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Arrays in C: C code example for using a variable sized array.

How to read a text file of any size into a variable sized, growing array, also allowing
text lines of differing length, demonstrating dynamic memory allocation.
   C array source code example:
   -  read text file into a variable sized array
   -  with a variable size per text line (up to a limit)
   -  sort the array
   -  write array content into another text file.

#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <stdlib.h>

// tell that these functions exist further below:
int perr(const char *pszFormat, ...);
int processData(char **apText, int iLinesUsed, int iLinesAlloced);

// maximum supported characters per line:
#define MAX_LINE_LENGTH 100000

// declare one global buffer to store a huge line:
char szLineBuf[MAX_LINE_LENGTH+100];
// +100 is safety space, to avoid crash on off-by-one errors.

int main(int argc, char *argv[])
   if (argc < 3)
      return 9+perr("specify input and output filename.\n");

   char *pszInFile  = argv[1];
   char *pszOutFile = argv[2];

   // preparation: make a clean empty buffer
   memset(szLineBuf, 0, sizeof(szLineBuf));

   // full flexible array for text storage:
   int iLinesAlloced  = 10;  // initial array size
   int iLinesUsed     =  0;  // number of lines actually used
   char **apTextData  = malloc(sizeof(char*) * (iLinesAlloced+10));
   // in C++, it's:   = new char*[iLinesAlloced+10];
   if (!apTextData)
      return 9+perr("out of memory\n");

   // read input file into the array. use binary mode "rb"
   // to avoid CR/LF line end conversions.
   FILE *fin = fopen(pszInFile , "rb");
   if (!fin )
      return 9+perr("cannot read %s\n" , pszInFile);

   // while not end of file, read another line
   while (fgets(szLineBuf, sizeof(szLineBuf)-10, fin) != 0)
      // space left in the array?
      if (iLinesUsed >= iLinesAlloced)
         // no: expand the array
         int iNewSize = iLinesAlloced * 2 + 10;
         printf("[expanding array to %d lines]\n", iNewSize);
         char **apNewData = malloc(sizeof(char*) * (iNewSize+10));
         // in C++:       = new char*[iNewSize+10];
         if (!apNewData)
           { perr("out of memory\n"); break; }
         memcpy(apNewData, apTextData, sizeof(char*) * iLinesUsed);
         free(apTextData); // in C++: delete [] apTextData;
         apTextData = apNewData;
         iLinesAlloced = iNewSize;

      // strip CR/LF from line endings so we get pure text
      char *psz = strchr(szLineBuf, '\r'); if (psz) *psz = '\0';
            psz = strchr(szLineBuf, '\n'); if (psz) *psz = '\0';

      // store the line
      apTextData[iLinesUsed] = strdup(szLineBuf);



   // do some processing on the array contents
   processData(apTextData, iLinesUsed, iLinesAlloced);

   // write the whole array to output file. use text mode "w"
   // so under windows it will create CRLF line endings.
   FILE *fout = fopen(pszOutFile, "w");
   if (!fout)
      return 9+perr("cannot write %s\n", pszOutFile);

   int iLine;
   for (iLine=0; iLine<iLinesUsed; iLine++)
      strncpy(szLineBuf, apTextData[iLine], MAX_LINE_LENGTH);
      szLineBuf[MAX_LINE_LENGTH-2] = '\0'; // safety w/ space for LF

      strcat(szLineBuf, "\n"); // produces CRLF under windows

      int nlen = strlen(szLineBuf);
      if (fwrite(szLineBuf, 1, nlen, fout) != nlen)
         return 9+perr("failed to fully write %s\n", pszOutFile);


   // cleanup
   free(apTextData); // in C++: delete [] apTextData;

   return 0;

// example 1: sort the array text lines alphabetically
// with a very simple sorting algorithm (bubblesort).
int processData(char **apText, int iLinesUsed, int iLinesAlloced)
   int iOuterIdx, iInnerIdx;

   for (iOuterIdx=0; iOuterIdx<iLinesUsed; iOuterIdx++)
      for (iInnerIdx=iOuterIdx+1; iInnerIdx<iLinesUsed; iInnerIdx++)
         char *pszLine1 = apText[iOuterIdx];
         char *pszLine2 = apText[iInnerIdx];
         if (strcmp(pszLine1, pszLine2) > 0)
            // swap both lines
            apText[iOuterIdx] = pszLine2;
            apText[iInnerIdx] = pszLine1;

      // show a progress indicator with percent info
      printf("[sorting ... %02d%%] \r",
         iOuterIdx * 100 / (iLinesUsed ? iLinesUsed : 1));

   printf("sorted %d lines.\n", iLinesUsed);

   return 0;

// helper function: print error with variable parameters.
int perr(const char *pszFormat, ...)
   va_list argList;
   va_start(argList, pszFormat);
   char szBuf[1024];
   vsprintf(szBuf, pszFormat, argList);
   fprintf(stderr, "error: %s", szBuf);
   return 0;

// See also: Simple C example for a fixed-size array .