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円';
 psz = strchr(szLineBuf, '\n'); if (psz) *psz = '0円';
 // store the line
 apTextData[iLinesUsed] = strdup(szLineBuf);
 iLinesUsed++;
 }
 fclose(fin);
 // 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円'; // 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);
 }
 fclose(fout);
 // 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;
}

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