Program compiles but stops responding during execution

I wrote a program that compiles, but it is not responding at runtime.

This program opens a PGM file and saves the convolution derivative in saida.pgm . The LerPGM() function receives a string with the filename and returns a 2D array dynamically allocated in img .

The Convolucao() function performs the convolution operation on the img array using a kernel already declared and saved on another array. SalvarPGM() receives this other array and a string and then writes the array to a PGM file whose name is the string received.

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

typedef struct{
    int c;
    int l;
    unsigned char maximo;
    unsigned char **imagem;
} PGM;

PGM *LerPGM(char* entrada);
void SalvarPGM(PGM *img, char* saida);
void Convolucao(PGM *img, char **kernel, PGM *saida);

int main()
{
    PGM *imgconv;
    char kernel[3][3]={{-1, 0, 1},{-1, 0, 1},{-1, 0, 1}};
    if(SalvarPGM(Convolucao(LerPGM("entrada.pgm"), kernel, imgconv), "saida.pgm")) printf("Operacao realizada com sucesso.");
    return 0;
}

PGM *LerPGM(char* entrada){
    PGM img;
    char tipo[3];
    int i, j;


    FILE *arq;
    arq = fopen(entrada, "r");
    if(arq == NULL){
        printf("Arquivo nao encontrado.");
        return 0;
    }

    fscanf(arq, "%s %d %d %d", &tipo, &img.c, &img.l, &img.maximo);
    if(strcmp(tipo, "P2")){
        printf("O arquivo nao e PGM.");
        return 0;
    }

    img.imagem = malloc(img.l * sizeof(char *));
    for(i=0; i<img.c; i++) img.imagem[i] = malloc(img.c * sizeof(char));
    if(img.imagem == NULL){
        printf("Falha na alocacao de memoria.");
        return 0;
    }

    for(i=0; i<img.l; i++){
        for(j=0; j<img.c; j++){
            fscanf(entrada, "%d", &img.imagem[i][j]);
        }
    }

    fclose(arq);

    return &img;
}

void Convolucao(PGM *img, char **kernel, PGM *saida){
    unsigned char aux[img->l+2][img->c+2];
    int i, j, k, l, soma;
    saida = img;

    for(i=1; i<img->l+1; i++){
        for(j=1; j<img->c+1; j++){
            aux[i][j] = img->imagem[i-1][j-1];
        }
    }

    for(i=1; i<img->l+1; i++){
        aux[i][0] = aux[i][1];
        aux[i][img->c+1] = aux[i][img->c];
    }
    for(j=0; j<img->c+2; j++){
        aux[0][j] = aux[1][j];
        aux[img->l+1][j] = aux[img->l][j];
    }

    for(i=1; i<img->l+1; i++){
        for(j=1; j<img->c+1; j++){
            soma=0;
            for(k=2; k>=0; k--){
                for(l=2; l>=0; l--){
                    soma+=kernel[k][l]*aux[i+(1-k)][j+(1-l)];
                }
            }
            if(soma<=0) saida->imagem[i-1][j-1]=0;
                else if(soma>=255) saida->imagem[i-1][j-1]=255;
                    else saida->imagem[i-1][j-1]=soma;
        }
    }

    free(img);
    free(aux);

    return saida;
}

void SalvarPGM(PGM *img, char* saida){
    int i, j;
    FILE *arq;

    arq = fopen("saida.pgm", "w");
    if(arq == NULL){
        printf("Ocorreu um erro.");
        return 0;
    }
    fprintf(arq, "%s\n%d %d %d", "P2", img->c, img->l, img->maximo);
    for(i=0; i<img->l; i++){
        fprintf(saida,"\n");
        for(j=0; j<img->c; j++){
            fprintf(saida, "%d ", img->imagem[i][j]);

        }
    }

    fclose(arq);
    free(img);
    return 1;
}

This program is not responding, and I do not know exactly why. I do not know if I'm using the pointers wrongly (or maybe structs ) or if I'm allocating them wrongly. When I do not use functions the program works perfectly. For example:

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

typedef struct{
    int c;
    int l;
    unsigned char maximo;
    unsigned char **imagem;
} PGM;

int main()
{
    PGM img;
    char tipo[10];
    int i, j, k, kk;
    FILE *entrada;
    entrada=fopen("entrada.pgm", "r");
    if(entrada==NULL){
        printf("Arquivo nao encontrado.");
        return 0;
    }
    fscanf(entrada, "%s %d %d %d", &tipo, &img.c, &img.l, &img.maximo);
    if(strcmp(tipo, "P2")){
        printf("O arquivo nao e PGM.");
        return 0;
    };

    img.imagem=malloc(img.l * sizeof(char *));
    for(i=0; i<img.c; i++) img.imagem[i]=malloc(img.c * sizeof(char));
    if(img.imagem==NULL) printf("Ocorreu um erro!");
    for(i=0; i<img.l; i++){
        for(j=0; j<img.c; j++){
            fscanf(entrada, "%d", &img.imagem[i][j]);
        };
    };

    int kernel[3][3]={{-1, 0, 1}, {-1, 0, 1},{-1, 0, 1}};
    unsigned char aux[img.l+2][img.c+2];
    for(i=1; i<img.l+1; i++){
        for(j=1; j<img.c+1; j++){
            aux[i][j]=img.imagem[i-1][j-1];
        };
    };


    for(i=1; i<img.l+1; i++){
        aux[i][0]=aux[i][1];
        aux[i][img.c+1]=aux[i][img.c];
    };
    for(j=0; j<img.c+2; j++){
        aux[0][j]=aux[1][j];
        aux[img.l+1][j]=aux[img.l][j];
    };

    int soma;
    for(i=1; i<img.l+1; i++){
        for(j=1; j<img.c+1; j++){
            soma=0;
            for(k=2; k>=0; k--){
                for(kk=2; kk>=0; kk--){
                    soma+=kernel[k][kk]*aux[i+(1-k)][j+(1-kk)];
                };
            };
            if(soma<=0) img.imagem[i-1][j-1]=0;
                else if(soma>=255) img.imagem[i-1][j-1]=255;
                    else img.imagem[i-1][j-1]=soma;
        };
    };

    fclose(entrada);
    FILE *saida;
    saida=fopen("saida.pgm", "w");
    fprintf(saida, "%s\n%d %d %d", tipo, img.c, img.l, img.maximo);
    for(i=0; i<img.l; i++){
        fprintf(saida,"\n");
        for(j=0; j<img.c; j++){
            fprintf(saida, "%d ", img.imagem[i][j]);

        };
    };

    fclose(saida);

    return 0;
}