Simple C example of doing an HTTP POST and consuming the response

Question

I would like to create a very simple C application that does an HTTP post   It will take a few parameters  and use these to construct a URL   I d just like to do a simple HTTP POST and get the response without the use of curl  the libraries are not and will not be installed on the machine this needs to run    Pseudo-code    Process 2 args Put args into template URL  http   api somesite com apikey ARG1 amp command ARG2 Do POST on generated URL Consume response   My Google and SO searches haven t yielded anything on this matter

User · Answer

Jerry s answer is great  However  it doesn t handle large responses  A simple change to handle this   memset response  0  sizeof response    total   sizeof response -1  received   0  do       printf  RESPONSE   s n   response          HANDLE RESPONSE CHUCK HERE BY  FOR EXAMPLE  SAVING TO A FILE      memset response  0  sizeof response        bytes   recv sockfd  response  1024  0       if  bytes  lt  0          printf  ERROR reading response from socket        if  bytes    0          break      received  bytes    while  1

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Handle added  Added Host header  Added linux   windows support  tested  XP WIN7   WARNING  ERROR    segmentation fault  if no host path or port as argument    include  lt stdio h gt     printf  sprintf     include  lt stdlib h gt     exit  atoi  malloc  free     include  lt unistd h gt     read  write  close     include  lt string h gt     memcpy  memset     ifdef   linux         include  lt sys socket h gt     socket  connect         include  lt netdb h gt     struct hostent  gethostbyname         include  lt netinet in h gt     struct sockaddr in  struct sockaddr     elif  WIN32      include  lt winsock2 h gt       include  lt ws2tcpip h gt       include  lt windows h gt       pragma comment lib  ws2 32 lib     Winsock Library   else   endif  void error const char  msg    perror msg   exit 0      int main int argc char  argv           int i      struct hostent  server      struct sockaddr in serv addr      int bytes  sent  received  total  message size      char  message  response 4096       int portno   atoi argv 2   gt 0 atoi argv 2   80      char  host   strlen argv 1   gt 0 argv 1   localhost       char  path   strlen argv 4   gt 0 argv 4           if  argc  lt  5    puts  Parameters   lt host gt   lt port gt   lt method gt   lt path gt    lt data gt    lt headers gt       exit 0            How big is the message         message size 0      if  strcmp argv 3   GET                          printf  Process 1 n            message size  strlen   s  s s s HTTP 1 0 r nHost   s r n              method                    message size  strlen argv 3                               path                      message size  strlen path                              headers                   if argc gt 5              message size  strlen argv 5                           query string              for i 6 i lt argc i                                          headers                       message size  strlen argv i   strlen   r n            message size  strlen   r n                                blank line                  else                       printf  Process 2 n            message size  strlen   s  s HTTP 1 0 r nHost   s r n            message size  strlen argv 3                               method                    message size  strlen path                                 path                      for i 6 i lt argc i                                          headers                       message size  strlen argv i   strlen   r n            if argc gt 5              message size  strlen  Content-Length   d r n   10     content length            message size  strlen   r n                                blank line                if argc gt 5              message size  strlen argv 5                           body                                printf  Allocating    n           allocate space for the message        message malloc message size           fill in the parameters        if  strcmp argv 3   GET                  if argc gt 5              sprintf message   s  s s s HTTP 1 0 r nHost   s r n                   strlen argv 3   gt 0 argv 3   GET                    method                            path                                              path                              strlen argv 5   gt 0                                                                   strlen argv 5   gt 0 argv 5     host                 query string              else             sprintf message   s  s HTTP 1 0 r nHost   s r n                   strlen argv 3   gt 0 argv 3   GET                    method                            path host                                         path                      for i 6 i lt argc i                                          headers                        strcat message argv i   strcat message   r n             strcat message   r n                                      blank line                  else               sprintf message   s  s HTTP 1 0 r nHost   s r n               strlen argv 3   gt 0 argv 3   POST                       method                        path host                                             path                      for i 6 i lt argc i                                          headers                        strcat message argv i   strcat message   r n             if argc gt 5              sprintf message strlen message   Content-Length   d r n   int strlen argv 5             strcat message   r n                                      blank line                if argc gt 5              strcat message argv 5                                 body                        printf  Processed n           What are we going to send         printf  Request  n s n  message              lookup the ip address         total   strlen message          create the socket         ifdef  WIN32 WSADATA wsa  SOCKET s   printf   nInitialising Winsock       if  WSAStartup MAKEWORD 2 2   amp wsa     0        printf  Failed  Error Code    d  WSAGetLastError         return 1     printf  Initialised  n       Create a socket if  s   socket AF INET   SOCK STREAM   0       INVALID SOCKET        printf  Could not create socket    d    WSAGetLastError        printf  Socket created  n     server   gethostbyname host   serv addr sin addr s addr   inet addr server- gt h addr   serv addr sin family   AF INET  serv addr sin port   htons portno   memset  amp serv addr 0 sizeof serv addr    serv addr sin family   AF INET  serv addr sin port   htons portno   memcpy  amp serv addr sin addr s addr server- gt h addr server- gt h length     Connect to remote server if  connect s    struct sockaddr    amp serv addr   sizeof serv addr    lt  0        printf  connect failed with error code    d    WSAGetLastError         return 1     puts  Connected    if  send s   message   strlen message    0   lt  0        printf  Send failed with error code    d    WSAGetLastError         return 1    puts  Data Send n       Receive a reply from the server if  received   recv s   response   2000   0      SOCKET ERROR        printf  recv failed with error code    d    WSAGetLastError        puts  Reply received n       Add a NULL terminating character to make it a proper string before printing response received      0   puts response    closesocket s   WSACleanup         endif      ifdef   linux        int sockfd      server   gethostbyname host       if  server    NULL  error  ERROR  no such host            sockfd   socket AF INET  SOCK STREAM  0           if  sockfd  lt  0  error  ERROR opening socket               fill in the structure            memset  amp serv addr 0 sizeof serv addr            serv addr sin family   AF INET          serv addr sin port   htons portno           memcpy  amp serv addr sin addr s addr server- gt h addr server- gt h length                      connect the socket            if  connect sockfd  struct sockaddr    amp serv addr sizeof serv addr    lt  0              error  ERROR connecting                       send the request         sent   0      do           bytes   write sockfd message sent total-sent           if  bytes  lt  0              error  ERROR writing message to socket            if  bytes    0              break          sent  bytes        while  sent  lt  total          receive the response        memset response  0  sizeof response        total   sizeof response -1      received   0      printf  Response   n        do          printf   s   response          memset response  0  sizeof response           bytes   recv sockfd  response  1024  0           if  bytes  lt  0             printf  ERROR reading response from socket           if  bytes    0             break         received  bytes        while  1        if  received    total          error  ERROR storing complete response from socket            close the socket        close sockfd        endif       free message        return 0

User · Answer

After weeks of research  I came up with the following code  I believe this is the bare minimum needed to make a secure connection with SSL to a web server      include  lt stdio h gt   include  lt openssl ssl h gt   include  lt openssl err h gt   include  lt openssl bio h gt    define APIKEY  YOUR API KEY   define HOST  YOUR WEB SERVER URI   define PORT  443   int main                     Initialize the variables            BIO  bio      SSL  ssl      SSL CTX  ctx                   Registers the SSL TLS ciphers and digests                  Basically start the security layer             SSL library init                    Creates a new SSL CTX object as a framework to establish TLS SSL         or DTLS enabled connections            ctx   SSL CTX new SSLv23 client method                     - gt  Error check            if  ctx    NULL                printf  Ctx is null n                           Creates a new BIO chain consisting of an SSL BIO            bio   BIO new ssl connect ctx                   Use the variable from the beginning of the file to create a          string that contains the URL to the site that you want to connect         to while also specifying the port             BIO set conn hostname bio  HOST     PORT                    Attempts to connect the supplied BIO            if BIO do connect bio   lt   0                printf  Failed connection n            return 1            else               printf  Connected n                          The bare minimum to make a HTTP request             char  write buf    POST   HTTP 1 1 r n                         Host    HOST   r n                         Authorization  Basic   APIKEY   r n                         Connection  close r n                          r n                    Attempts to write len bytes from buf to BIO            if BIO write bio  write buf  strlen write buf    lt   0                               Handle failed writes here                    if  BIO should retry bio                            Not worth implementing  but worth knowing                                    - gt  Let us know about the failed writes                    printf  Failed write n                          Variables used to read the response from the server            int size      char buf 1024                   Read the response message            for                                  Get chunks of the response 1023 at the time                     size   BIO read bio  buf  1023                           If no more data  then exit the loop                    if size  lt   0                        break                                    Terminate the string with a 0  to let know C when the string              ends                     buf size    0                          - gt   Print out the response                    printf   s   buf                         Clean after ourselves            BIO free all bio       SSL CTX free ctx        return 0      The code above will explain in details how to establish a TLS connection with a remote server    Important note  this code doesn t check if the public key was signed by a valid authority  Meaning I don t use root certificates for validation  Don t forget to implement this check otherwise you won t know if you are connecting the right website  When it comes to the request itself  It is nothing more then writing the HTTP request by hand   You can also find under this link an explanation how to instal openSSL in your system  and how to compile the code so it uses the secure library

User · Answer

A message has a header part and a message body separated by a blank line   The blank line is ALWAYS needed even if there is no message body   The header starts with a command and has additional lines of key value pairs separated by a colon and a space   If there is a message body  it can be anything you want it to be   Lines in the header and the blank line at the end of the header must end with a carraige return and linefeed pair  see HTTP header line break style  so that s why those lines have  r n at the end   A URL has the form of http   host port path query string  There are two main ways of submitting a request to a website    GET  The query string is optional but  if specified  must be reasonably short  Because of this the header could just be the GET command and nothing else  A sample message could be   GET  path query string HTTP 1 0 r n  r n  POST  What would normally be in the query string is in the body of the message instead  Because of this the header needs to include the Content-Type  and Content-Length  attributes as well as the POST command   A sample message could be   POST  path HTTP 1 0 r n Content-Type  text plain r n Content-Length  12 r n  r n query string    So  to answer your question  if the URL you are interested in POSTing to is http   api somesite com apikey ARG1 amp command ARG2 then there is no body or query string and  consequently  no reason to POST because there is nothing to put in the body of the message and so nothing to put in the Content-Type  and Content-Length   I guess you could POST if you really wanted to   In that case your message would look like   POST  apikey ARG1 amp command ARG2 HTTP 1 0 r n  r n   So to send the message the C program needs to    create a socket lookup the IP address  open the socket send the request wait for the response close the socket   The send and receive calls won t necessarily send receive ALL the data you give them - they will return the number of bytes actually sent received   It is up to you to call them in a loop and send receive the remainder of the message   What I did not do in this sample is any sort of real error checking - when something fails I just exit the program   Let me know if it works for you    include  lt stdio h gt     printf  sprintf     include  lt stdlib h gt     exit     include  lt unistd h gt     read  write  close     include  lt string h gt     memcpy  memset     include  lt sys socket h gt     socket  connect     include  lt netinet in h gt     struct sockaddr in  struct sockaddr     include  lt netdb h gt     struct hostent  gethostbyname     void error const char  msg    perror msg   exit 0      int main int argc char  argv             first what are we going to send and where are we going to send it         int portno          80      char  host           api somesite com       char  message fmt    POST  apikey  s amp command  s HTTP 1 0 r n r n        struct hostent  server      struct sockaddr in serv addr      int sockfd  bytes  sent  received  total      char message 1024  response 4096        if  argc  lt  3    puts  Parameters   lt apikey gt   lt command gt     exit 0             fill in the parameters        sprintf message message fmt argv 1  argv 2        printf  Request  n s n  message           create the socket        sockfd   socket AF INET  SOCK STREAM  0       if  sockfd  lt  0  error  ERROR opening socket            lookup the ip address        server   gethostbyname host       if  server    NULL  error  ERROR  no such host            fill in the structure        memset  amp serv addr 0 sizeof serv addr        serv addr sin family   AF INET      serv addr sin port   htons portno       memcpy  amp serv addr sin addr s addr server- gt h addr server- gt h length           connect the socket        if  connect sockfd  struct sockaddr    amp serv addr sizeof serv addr    lt  0          error  ERROR connecting            send the request        total   strlen message       sent   0      do           bytes   write sockfd message sent total-sent           if  bytes  lt  0              error  ERROR writing message to socket            if  bytes    0              break          sent  bytes        while  sent  lt  total           receive the response        memset response 0 sizeof response        total   sizeof response -1      received   0      do           bytes   read sockfd response received total-received           if  bytes  lt  0              error  ERROR reading response from socket            if  bytes    0              break          received  bytes        while  received  lt  total        if  received    total          error  ERROR storing complete response from socket            close the socket        close sockfd           process response        printf  Response  n s n  response        return 0      Like the other answer pointed out  4096 bytes is not a very big response   I picked that number at random assuming that the response to your request would be short   If it can be big you have two choices    read the Content-Length  header from the response and then dynamically allocate enough memory to hold the whole response  write the response to a file as the pieces arrive   Additional information to answer the question asked in the comments   What if you want to POST data in the body of the message   Then you do need to include the Content-Type  and Content-Length  headers   The Content-Length  is the actual length of everything after the blank line that separates the header from the body   Here is a sample that takes the following command line arguments    host port command  GET or POST  path  not including the query data  query data  put into the query string for GET and into the body for POST   list of headers  Content-Length  is automatic if using POST    So  for the original question you would run   a out api somesite com 80 GET   apikey ARG1 amp command ARG2    And for the question asked in the comments you would run   a out api somesite com 80 POST    name ARG1 amp value ARG2   Content-Type  application x-www-form-urlencoded    Here is the code    include  lt stdio h gt     printf  sprintf     include  lt stdlib h gt     exit  atoi  malloc  free     include  lt unistd h gt     read  write  close     include  lt string h gt     memcpy  memset     include  lt sys socket h gt     socket  connect     include  lt netinet in h gt     struct sockaddr in  struct sockaddr     include  lt netdb h gt     struct hostent  gethostbyname     void error const char  msg    perror msg   exit 0      int main int argc char  argv          int i          first where are we going to send it         int portno   atoi argv 2   gt 0 atoi argv 2   80      char  host   strlen argv 1   gt 0 argv 1   localhost        struct hostent  server      struct sockaddr in serv addr      int sockfd  bytes  sent  received  total  message size      char  message  response 4096        if  argc  lt  5    puts  Parameters   lt host gt   lt port gt   lt method gt   lt path gt    lt data gt    lt headers gt       exit 0             How big is the message         message size 0      if  strcmp argv 3   GET                  message size  strlen   s  s s s HTTP 1 0 r n              method                    message size  strlen argv 3                               path                      message size  strlen argv 4                               headers                   if argc gt 5              message size  strlen argv 5                           query string              for i 6 i lt argc i                                          headers                       message size  strlen argv i   strlen   r n            message size  strlen   r n                                blank line                  else               message size  strlen   s  s HTTP 1 0 r n            message size  strlen argv 3                               method                    message size  strlen argv 4                               path                      for i 6 i lt argc i                                          headers                       message size  strlen argv i   strlen   r n            if argc gt 5              message size  strlen  Content-Length   d r n   10     content length            message size  strlen   r n                                blank line                if argc gt 5              message size  strlen argv 5                           body                            allocate space for the message        message malloc message size           fill in the parameters        if  strcmp argv 3   GET                  if argc gt 5              sprintf message   s  s s s HTTP 1 0 r n                   strlen argv 3   gt 0 argv 3   GET                    method                            strlen argv 4   gt 0 argv 4                          path                              strlen argv 5   gt 0                                                                   strlen argv 5   gt 0 argv 5                          query string              else             sprintf message   s  s HTTP 1 0 r n                   strlen argv 3   gt 0 argv 3   GET                    method                            strlen argv 4   gt 0 argv 4                          path                      for i 6 i lt argc i                                          headers                        strcat message argv i   strcat message   r n             strcat message   r n                                      blank line                  else               sprintf message   s  s HTTP 1 0 r n               strlen argv 3   gt 0 argv 3   POST                       method                        strlen argv 4   gt 0 argv 4                              path                      for i 6 i lt argc i                                          headers                        strcat message argv i   strcat message   r n             if argc gt 5              sprintf message strlen message   Content-Length   d r n  strlen argv 5             strcat message   r n                                      blank line                if argc gt 5              strcat message argv 5                                 body                            What are we going to send         printf  Request  n s n  message           create the socket        sockfd   socket AF INET  SOCK STREAM  0       if  sockfd  lt  0  error  ERROR opening socket            lookup the ip address        server   gethostbyname host       if  server    NULL  error  ERROR  no such host            fill in the structure        memset  amp serv addr 0 sizeof serv addr        serv addr sin family   AF INET      serv addr sin port   htons portno       memcpy  amp serv addr sin addr s addr server- gt h addr server- gt h length           connect the socket        if  connect sockfd  struct sockaddr    amp serv addr sizeof serv addr    lt  0          error  ERROR connecting            send the request        total   strlen message       sent   0      do           bytes   write sockfd message sent total-sent           if  bytes  lt  0              error  ERROR writing message to socket            if  bytes    0              break          sent  bytes        while  sent  lt  total           receive the response        memset response 0 sizeof response        total   sizeof response -1      received   0      do           bytes   read sockfd response received total-received           if  bytes  lt  0              error  ERROR reading response from socket            if  bytes    0              break          received  bytes        while  received  lt  total        if  received    total          error  ERROR storing complete response from socket            close the socket        close sockfd           process response        printf  Response  n s n  response        free message       return 0

[c] Simple C example of doing an HTTP POST and consuming the response

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