTP response including headers, and the program will add/remove/modify one of those headers, and pass on the rest on as-is. Given no arguments, stdin will just pass to stdout.

E.g.

$ curl -i example.com | headshape              # pass through$ curl -i example.com | headshape Server       # remove the Server header$ curl -i example.com | headshape Via 'foobar' # add/modify the Via header

So it does a bit of HTTP parsing to find the right header - or add it if it's not there - and to figure out what the response code is. Only responses with a 2xx status will be altered, 1xx responses will be ignored, and 3xx and above will cause it to just forward all remaining data unaltered (this partly an arbitrary choice; it's just a code exercise after all). It also defaults to forwarding everything as-is if it sees something it can't make sense of.

Below is the code. Much to my surprise it seems to work exactly as intended but, again,I've never written C code, so it's no doubt horrible. Any and all tips are welcome.

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <unistd.h>#define CRLF "\r\n"            // HTTP line ending#define BLOCK_SIZE 1024        // Any reason to make this larger/smaller?#define HEADER_DELIM ": "      // Used to spot header lines#define MAX_HEADER_LENGTH 100  // max length of a header *name*; not the whole header linestatic int   parseStatus(char* line);static char* getHeaderName(char* line);static int   isDelimited(char* line, int continuation);static int   isBoundary(char* line);void         writeLine(char* line);void         passthru();int main(int argc, char const *argv[]) {    // states    int should_parse = 0;    int effective_status = 0;    // options/    char* target_header = NULL;    char* payload_line  = NULL;    // loop variables    int   line_continued = 0;    int   current_status = 0;    char* current_header = NULL;    char* line_out       = NULL;    char  line[BLOCK_SIZE];    // exit with >0 if nothing's being piped    if(isatty(fileno(stdin))) {        return 1; // TODO: is there a better code for this?    }    // Get the target header argument, if any    if(argc > 1) {        should_parse = 1; // could just check target_header, but this seems more descriptive        target_header = (char*) argv[1];    }    // Get the target value argument, if any    if(argc > 2) {        // build the payload header line (i.e. "Header: value")        size_t payload_length = 1;        payload_length += strlen(target_header);        payload_length += strlen(HEADER_DELIM);        payload_length += strlen(argv[2]);        payload_length += strlen(CRLF);        payload_line = (char *) malloc(payload_length);        strcpy(payload_line, target_header);        strcat(payload_line, HEADER_DELIM);        strcat(payload_line, argv[2]);        strcat(payload_line, CRLF);    }    // heading parsing loop    while(should_parse && fgets(line, BLOCK_SIZE, stdin)) {        // if previous fgets didn't get a complete, CRLF-delimited line        // (i.e. a line exceeded BLOCK_SIZE), assume the current chunk        // is the continuation, and output it without parsing        if(line_continued) {            writeLine(line_out);            line_continued = !isDelimited(line, line_continued);            continue;        }        // check if this is a "complete" CRLF-delimited line        line_continued = !isDelimited(line, line_continued);        // reference our input for later        line_out = line;        if((current_status = parseStatus(line))) {            // set the status code            effective_status = current_status;            // we're not interested in rewriting 3xx and higher responses            if(effective_status >= 300) {                // pass the remaining data directly to stdout                should_parse = 0;            }        } else if((current_header = getHeaderName(line))) {            // check if this is the header we're looking for            if(strcmp(current_header, target_header) == 0) {                // replace or remove the header                line_out = payload_line;                // pass the remaining data directly to stdout                should_parse = 0;             }        } else if(isBoundary(line)) {            // We've reached the end of the HTTP header section.            // If effective_status is 1xx, parsing continues but            // 2xx will stop the parsing and insert the header            // if need be            if(effective_status >= 200) {                // add the header, if this is a 2xx response                if(payload_line && effective_status < 300) {                    writeLine(payload_line);                }                // pass the remaining data directly to stdout                should_parse = 0;            }        } else {            // stop parsing if the line isn't recognized and            // pass the remaining data directly to stdout            should_parse = 0;        }        // output the line (unless it's NULL)        if(line_out) writeLine(line_out);    }    // pass the remaining data directly to stdout    passthru();    // free the payload line, if necessary    if(payload_line) free(payload_line);    return 0;}// Gets the header name in the line, if any.static char* getHeaderName(char* line) {    static char header[MAX_HEADER_LENGTH];    char* delim = strstr(line, HEADER_DELIM);    if(delim) {        size_t count = delim - line;        count = count >= MAX_HEADER_LENGTH ? MAX_HEADER_LENGTH - 1 : count;        strncpy(header, line, count);        header[count] = '\0';        return header;    }    return NULL;}// Match a line like "HTTP/1.x xxx..." and extract the status code// Not exactly regex-like precision here, though. Maybe use strtok()?static int parseStatus(char* line) {    size_t code_length = 4;    char* http  = strstr(line, "HTTP/1.");    char* delim = strchr(line, ' ');    size_t remaining = strlen(line) - (delim - line) + 1;    if(http && delim && remaining > code_length) {        char code[remaining];        strncpy(code, delim, remaining);        return atoi(code);    }    return 0;}// Check whether a string ends with CRLF// Note: This function keeps the last char of the line from its previous// invocation in memory in order to spot the 2-character CRLF even if it's// been split into separate fgets lines.static int isDelimited(char* line, int continuation) {    static char prev = 0;    char tail[3] = {'\0', '\0', '\0'};    int offset = strlen(line) - 2;    if(offset >= 0) {        // grab last 2 chars        tail[0] = line[offset];        tail[1] = line[offset + 1];    } else if(continuation && offset == -1) {        // grab char carried over from earlier, and the line's single char        tail[0] = prev;        tail[1] = line[0];    }    prev = tail[1];    return !strcmp(tail, CRLF);}// is this line a "blank" CRLF line?static int isBoundary(char* line) {    return strlen(line) == strlen(CRLF) && strcmp(line, CRLF) == 0;}// write to stdout & flushvoid writeLine(char* line) {    fwrite(line, sizeof(char), strlen(line), stdout);    fflush(stdout);}// pass stdin through to stdoutvoid passthru() {    char buffer[BLOCK_SIZE];    while(1) {        size_t bytes = fread(buffer, sizeof(char), BLOCK_SIZE, stdin);        fwrite(buffer, sizeof(char), bytes, stdout);        fflush(stdout);        if(bytes < BLOCK_SIZE && feof(stdin)) break;    }}

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