GDB in a programming in C - gdb

Question

I have a problem with a program in C. I need the absolute memory address of the local variables to the function main. I am debugging with the gdb.
How to know if there is an overflow in the variables.
Thank you
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <ctype.h>
typedef int16_t int16;
#include "mulaw.h"
void decodeFile(FILE * fIn, FILE * fOut, uint32_t samples) {
uint32_t i;
uint8_t * inSamples = malloc(samples * sizeof(uint8_t));
int16_t * outSamples = malloc(samples * sizeof(int16_t));
fread(inSamples, sizeof(uint8_t), samples, fIn);
for (i = 0; i < samples; i++) {
outSamples[i] = 4 * muLaw[inSamples[i]];
}
fwrite(outSamples, sizeof(int16_t), samples, fOut);
free(inSamples);
free(outSamples);
}
#define MAX_FILE_SIZE 256
int main(int argc, char **argv)
{
char inputFile[MAX_FILE_SIZE];
char outputFile[MAX_FILE_SIZE];
FILE * fIn = NULL, * fOut = NULL;
struct header_t {
char ChunkID[4];
int32_t ChunkSize;
char Format[4];
} header;
char SubchunkID[4];
uint32_t SubchunkSize;
struct subheader_t {
int16_t AudioFormat;
int16_t NumChannels;
int32_t SampleRate;
int32_t ByteRate;
int16_t BlockAlign;
int16_t BitsPerSample;
int16_t ExtraParamSize;
int16_t Padding;
} subheader;
/* Usage */
if (argc != 3) {
puts("Usage is: mulaw INFILE OUTFILE\n");
exit(EXIT_FAILURE);
}
/* Careful here!!!!! */
strncpy(inputFile, argv[1], MAX_FILE_SIZE);
strncpy(outputFile, argv[2], MAX_FILE_SIZE);
/* Open input file */
fIn = fopen (inputFile, "rb");
/* Read main header */
fread(&header, sizeof(struct header_t), 1, fIn);
if (memcmp(header.ChunkID, "RIFF", 4) != 0
|| memcmp(header.Format, "WAVE", 4) != 0) {
fprintf(stderr, "Unknown input format\n");
exit(EXIT_FAILURE);
}
/* Read sub header */
while (fread(SubchunkID, sizeof(SubchunkID), 1, fIn)) {
fread(&SubchunkSize, sizeof(SubchunkSize), 1, fIn);
printf("Reading chunk of type %c%c%c%c (%d bytes)\n",
isprint(SubchunkID[0]) ? SubchunkID[0] : '?',
isprint(SubchunkID[1]) ? SubchunkID[1] : '?',
isprint(SubchunkID[2]) ? SubchunkID[2] : '?',
isprint(SubchunkID[3]) ? SubchunkID[3] : '?',
(int) SubchunkSize);
if (memcmp(SubchunkID, "fmt ", 4) == 0) {
/* read a fmt_ header */
fread(&subheader, SubchunkSize, 1, fIn);
/* we are going to adjust this header now to change the audio format */
if (subheader.AudioFormat != 7) {
fprintf(stderr, "Only mu-law audio input is supported\n");
exit(EXIT_FAILURE);
}
/* adjust audio format and bit depth */
subheader.AudioFormat = 1;
subheader.BitsPerSample = 16;
/* fix derivative fields */
subheader.ByteRate = subheader.SampleRate * subheader.NumChannels * subheader.BitsPerSample / 8;
subheader.BlockAlign = subheader.NumChannels * subheader.BitsPerSample / 8;
/* we don't write ExtraParamSize, because for AudioFormat == 1 it is not needed */
SubchunkSize -= 2;
/* Open file and write the header for our updated fmt_ chunk */
fOut = fopen (outputFile, "wb");
fwrite(&header, sizeof(struct header_t), 1, fOut); /* Main header */
fwrite(SubchunkID, sizeof(SubchunkID), 1, fOut); /* Subheader */
fwrite(&SubchunkSize, sizeof(SubchunkSize), 1, fOut);
fwrite(&subheader, SubchunkSize, 1, fOut);
} else if (memcmp(SubchunkID, "data", 4) == 0) {
/* here is our mu-law data */
/* write the header for our new chunk (it is twice as large) */
int32_t tSubchunkSize = SubchunkSize * 2;
fwrite(SubchunkID, sizeof(SubchunkID), 1, fOut);
fwrite(&tSubchunkSize, sizeof(SubchunkSize), 1, fOut);
/* process the data */
(fIn, fOut, SubchunkSize);
} else {
/* unknown chunk, skipping */
fseek(fIn, SubchunkSize, SEEK_CUR);
}
}
/* Cleanup and exit */
fclose(fIn);
fclose(fOut);
exit(EXIT_SUCCESS);
}
}

Answer 1

Just run gdb yourexe
Then in gdb:
break main
run
p &inputFile
That will give you the address of inputFile: ex $2 = (char (*)[256]) 0x62fd60
To get the size of a variable you can use p sizeof(variable) or p sizeof(type_of_the_variable>)
That said, not sure if it will help you to track down your problem.