Toy implementation of netstrings

netstrings.c

#include <stdio.h>
#include <ctype.h>

#define MAX_BYTES 1 << 24

int
read_netstring(char * buffer, size_t limit, FILE * f){
    size_t msgSize = 0;
    size_t readSize = 0;
    int c = 0;
    
    /* read header (and at most MAX_BYTES - 1 bytes later) */
    while ((c = fgetc(f)) != EOF && msgSize < MAX_BYTES){
        /* We've got a malformed message and this channel can no longer be
           trusted, so we just discard everything. */        
        if (!isdigit(c) && c != ':'){
            return EOF;
        }
        msgSize += (char)c - '0';
    }
    
    limit = limit < MAX_BYTES? limit : MAX_BYTES;
    
    if (msgSize > limit){
        return EOF;
    }
    
    /* consume the rest of the message */
    readSize = (int)fread((void *) buffer, sizeof(char), msgSize, f);
    if (readSize < msgSize){
        return EOF;
    }
    /* consume the string indicator (optional) */
    c = fgetc(f);
    if (c != ',' && c != EOF){
        ungetc(c, f);
    }
    return readSize;
}

int
write_netstring(char * buffer, size_t limit, FILE * f){
    int header = 0;
    int body = 0;
    if (limit > MAX_BYTES - 1){
        return EOF;
    }
    if ((header = fprintf(f, "%d:", (int)limit)) < 0){
        return EOF;
    }
    if ((body = fwrite((void *) buffer, sizeof(char), limit, f)) < limit){
        return EOF;
    }
    if (fputc(',', f) == EOF){
        return EOF;
    }
    return header + body + 1;
}