Below C program provides API’s for receiving http GET response JSON to C buffer which you can then parse using JSON parsing library like JSMN as we mentioned in our another post. $ vim receive_http_get_response.c #include <stdio.h> #include <string.h> #include <stdlib.h> #include <curl/curl.h> #include <unistd.h> #include <stdbool.h> #include <getopt.h> struct curl_fetch_st { char *payload; size_t size; }; /* callback for curl fetch */ size_t curl_callback (void *contents, size_t size, size_t nmemb, void *userp) { size_t realsize = size * nmemb; /* calculate buffer size */ struct curl_fetch_st *p = (struct curl_fetch_st *) userp; /* cast pointer to fetch struct */ p->payload = (char *) realloc(p->payload, p->size + realsize + 1); if (p->payload == NULL) { fprintf(stderr, "ERROR: Failed to expand buffer in curl_callback"); free(p->payload); return -1; } /* copy contents to buffer */ memcpy(&(p->payload[p->size]), contents, realsize); p->size += realsize; p->payload[p->size] = 0; return realsize; } /* fetch and return url body via curl */ CURLcode curl_fetch_url(CURL *ch, const char *url, struct curl_fetch_st *fetch) { CURLcode rcode; /* curl result code */ fetch->payload = (char *) calloc(1, sizeof(fetch->payload)); if (fetch->payload == NULL) { fprintf(stderr, "ERROR: Failed to allocate payload in curl_fetch_url"); return CURLE_FAILED_INIT; } fetch->size = 0; curl_easy_setopt(ch, CURLOPT_URL, url); curl_easy_setopt(ch, CURLOPT_WRITEFUNCTION, curl_callback); curl_easy_setopt(ch, CURLOPT_WRITEDATA, (void *) fetch); curl_easy_setopt(ch, CURLOPT_USERAGENT, "libcurl-agent/1.0"); curl_easy_setopt(ch, CURLOPT_TIMEOUT, 5); curl_easy_setopt(ch, CURLOPT_FOLLOWLOCATION, 1); curl_easy_setopt(ch, CURLOPT_MAXREDIRS, 1); /* fetch the url */ rcode = curl_easy_perform(ch); return rcode; } struct curl_fetch_st curl_fetch; /* curl fetch struct */ char *curl_receive_reply_json_to_buffer(char *url, char *userpwd) { CURL *ch; /* curl handle */ CURLcode rcode; /* curl result code */ struct curl_fetch_st *cf = &curl_fetch; /* pointer to fetch struct */ struct curl_slist *headers = NULL; /* http headers to send with request */ /*TODO: when to free above malloced url*/ /* init curl handle */ if ((ch = curl_easy_init()) == NULL) { /* log error */ fprintf(stderr, "ERROR: Failed to create curl handle in fetch_session"); /* return error */ return NULL; } /* set content type */ headers = curl_slist_append(headers, "Accept: application/json"); headers = curl_slist_append(headers, "Content-Type: application/json"); curl_easy_setopt(ch, CURLOPT_HTTPGET, 1L); if (userpwd != NULL) { curl_easy_setopt(ch, CURLOPT_USERPWD, userpwd); curl_easy_setopt(ch, CURLOPT_HTTPHEADER, headers); curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 0L); curl_easy_setopt(ch, CURLOPT_SSL_VERIFYHOST, 0); } /* fetch page and capture return code */ rcode = curl_fetch_url(ch, url, cf); /* cleanup curl handle */ curl_easy_cleanup(ch); /* free headers */ curl_slist_free_all(headers); /* check return code */ if (rcode != CURLE_OK || cf->size < 1) { /* log error */ fprintf(stderr, "ERROR: Failed to fetch url (%s) - curl said: %s", url, curl_easy_strerror(rcode)); /* return error */ return NULL; // TODO: how to handle "curl said: Timeout was reached" } /* check payload */ if (cf->payload != NULL) { /* print result */ return cf->payload; } /* error */ fprintf(stderr, "ERROR: Failed to populate payload"); /* free payload */ free(cf->payload); /* return */ return NULL; } char *curl_download_json_to_buffer(char *remote_url) { char *reply_json_received; int wait_count = 0; while(wait_count < 10) { reply_json_received = curl_receive_reply_json_to_buffer(remote_url, NULL); if (json == NULL) { i++; printf("failed for %d th time, retrying after 3 sec...\n", i); sleep(5); //sleep for 5 sec and retry continue; } else break; } return reply_json_received; }
Command line execution
curl fetch */ size_t curl_callback (void *contents, size_t size, size_t nmemb, void *userp) { size_t realsize = size * nmemb; /* calculate buffer size */ struct curl_fetch_st *p = (struct curl_fetch_st *) userp; /* cast pointer to fetch struct */ p->payload = (char *) realloc(p->payload, p->size + realsize + 1); if (p->payload == NULL) { fprintf(stderr, "ERROR: Failed to expand buffer in curl_callback"); free(p->payload); return -1; } /* copy contents to buffer */ memcpy(&(p->payload[p->size]), contents, realsize); p->size += realsize; p->payload[p->size] = 0; return realsize; } /* fetch and return url body via curl */ CURLcode curl_fetch_url(CURL *ch, const char *url, struct curl_fetch_st *fetch) { CURLcode rcode; /* curl result code */ fetch->payload = (char *) calloc(1, sizeof(fetch->payload)); if (fetch->payload == NULL) { fprintf(stderr, "ERROR: Failed to allocate payload in curl_fetch_url"); return CURLE_FAILED_INIT; } fetch->size = 0; curl_easy_setopt(ch, CURLOPT_URL, url); curl_easy_setopt(ch, CURLOPT_WRITEFUNCTION, curl_callback); curl_easy_setopt(ch, CURLOPT_WRITEDATA, (void *) fetch); curl_easy_setopt(ch, CURLOPT_USERAGENT, "libcurl-agent/1.0"); curl_easy_setopt(ch, CURLOPT_TIMEOUT, 5); curl_easy_setopt(ch, CURLOPT_FOLLOWLOCATION, 1); curl_easy_setopt(ch, CURLOPT_MAXREDIRS, 1); /* fetch the url */ rcode = curl_easy_perform(ch); return rcode; } struct curl_fetch_st curl_fetch; /* curl fetch struct */ char *curl_receive_reply_json_to_buffer(char *url, char *userpwd) { CURL *ch; /* curl handle */ CURLcode rcode; /* curl result code */ struct curl_fetch_st *cf = &curl_fetch; /* pointer to fetch struct */ struct curl_slist *headers = NULL; /* http headers to send with request */ /*TODO: when to free above malloced url*/ /* init curl handle */ if ((ch = curl_easy_init()) == NULL) { /* log error */ fprintf(stderr, "ERROR: Failed to create curl handle in fetch_session"); /* return error */ return NULL; } /* set content type */ headers = curl_slist_append(headers, "Accept: application/json"); headers = curl_slist_append(headers, "Content-Type: application/json"); curl_easy_setopt(ch, CURLOPT_HTTPGET, 1L); if (userpwd != NULL) { curl_easy_setopt(ch, CURLOPT_USERPWD, userpwd); curl_easy_setopt(ch, CURLOPT_HTTPHEADER, headers); curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 0L); curl_easy_setopt(ch, CURLOPT_SSL_VERIFYHOST, 0); } /* fetch page and capture return code */ rcode = curl_fetch_url(ch, url, cf); /* cleanup curl handle */ curl_easy_cleanup(ch); /* free headers */ curl_slist_free_all(headers); /* check return code */ if (rcode != CURLE_OK || cf->size < 1) { /* log error */ fprintf(stderr, "ERROR: Failed to fetch url (%s) - curl said: %s", url, curl_easy_strerror(rcode)); /* return error */ return NULL; // TODO: how to handle "curl said: Timeout was reached" } /* check payload */ if (cf->payload != NULL) { /* print result */ return cf->payload; } /* error */ fprintf(stderr, "ERROR: Failed to populate payload"); /* free payload */ free(cf->payload); /* return */ return NULL; } char *curl_download_json_to_buffer(char *remote_url) { char *reply_json_received; int wait_count = 0; while(wait_count < 10) { reply_json_received = curl_receive_reply_json_to_buffer(remote_url, NULL); if (json == NULL) { i++; printf("failed for %d th time, retrying after 3 sec...\n", i); sleep(5); //sleep for 5 sec and retry continue; } else break; } return reply_json_received; }Gotchas and common issues
Permission checks - verify user access rights and sudo privileges before executing system-level operations.
Environment configuration - double-check path variables and dependency versions to prevent runtime failures.
Backup safeguards - maintain configuration backups before applying system or database modifications.
Following these steps ensures clean configuration and reliable execution for receive http get response json to buffer using curl api – c program.
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